the fate of methyl bromide, ethylene glycol, and propylene

Retrospective Theses and Dissertations Iowa State University Capstones, Theses andDissertations

1996

The fate of methyl bromide, ethylene glycol, andpropylene glycol in soil and surface water: influenceof soil variables and vegetation on degradation andoffsite movementPatricia Jane RiceIowa State University

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The fate of methyl bromide, ethylene glycol, and propylene glycol in soil and surface water:

influence of soil variables and vegetation on degradation and offsite movement

by

Patricia Jane Rice

A dissertation submitted to the graduate faculty

in partial fulfillment of the requirements for the degree of

DOCTOR OF PfflLOSOPHY

Major: Toxicology

Major Professors: Joel R. Coats and Todd A. Anderson

Iowa State University

Ames, Iowa

1996

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Graduate College Iowa State University

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Patricia Jane Rice

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iii

TABLE OF CONTENTS

LISTOFFIGURES v

LIST OF TABLES viii

ACKNOWLEDGMENTS ix

CHAPTER 1. GENERAL INTRODUCTION 1 Introduction 1 Methyl Bromide 1 Ethylene Glycol and Propylene Glycol 3 Rhizosphere 6 Dissertation Objectives 8 Dissertation Organization 8

CHAPTER 2. THE INFLUENCE OF SOIL ENVIRONMENTAL VARIABLES ON THE DEGRADATION AND VOLATILITY OF METHYL BROMIDE IN SOIL 10

Abstract 10 Introduction 11 Experimental Procedures 13 Results and Discussion 22 Conclusions 36 Acknowledgment 37 References 37

CHAPTER 3. EVALUATION OF THE USE OF VEGETATION FOR REDUCING THE ENVIRONMENTAL IMPACT OF DEICING AGENTS 42

Abstract 42 Introduction 43 Materials and Methods 45 Results 51 Discussion 63 Conclusion 66 Acknowledgment 66 References 67

CHAPTER 4. THE USE OF AQUATIC PLANTS TO REMEDIATE SURFACE WATERS CONTAMINATED WTTH AIRCRAFT DEICING AGENTS 71

Abstract 71

iv

Introduction 72 Materials and Methods 73 Results 78 Discussion 83 Conclusion 86 Acknowledgment 86 References 86

GENERAL CONCLUSION 89 Influence of soU environmental variables on the fate of methyl bromide in soil 89 Use of vegetation to reduce the environmental impact of deicing agents 90

APPENDIX. THE INFLUENCE OF VEGETATION ON THE MOBILITY OF PROPYLENE GLYCOL THROUGH THE SOIL PROFILE 92

GENERAL REFERENCES 103

V

LISTOFHGURES

CHAPTER 1. GENERAL INTRODUCTION

Figure 1. D^radation products of methyl bromide 4

Figure 2. Structure of ethylene glycol and propylene glycol 4

CHAPTER 2. THE INFLUENCE OF SOIL ENVIRONMENTAL VARIABLES ON THE DEGRADATION AND VOLATILITY OF METHYL BROMIDE IN SOIL

Figure 1. Undisturbed soil column used to study the volatility, movement, and degradation of methyl bromide 17

Figure 2. \ficrobial respiration in soil fumigated with 2,733 ^ig/g methyl bromide. Data points are the mean ± one standard deviation 26

Figure 3. Microbial respiration in soil fumigated with 350 |ig/g methyl bromide. Data points are the mean ± one standard deviation 27

Figure 4. Volatility of methyl bromide in undisturbed soil colunms following a 48-h fiimigation period. Data points are the mean ± one standard deviation 29

Figure 5. Bromide ion breakthrough from an undisturbed soil column treated with methyl bromide. Soil columns were leached weekly after the 48-h fumigation period 31

Figure 6. Volatilization of field-applied methyl bromide 32

Figure 7. Concentration of gaseous methyl bromide detected in soil from three fumigated fields 33

Figure 8. Mean concentrations of bromide ion detected in soil from three methyl bromide-fumigated fields 35

vi

CHAPTERS.

Figure 1.

Figure 2.

Figure 3.

Figure 4.

Figure 5.

Figure 6.

CHAPTER 4.

Figure 1.

Figure 2.

Figure 3.

EVALUATION OF THE USE OF VEGETATION FOR REDUCING THE ENVIRONMENTAL IMPACT OF DEICING AGENTS

Sampling sites at Offutt Air Force Base, Omaha, NE 46

Mineralization of ['''Cjethylene glycol in nonvegetated soils and bluegrass (P. pratensis), fescue (E arundimcea), rye (Z. perenne), trefoil (L. comiculatus), and mixed rhizosphere soils at -10 "C, 0 "C, and 20 °C. Mixed rhizosphere soils were collected from soil that contained M sativa, F. arundinacea, L. pererme, and P. pratensis 50

Mineralization of 100 ng/g, 1,000 ng/g, and 10,000 |ig/g ["C]ethylene glycol in nonvegetated soils incubated at 0 "C. Data points are the mean of three replicated ± one standard deviation.. 52

Mineralization of 100 pig/g, 1,000 |ig/g, and 10,000 |ig/g ['"•CJethylene glycol inM sativa rhizosphere soils incubated at 0 "C. Data points are the mean of three replicated + one standard deviation 53

The effects of vegetation and soil temperature on the mineralization of ['"CJethylene glycol after a 15 d incubation period. Each bar is the mean of three replicates. Bars followed by the same letter are not significantly diflferent (p=0.05) 58

Mineralization of ethylene glycol and propylene glycol in different rhizosphere soils incubated at -10 "C, 0 "C, and 20 °C. The symbols represent the following treatments EG (ethylene glycol), PG (propylene glycol), FA {F. arundinacea rhizosphere soil),

T (L. comiculatus rhizosphere soil), and M (mixed rhizosphere soil)... 60

THE USE OF AQUATIC PLANTS TO REMEDIATE SURFACE WATERS CONTAMINATED WITH AIRCRAFT DHCING AGENTS

Apparatus used to measure the fate of ["C]ethylene glycol and ['"Cjpropylene glycol in the aquatic emergent whole-plant system 75

Glass exposure chamber used to collect radiocarbon released by the aquatic emergent plants 77

Mineralization of [''*C]ethyIene glycol in nonvegetated soil, sterile soil, and soil that contained either Scirpus fluniatilis, Scirpus acutus, or Scirpus validus. Data points (cumulative '"COj) followed by the same letter are not significantly different (^0.05) 80

vii

Figure 4. Mineralization of P'^jpropylene glycol in nonvegetated soil, sterile soil, and soil that contained either Scirpusflimiatilis, Scirpus acutus, or Scirpus validus. Data points (cumulative ''KZOj) followed by the same letter are not significantly different (/)=0.05) 81

Figure 5. The distribution of recovered in the plant shoots and roots. The total quantity of applied detected in the plant tissues was less than 8% of the radiocarbon applied. Data points are the mean of three to five replicates ± one standard deviation 84

APPENDIX. THE INFLUENCE OF VEGETAHON ON THE MOBILITY OF PROPYLENE GLYCOL THROUGH THE SOIL PROFILE

Figure I. Vegetated undisturbed soil column used to study the influence of plants on the mobility of aircraft deicers through the soil profile 96

Figure 2. Concentration of propylene glycol detected in the leachate of vegetated and nonvegetated soil colunms 98

Figure 3. Cumulative concentration of propylene glycol detected in the leachate of vegetated and nonvegetated soil columns 100

viii

LIST OF TABLES


Table I. Volatility of methyl bromide as influenced by soil temperature and soil moisture. Volatility is reported as the percentage of methyl bromide applied 0, 3, and 72 hours after the 48-h fumigation period 15

Table 2. Soil characteristics of surface soil (0-10 cm) from three fields professionally fiimigated with methyl bromide 20

Table 3. Degradation of methyl bromide to bromide ion as influenced by soil moisture and soil temperature. Values are reported as percentage of the initially applied methyl bromide after a 48-h fumigation period (0 h post fumigation) 24

CHAPTER 3. EVALUATION OF THE USE OF VEGETATION FOR REDUCING THE ENVIRONMENTAL IMPACT OF DEICING AGENTS

Table L Soil characteristics of Offutt Air Force Base sampling sites 47

Table 2. Calculated MT50s for ['"'Cjethylene glycol. MTSOs represent the time estimated for 50% of the applied ["C]ethylene glycol to transform to '"COj 55

Table 3. Calculated MT50s for [•''Cjpropylene glycol. MT50s represent the time estimated for 50% of the applied ['''C]propylene glycol to transform to '"CO^ 56

Table 4. Calculated MTSOs for site soils collected at OflEutt Ah" Force Base 57

Table 5. Calculated MT50s for [''*C]ethylene glycol. MT50s represent the time estimated for 50% of the applied ['^'CJethylene glycol to transform to '^O^ -61

CHAPTER 4. THE USE OF AQUATIC PLANTS TO REMEDIATE SURFACE WATERS CONTAMINATED WITH AIRCRAFT DEICING AGENTS

Table 1. Distribution of "C in the ["C]ethylene glycol and ["C]propylene glycol soil-pl2uit systems 79

APPENDIX. THE INFLUENCE OF VEGETATION ON THE MOBILITY OF PROPYLENE GLYCOL THROUGH THE SOIL PROFILE

Table L Soil characteristics of the undisturbed soil columns 95

ix

ACKNOWLEDGMENTS

I would like to express my sincere gratitude and appreciation to my major professors

Dr. Joel R. Coats and Dr. Todd A. Anderson for their guidance, support, and encouragement

throughout my graduate research program. Thanks to all my good friends and coworkers for

your support and all the laughs throughout the years, you all truly helped make graduate

school a very positive experience. I would also like thank my family for all their love and

encouragement during my PhJD. research.

I thank all my committee members Dr. Ramesh Kanwar, Dr. Tom Loynachan, Dr. Tom

Moorman, and Dr. Wendy Wintersteen for their guidance and valuable input during my

research program. I would also like to express my appreciation to Jennifer Anhalt, Karin

Tollefson, Ellen Kruger, Pam Rice, Todd Anderson, Tmi Cink, John Ramsey, Brett Nelson,

and Piset Khuon for all their assistance.

Funding for this research was provided by the North Central Regional Pesticide

Impact Assessment Program (NAPIAP) and U. S. Air Force OflBce of Scientific Research.

The methyl bromide used in this research was supplied by the Great Lakes Chemical

Company.

1

CHAFFER 1. GENERAL INTRODUCTION

Introduction

Methyl bromide (MeBr), ethylene glycol (EG), and propylene glycol (PG) are widely

used chemicals in North America. Within the last few years these compounds have become

environmental concerns. Controversy over the potential role of MeBr in damaging the ozone

layer necessitates the reduction of emissions of MeBr into the atmosphere. Large quantities

of ethylene glycol and propylene are released into the environment through the use of glycol-

based deicing agents used to remove and prevent ice from accumulating on aircraft.

Significant quantities of these fluids spill to the ground and contaminate soil and water

environments. Surface waters contaminated with airport runoflThave been shown to be

harmful to aquatic communities. The aim of our research was to study each compound within

the framework of where and how they are of environmental concern. Our experiments

evaluated MeBr degradation and movement in the environment and indicated potential

methods for reducing the flux of MeBr into the atmosphere from fumigated soils.

Furthermore, our investigations show plants can be used to remediate soils and surface waters

contaminated with aircraft deicing agents.

Methyl Bromide

Methyl bromide is a biocidal tumigant used to control a broad spectrum of pests and

diseases including nematodes, insects, weed seeds, viruses, and fungi [2]. By volume, it is the

second most widely applied insecticide in the world [I]. The average armual use rate of MeBr

has increased by 7% since 1984, and it is currently the fifth most widely used pesticide in U.S.

agriculture [1,14]. Over 55 million pounds of MeBr were used in the U.S. in 1990.

Approximately 80% was applied as a soil fiimigant and an additional 15% was employed as a

fumigant for agricultural commodities (food and packaging materials) and facilities [1-3].

2

Atmospheric MeBr is believed to be the primary source of atmospheric bromine

radicals that contribute to catalytic destruction of the ozone layer [15], Photolysis of MeBr

at high elevations (stratosphere) produces bromine radicals. MeBr's atmospheric life-span

(~ 2 years) is relatively short compared with chlorofluorocarbons (50 to 100 years), for

which a phase-out was initiated in 1985; however, bromine radicals can scavenge ozone 40

times more efRcientiy than chlorine radicals [1,16-18], The quantity of MeBr released into

the atmosphere is estimated to be 65% from natural sources (ocean, burning biomass) and

35% from anthropogenic sources (agricultural fumigants, chemical manufacturing activities,

and car exhaust) [1,3,15,19], Mano and Andreae [15] studied the emissions of MeBr during

the smoldering and flaming phase of grass fires. They estimated approximately 30% of

global emissions of MeBr resulted from burning biomass. Large quantities of field-applied

MeBr (> 80%) have been shown to volatilize into the atmosphere [19]. Naturally occurring

MeBr sinks help decrease the amount of bromine radicals that react with stratospheric ozone.

These sinks include the reaction of MeBr with tropospheric OH radicals, the oceans, and

potentially forest canopies and soils [16,20,21],

Recent dispute over MeBr's potential to deplete the ozone layer has led the

Environmental Protection Agency (EPA) to propose a phasing out of its use [2]. In 1991 the

United Nations Environment Program (UNEP) Montreal Protocol committee classified

methyl bromide as a Class I ozone depleter. The EPA is responsible for enforcing a phase-

out of all Class I ozone depleter chemicals by the year 2000 [1,2], Presently, there are few

viable alternatives to replace this fiimigant. Banning of MeBr may, by one estimate, result in

an annual loss of over $1.3 billion to U.S. consumers and producers [2],

Despite MeBr's extensive use, there are only a few recent publications describing its

fate in soil [19,22-25]. Previous research primarily focused on evaluating the toxicity of

MeBr and measuring levels of residues on food [26], MeBr is considered a minor surface

and groundwater contaminant and a major air contaminant [27]. During fumigation, MeBr

3

penetrates into the soil and is partitioned into the liquid, gas, and adsorbed solid phases

[19,22]. Degradation in the soil may occur by abiotic or biotic reactions. These processes

include substitution (hydrolysis, conjugation), as well as reduction and oxidation reactions

[23,28-30]. Previous studies have shown MeBr is degraded in soil (methanotrophic bacteria)

[31] and anaerobic sediment [23]. Degradation products of MeBr include bromide ion (Br*),

methanol, formaldehyde, hydrobromic acid, and carbon dioxide [29,30,32,33] (Fig. 1).

The persistence, volatility, degradation, and mobility of MeBr in the soil is influenced

by chemical properties, soil properties, and environmental conditions. MeBr is a water-

soluble pesticide (>17,000 mg/L) that may potentially move through the soil and contaminate

groundwater [27]. Previous studies have detected levels of MeBr in surface water due to

leaching and surface runoff of MeBr-fumigated soils [33]. Information on the fate of MeBr

under various conditions is needed to make educated decisions involving its use and

regulation.

Ethylene Glycol and Propylene Glycol

Deicing-fluids are used to remove ice and snow that accumulates on aircraft and

airfield runways. Type I deicers that are widely used in North America consist of a minimum

of 80% glycol by weight, primarily EG- or PG- based [4,34,35] (Fig. 2). Ethylene glycol is

also used in vehicular antifreeze, industrial solvents, antifreeze in heating and cooling systems,

and in production of plastics and inks [36-39], Over 5.5 billion pounds of EG was produced

in 1994. Ethylene glycol was ranked 30th in the top 50 of the largest volume chemicals

manufactured in the United States [40].

Vast quantities of glycols enter the environment through deicing of aircraft, spills, and

improper disposal of used antifreeze. Approximately 43 million L/yr of aircraft deicing

products are used nationwide. During severe storms, large planes may require thousands of

gallons of deicing-fluid per deicing event [4]. An estimated 80% of the fluids spill onto the

4

Methyl Bromide Transformation

H Br-I

H — C — H

_ I Br-

Br- o

/ " —*• • H — C — H

Br Br- ^

H H o

I I II H — C — H - • H — C — H - • H — C — O H - ^ C O j

I I H OH

Fig. 1. Degradation products of methyl bromide.

HOCH^CH^OH CH3CH(0H)CH20H Ethylene glycol Propylene glycol

Fig. 2. Structure of ethylene glycol and propylene glycol

5

ground, which may lead to the contamination of soil, surface water, and groundwater

[4,36,41]. Runoff may also be collected in airport storm-sewer systems and directly released

(untreated) into streams, rivers, or on-site retention basins [4,6,35,36], Airport runoff and

storm-sewer discharge have been found to contain concentrations of EG ranging from

70 mg/L to > 5,000 mg/L [4]. Hartwell et al. [41] reported 4,800 mg/L EG in a creek which

had received drainage from an airport storage basin. Ethylene glycol has been detected in

groundwater at 415 mg/L [4] and 2,100 mg/L [43]. Surface waters contaminated with airport

runoff have been shown to be harmful to aquatic communities [4-6], Fisher and co-workers

[34] studied the acute impact of airport storm-water discharge on aquatic life and reported a

48-h LC50 of 34.3 and 69.3% effluent for Pimephalespromelas and Daphnia magna,

respectively. The primary concern of untreated runoff released into surface waters is the high

BOD produced by the rapid biodegradation of EG and PG. Even dilute levels of

contamination may deplete the available dissolved oxygen, resulting in asphyxiation [4-6,36].

Fish kills have been observed in waters with direct discharge of airport runoff and waste [4].

Previous research has revealed that microbial degradation of EG can occur in both

aerobic and anaerobic environments. Bacteria isolated from water, soil, and sewage sludge

were found to use EG as a source of carbon and energy for growth [3,38,44,45]. McGahey

and Bouwer [37] studied the biodegradation of EG in simulated subsurface environments,

utilizing inoculum from soil, groundwater, and wastewater. They concluded that naturally

occurring microorganisms were capable of degrading EG and substrate concentration, soil

type, temperature, and quantity of oxygen affects the rate of biodegradation. Evans and

David [36] reported the biodegradation rate of EG in river waters under controlled laboratory

conditions. Degradation rates varied in the different samples, depending on the water

temperature, type of microorganisms available, and their biomass. Wthin 14 d, 2 mg/L EG

was completely degraded in river waters at 8 "C. Klecka and co-workers [35] measured the

biodegradation rates of five different aircraft deicing-fluids in soil collected near an airport

6

runway. Rates of degradation for the deicers ranged from 2.3 to 4.5 mg/kg soil per day and

66.3 to 93.3 mg/kg soil per day for samples at -2 "C and 25 "C, respectively.

Rhizosphere

The rtiizosphere is the region of soil directly influenced by the roots. Plant roots

secrete energy rich exudates (sugars, amino acids, vitamins, and keto acids) and mucilages

(polysaccharides) which support large and diverse populations of microorganisms. Root-

influenced soils have a greater microbial biomass (10 to 100 times) and activity than bulk

soils, therefore enhanced degradation of organic compounds may occur in the rhizosphere

[10,46-48], In addition, the interaction between plants and their associated microbial

communities is mutually beneficial for both types of organisms. Soil microorganisms have a

positive influence on plants by 1) solubilizing inorganic nutrients and secreting organic

compounds (gibberellins, auxins, amino acids, and vitamins) which stimulate plant growth and

2) potentially deterring plant pathogens through competition and production of antibiotics

[10,47].

Vegetation can enhance the removal of human-made organic compounds and

pollutants in soil environments by microbial degradation in the rhizosphere and plant uptake

[12,13]. Previous research has shown enhanced degradation of industrial chemicals such as

trichloroethylene [7], polycyclic aromatic hydrocarbons [8], and petroleum [49] in the

rhizosphere soil as compared to root-free soil. Increased mineralization of the pesticides

parathion [50] and carbofuran [51] has been reported in the rhizosphere of rice plants. Hsu

and Bartha [52] noted similar results for parathion in the bean rhizosphere. Accelerated

mineralization of pesticides has also been found in the rhizosphere of plants from pesticide-

contaminated sites. Anderson and co-workers [22,53] observed greater microbial biomass

and enhanced degradation of atrazine, trifluralin, and metolachlor (after 14 d) in the

rhizosphere soil of herbicide-resistant Kochia sp. in comparison to nonrhizosphere and sterile

7

soils, respectively. In addition to enhanced degradation in the rhizosphere, plants may take up

contaminants as part of their transpiration stream [14], Lee and Kyung [SI] monitored the

uptake of fresh and aged carbofuran residues by rice plants. Approximately 60 to 70 % of the

detected in the shoots was the intact parent compound in both the freshly applied and aged

soils. Anderson and Walton [9] studied the fate of ['^]TC£ in soil-plant systems collected

from a contaminated site. Th^r reported 1 to 21% of the recovered radiocarbon (depending

on the plant species) was detected in the plant tissues, particularly in the roots. Vegetation

may play a vital role in reclaiming polluted ecosystems and preventing further contamination

by enhancing degradation and uptake into tissues, thereby reducing migration to surface

waters and groundwater aquifers.

Wetland plants may also be utilized to remediate contaminated water and soil. Like

terrestrial plants, aquatic macrophytes are capable of taking contaminants up in their tissues

and enhancing biodegradation in the rhizosphere. Aquatic plants have an adaptation that

enables efficient translocation of oxygen from the shoots to the roots, thereby forming

oxidized microzones in a saturated anaerobic environment [13,54]. The rhizosphere of an

emergent aquatic macrophyte is more conducive for microbial growth (aerobes and facultative

anaerobes) and activity than saturated root-free soil, thus creating a better environment for

enhance biodegradation. Within the past fifteen years, aquatic macrophytes have been utilized

for wastewater treatment. Wetland plants have been shown to reduce nutrients, organic

contaminants, and BOD from industrial, municipal, and agricultural wastewater [54-60].

Gersber et al. [56] observed aquatic emergent macrophytes, bulrush (Scirpus validus),

common reed (Phragmites communis), and cattail (Typha latifola) reduced the BOD and

ammonia levels in primary effluents. The artificial wetland beds cultured with

S. validus were superior to the other vegetated and nonvegetated beds. S. validus had

reduced the BOD level in the primary wastewater inflow from 118 mg/L to 5.3 mg/L. Reddy

et al. [55] also noted emergent and floating aquatic macrophytes were able to improve sewage

8

effluent by decreasing the BOD and increasing the concentration of dissolved oxygen.

Artificial wetlands and shallow storage basins cultured with aquatic macrophytes may be

useful for treating airport runoff thus reducing the BOD and glycol concentration in receiving

waters.

Dissertation Objectives

The overall objective of our research is to monitor the &te of MeBr and aircraft

deicers (['^]EG and ['"^CIPG) in soil as influenced by diflferent soil conditions (moisture and

temperature) and vegetation and to contribute evidence to support the following hypotheses:

1) Soil temperature and moisture will influence the degradation and volatility of methyl

bromide in soil; 2) Vegetation will enhance the degradation of ethylene glycol and propylene

glycol fi-om contaminated soils and surface waters and, therefore reduce their offsite

movement. Specific objectives of my research are:

1. Determine the influence of soil moisture and soil temperature on the volatility

and degradation of methyl bromide in soil.

2. Study the volatility and mobility of methyl bromide in undisturbed soil

columns.

3. Study the volatility and degradation of methyl bromide in professionally

fumigated fields and compare the results with laboratory data.

4. Determine the degradation of aircraft deicing agents ([''*C]EG and ['"CIPG) in

rhizosphere and nonrhizosphere soils at dififerent temperatures to represent

seasonal changes.

5. Investigate the fate of ['•*C]EG and/or ['•'CjPG in aquatic emergent plant-soil

systems.

9

Dissertation Organization

This dissertation is composed of a general introduction, three journal papers, and an

appendix. The first paper addresses the influence of soil environmental variables on the

degradation and volatility of methyl bromide in soil. In addition, the mobility of methyl

bromide through the soil profile and its potential to contaminate groundwater was determined.

Volatility and degradation of MeBr fi'om professionally fumigated fields were also compared

with the laboratory data. A portion of this paper has been published in Environmental

Toxicology and Chemistry. The second paper evaluates the use of vegetation to enhance the

degradation of aircraft deicing agents in the soil. The influence of different rhizosphere soils

and soil temperatures are further discussed. This paper will be submitted to Environmental

Toxicology and Chemistry. The third paper investigates the use of different aquatic emergent

plants to phytoremediate surface waters contaminated with aircraft deicing agents. This

information further supports the benefits of vegetation as a method to enhance the

degradation of organic pollutants in the environment. This paper will be submitted to

Environmental Toxicology and Chemistry. General Conclusion, Appendix (which is an

additional paper), and General Reference chapters along with an Acknowledgment section

will follow the third paper.

10


A portion of this paper has been published in Environmental Toxicology and Chemistry

Patricia J. Rice^ Todd A. Anderson*, James H. Cinld, and Joel R. Coats^

Abstract • Recent controversy over the potential role of methyl bromide ^^r) in damaging

the ozone layer has spurred interest in qualitatively increasing our understanding of the

transformation and movement of this flimigant. In contrast to the extensive uses of this

common agricultural fiimigant, there is a paucity of data on the environmental fate of MeBr.

Our research indicates MeBr is rapidly volatilized from fumigated soil (within the first 24

hours) and volatility significantly increases with temperature (35 °C > 25 "C > 15 "C) and

moisture (-3 kPa > -33 kPa > -300 kPa). Degradation of MeBr, measured by production of

bromide ion (Br*), was also positively related to temperature and moisture. Undisturbed soil

column studies uidicated that MeBr rapidly volatilized (> 50% of the M^r flux occurred in

48 hours) and did not leach into subsurface soil. Residual MeBr was degraded in the soil

column, evident by the high concentrations of Br* in the leachate water. These studies provide

valuable information for assessing the fate of MeBr in soil, which should lead to more

informed decisions regulating its use.

Keywords - Methyl bromide Degradation Volatility Mobility

fPesticide Toxicology Laboratory, Iowa State University, Ames, lA 50011

{The Institute of Wildlife and Environmental Toxicology, Clemson University, Pendleton, SC 29670

§Bayer Corporation, Agriculture Division, Research and Development Dept ARC2, Stilwell, KS 66085-9104

11

INTRODUCTION

Methyl bromide ^eBr) is a biocidal fiimigant used to control a broad spectrum of

pests and diseases including nematodes, insects, weed seeds, viruses, and fiingi [1], By

volume, it is the second most widely applied insecticide in the world [2]. The average annual

use rate of MeBr has increased by 7% since 1984, and it is currently the fifth most widely

used pesticide in U.S. agriculture [2-3]. Over 55 million pounds of MeBr were used in the

U.S. in 1990. Approximately 80% was applied as a soil fumigant and an additional 15% was

employed as a fumigant for agricultural commodities (food and packaging materials) and

facilities [1,2,4].

Atmospheric MeBr is believed to be the primary source of atmospheric bromine

radicals that catalytically destroy the ozone layer [5]. Photolysis of MeBr at high elevations

(stratosphere) produces bromine radicals. MeBr's atmospheric life-span (~ 2 years) is

relatively short compared with chlorofluorocarbons (50 to 100 years), which were banned in

1985; however, bromine radicals can scavenge ozone 40 times more eflBciently than chlorine

radicals [2,6-8]. The quantity of MeBr released into the atmosphere is estimated to be 65%

fi-om natural sources (ocean, burning biomass) and 35% fi'om anthropogenic sources

(agricultural fiimigant, chemical manufacturing activities, and car exhaust) [2,4-5,9]. Mano

and Andreae [5] studied the emissions of MeBr during the smoldering and flaming phase of

grass fires. They estimated approximately 30% of global emissions of MeBr resulted fi-om

burning biomass. Large quantities of field-applied MeBr (> 80%) have been shown to

volatilize into the atmosphere [9]. Naturally occurring MeBr sinks help decrease the amount

of bromine radicals that react with stratospheric ozone. These sinks include the reaction of

MeBr with tropospheric OH radicals, the oceans, and potentially forest canopies and soils

[6,10-11].

Recent dispute over MeBr's potential to deplete the ozone layer has led the

Environmental Protection Agency (EPA) to propose a phasing out of its use [1]. In 1991 the

12

United Nations Environment Program (UNEP) Montreal Protocol committee classified methyl

bromide as a Class I ozone depleter. The EPA is responsible for enforcing a phase out of all

Class I ozone depleter chemicals by the year 2000 [1-2]. Presently, there are few viable

alternatives to replace this fumigant. Banning of MeBr may, by one estimate, result in an

annual loss of over $1.3 billion to U.S. consumers and producers [1].

Despite MeBr's extensive use, there are only a few recent publications describing its

fate in soil [9,12-15]. Previous research primarily focused on evaluating the toxicity of MeBr

and measuring levels of residues on food [16]. MeBr is considered a minor surface and

ground water contaminant and a major air contaminant [17]. During fumigation, MeBr

penetrates into the soil and is partitioned into the liquid, gas, and adsorbed solid phases [9,12].

Degradation in the soil may occur by abiotic or biotic reactions. These processes include

substitution (hydrolysis, conjugation), as well as reduction and oxidation reactions [13,18-20].

Previous studies have shown MeBr is degraded in soil (methanothrophic bacteria) [21] and

anaerobic sediment [13], Degradation products ofMeBr include bromide ion (Br ), methanol,

formaldehyde, hydrobromic acid, and carbon dioxide [19,20,22-23].

The persistence, volatility, degradation, and mobility of MeBr in the soil is influenced

by chemical properties, soil properties, and environmental conditions. MeBr is water-soluble

(>17,000 mg/L) and may potentially move through the soil and contaminate ground water

[17], Previous studies have detected levels of MeBr in surface water due to leaching and

surface runoff of MeBr-fumigated soils [23], Information on the fate of MeBr under various

conditions is needed to make educated decisions involving its use and regulation. We report

herein on the influence of environmental and soil variables on the degradation and volatility of

MeBr in soil. In addition, large undisturbed soil columns were utilized to asses the movement,

degradation, and leaching potential of MeBr under controlled laboratory conditions. Volatility

and degradation of field-applied MeBr were studied and compared with laboratory results.

13

EXFEIUMENIAL PROCEDURES

Chemical

Methyl bromide was obtained from Great Lakes Chemical Co. (West Lafayette, IN)

and stored as a liquid at -60 "C. Pure MeBr was used for analytical standards and fumigation

of laboratory samples.

Soil collection and treatment

The pesticide-free soil used in the laboratory studies was obtained from the Iowa State

University Agronomy and Agricultural Engineering Farm near Ames, (Boone County) Iowa.

Samples were collected using a golf-cup cutter (10.5 cm x 10 cm, Paraide Products Co.),

sieved (2.0 mm), and stored in the dark at 4°C until needed. Ten golf-cup cutter samples

were randomly collected from the field and combined for each replicate. Soil was analyzed by

standard methods to determine physicochemical properties. The sandy loam soil had a

measured pH of 6.6 and consisted of 54% sand, 29% silt, 17% clay, 3.1% organic matter. In

all the studies described below, liquid MeBr was applied to the soil and allowed to incubate

(sealed) for 48 h (48-h equilibration period) before initial experimental monitoring to allow

MeBr to difi^se throughout the soil and reach an equilibrium between air/soil/water.

Volatility study

Soil (10 g dry weight) was placed in 45-ml glass bottles equipped with

polytetrafluoroethylene-lined septa. Moisture tension was adjusted to -300 kPa, -33 kPa, and

-3 kPa with an actual water content of 0.1 g/g, 0.3 g/g, and 0.6 g/g. respectively. MeBr was

applied to the soil surface as a liquid [24] at a concentration of2,733 |ig MeBr/g soil

(594 g/m'), which represents the typical staictural fumigation rate. This rate was used instead

of field fijmigation rate (392 kg/ha = 132 )ig MeBr/g soil) because of the difiQculty of applying

small quantities of this highly volatile compound (Henry's law constant = 6.2 x 10-^ atm*mV

14

mol, vapor pressure at 25 "C = 1633.0 mm Hg, ~ 2.2 kPa) [17], Samples were incubated in

the dark at 15 "C, 25 "C, or 35 "C. Each treatment consisted of four replicates. After the 48-h

equilibration period, concentrations of MeBr were measured at different time intervals using

headspace gas chromatography [24]. The first analysis immediately following the 48-h

equilibration period was considered time 0 h (Table 1). Headspace above the soil samples was

purged with Nj following each analysis. MeBr flux from soUs was determined from headspace

concentrations. The data were statistically analyzed using analysis of variance (ANOVA) and

least significant difference (LSD) at 5% [25].

Degradation study

Samples were analyzed for Br- to assess the influence of temperature and moisture on

MeBr degradation. Soil treatments were identical to those previously stated. Samples were

fiimigated as described above. Soils were extracted with 20 mL deionized water by

mechanical agitation and centrifijgation. The supernatant was removed and analyzed for Br'

using a bromide-specific electrode (Model 94-35, Orion Research Inc., Boston, MA).

Analysis of variance and LSD (5%) were used to determine the significant differences

between treatments [25].

Microbial toxicity study

Soil respiration was measured to determine the effect of MeBr on microbial activity.

Twenty grams of soil (dry weight) was placed in stoppered, 250-mL glass jars, and soil

moisture was adjusted to -33 kPa. MeBr was applied at concentrations of 2,733 jig MeBr/g

soil and 350 fig MeBr/g soil to represent structural fumigation and 2.6 times the rate of field

fiimigation, respectively. Field rate was not utilized because of application diflSculties as

stated previously. Soils were incubated in the dark at 25 °C, Carbon dioxide efflux was

measured at 24-h intervals after the initial 48-h fumigation period. The sample headspace was

Table i. Volatility of methyl bromide as influenced by soil temperature and soil moisture. Volatility is reported as

the percentage of methyl bromide applied 0, 3, and 72 hours after the 48-h fumigation period

Soil Moisture Soil Temperature % Volatilized % Volatilized Total % Volatilized

(kPa)' CC) (Oh)'' (3h) (72 h)

Hg/g±SD"' (%)" Mg/g±SD (%) Mg/g±SD (%)

-33 15 537 ± 125 (20)^^ 746+ 143 {llf 865 ±131 (32)^

-33 25 565 ±266 (21)'^ 832 ±386 (30)^ 953 ±435 (35)^^

-33 35 961 ±748 (35f 1391 ±1151 (51f 1488± 1208 (54)®

-300 25 90±39 (3)^ 111 ±34 (4)° 114±34 (4f

-33 25 565 ±266 (21)'^ 832 ±386 (30f 953 ±435 (35)^^

-3 25 1416 ± 165 (52)« 1768 ±239 (65)" 1781 ±241 (65)»

'Moistures are determined by applying suction to the soil therefore, pressure values are reported as negative.

''Percent volatilized based on quantity of MeBr in the sample headspace 48-h after treatment.

" Data for treatments are means ± standard deviation (^g MeBr/g soil) of four replicates.

''Means followed by the same letter are not significantly different (p < 0.05).

16

puiged with moist, COj-free air and was analyzed using an infrared gas analyzer (Model 300,

NCne Safety Appliances Co., Pittsburgh, PA) [26]. Microbial respiration in the fumigated and

untreated samples was compared. Treatments were considered significantly different when

the SD of the means did not overlap.

Column study

Two undisturbed soil columns (IS cm diameter x 38 cm length) were obtained from an

agricultural field site (no previous pesticide history) near Ames, lA The procedures for

collection and removal of the colunms were previously described [27]. Columns were stored

in the dark at 4 °C until needed. Additional soil samples were collected at the same depths as

the colunm, and soil physicochemical properties were determined. A composite of the soil

samples comparable to the soil column consisted of sandy clay loam soil with a pH of 5.4 and

54% sand, 25% silt, 21% clay, and 2.5% organic matter.

Soil columns were prepared for laboratory studies as described by Kruger et al. [28],

Modifications were made to collect volatilized MeBr from the soil (Figure 1). The PVC pipe

surrounding the sides of the column was longer than the soil column to insure suf5cient

headspace. A Plexiglass® plate with 3 openings was mounted to the top of the PVC pipe.

These openings were sealed with polytetrafiuoroethylene-covered neoprene stoppers

containing either a resazurin trap, metal hook (for granular activated carbon traps), or glass

tube (for addition of water). Soil columns were initially saturated with 0.005 M CaSO^then

drained to field capacity. Four 500-mL increments of deionized water were leached through

the colunms to determine any background concentrations of Br* and MeBr.

Liquid MeBr was applied to the soil surface and the columns were immediately sealed.

Soil columns were incubated for 48 h to allow MeBr to penetrate the soil and reach an

equilibrium between the air/soil/water. During the MeBr treatment and the following 48-h

equilibration period, soil columns were placed on a flat, solid surface to help seal the base and

17

Resazurin

1 IMS

I Charcoal Trap

Aluminum collar

PVC Pipe

Paraffin Wax

Perforated Plexiglas'

Soil

,TM

38 cm.

Spacers Wire Screen

Funnel For Leachate Collection

Figure 1. Undisturbed soil colunm used to study the volatility, movement, and degradation of

methyl bromide.

18

prevent potential loss of MeBr through the bottom of the columns. Steps were not taken to

determine the loss of MeBr through the base since minimal loss was expected. The MeBr-

fumigated columns were placed in column stands and maintained at 24 + 1 °C. Soil columns

were leached weekly with 500 mL deionized water to represent 1 inch of rainfall. A 1000-mL

separatory flmnel with a piece of Tygon® tubing (5 cm in length) was attached to the glass

tube during a leaching event Water was slowly dripped (2 mL/min) onto the column to

prevent pooling at the soil surface. After each rain event, the funnel was removed and

replaced with a septum to help prevent any loss of MeBr. Leachate was collected at the

bottom of the column and analyzed for Br' and MeBr by using a bromide-specific electrode,

and gas chromatography (GC), respectively.

Resazurin and granular activated carbon traps were suspended in the headspace of the

columns after the 48-h fiimigation. Resazurin traps containing S mL resazurin solution (0.5

mL 4% resazurin in ethanol, 4.95 mL deionized water) were used to indicate if the headspace

above the columns was becoming anaerobic. Carbon traps consisted of 8 g activated charcoal

wrapped in 5 cm x 5 cm, 100% cotton net (1-mm mesh). These traps were changed

periodically and used to determine the amount of MeBr in the headspace of the column.

Upon removal, the traps were placed in 45-mL glass bottles equipped with screw caps and

poly te t ra f luoroe thy lene- I ined sep ta and s to red a t -60 °C un t i l ana lys i s . MeBr was desorbed of t

the carbon traps by the procedure of Woodrow et al. [24] with modifications. Two grams of

carbon (trap) was placed in a 7-ml glass vial and sealed with a polytetrafluoroethylene-lined

septa. Three mL of air was removed fi'om the vial with a gas-tight syringe and replaced with

3 mL benzyl alcohol (Fisher Scientific, Pittsburgh, PA). Samples were warmed to 110°C for

15 minutes and the headspace was analyzed by GC. Headspace of the 45-mL glass bottles

was also analyzed before the desorption of MeBr. Quantities of MeBr detected were

considered in the final calculation of MeBr that volatilized fi'om the soil.

19

At the conclusion of the study, the undisturbed soil columns were cut into S-cm

increments and detracted with water as stated above in the degradation study. The soil

extracts were analyzed by using a bromide-specific electrode.

Field Study

Three adjacent fields at the Johnson Research Farm (Ames, Iowa) were professionally

fumigated (Hendrix & Dail Inc., Greenville, NC) with 392 kg/ha MeBr-chloropicrin mbcture

(98:2 by weight). During fiimigation, MeBr was injected (20-25 cm) into the soil and

immediately covered with a thin polyvinyl tarp. The tarp remained on the field for 48 h to

decrease the flux of MeBr into the atmosphere and allowed it to penetrate into the soil. Five

golf-cup cutter (10.5 cm x 10 cm) soil samples were randomly collected fi"om each field prior

to MeBr fiimigation. Soil was analyzed by standard methods to determine the

physicochemical properties (Table 2).

Flux chambers, equipped with an granular activated carbon trap (12-20 mesh, Aldrich

Chemical Company, Milwaukee, WI), were utilized to determine the amount of MeBr that

volatilized fi-om the soil. These chambers were constructed out of 4 L (Fisher Scientific)

brown glass solvent bottles. The bottom portion of the bottles were removed (leaving 20 cm

in height) and the caps were replaced with a polytetrafluoroethylene-covered #6 one-hole

rubber stopper, equipped with an activated carbon trap. The traps consisted of plastic drying

tubes filled with 8 g granular activated carbon. Each flux chamber was covered with

aluminum foil to minimize an increase in temperature within the chamber. Parafilm was

wrapped around the stoppers and carbon traps to ensure a tight seal. Five flux chambers were

randomly place on the polyvinyl tarp after fiimigation to monitor the quantity of MeBr being

released through the tarp. Just before removal of the tarp, the plastic was cut with a razor

blade and the flux chambers were placed 2 inches into the soil. The tarp was then carefiilly

removed so as not to disturb the flux chambers. Activated carbon traps were replaced at

Table 2. Soil characteristics of surface soil (0-10 cm) from three fields professionally fumigated with methyl bromide

Sand Silt Clay O.M." C.E.C.*'

Texture (%) (%) (%) (%) (meq/lOOg) PH®

Field 1 Clay loam 42 28 30 3.3 18.4 7.4

Field 2 Sandy clay loam 48 28 24 2.9 14.2 7.2

Field 3 Clay loam 40 32 28 3.4 17.2 7.1

"Organic matter.

''Cation exchange capacity.

*^1:1 (soilidistilled water).

21

various time intervals. Upon removal, the traps were placed in ziplock bags and stored in an

ice chest. Once the samples were taken back to the lab, the activated carbon samples were

placed in 45-mL glass bottles equipped with screw caps and polytetrafluoroethylene-lined

septa and stored at -60 °C. The carbon traps were analyzed following the procedures

previously mentioned.

Several soil probe samples (2-cm diameter x 2S-cm length) were randomly collected

from each field at various time intervals (prior to fumigation, 48 h after fumigation, and

several times after the removal of the tarp). Soil samples were placed in 100-ml and 250-mL

glass jars equipped with polytetrafluoroethylene-covered rubber stoppers containing two glass

tubes with septa. Six (100-ml glass jar) of the nine samples per field were analyzed for MeBr

(headspace analysis) followed by Br' (soil extraction) as previously stated. In the remaining

samples (250-ml jar), concentrations of COj were measured on the IR gas analyzer (as

described) to determine the potential toxicity of 392 kg/ha field-applied MeBr to the soil

microorganisms.

Analysis with bromide-specific electrode

Supernatant and leachate samples fi'om degradation and column studies, respectively,

were measured for Br* using a bromide-specific electrode attached to a pH meter (Fisher

Scientific, Pittsburgh, PA). Br* standards were prepared with NaBr, deionized water, and 5 M

NaNOj (ionic strength buflfer). Calibration curves were constructed fi-om the standards and

used to determine the sample concentrations.

Analysis of MeBr by gas chromatography

Procedures for the analytical standards and analysis of sample and standard headspace

were modified from Woodrow et al. [24]. Methyl bromide standards were made in benzyl

alcohol, stored at -60 "C, and replaced every 2 weeks. Samples were analyzed on a Varian

22

3700 gas chromatograph equipped with a°Ni electron-capture detector at 350 °C. Injector

temperature was 170 °C with column temperatures of 160 "C and 140 "C for the volatility and

column studies, respectively. The glass colunrn (0.912 m x 2.0 mm i.d.) was packed with 100/

120 mesh Porapak Q (Supelco Inc., Bellefonte, PA) on Carbopack with a carrier gas

consisting of ultra pure nitrogen (26 mL/min). Recently the U. S. Environmental Protection

Agency has proposed using the static headspace technique as an alternative to the purge-and-

trap method [29]. Riga and Lewis [29] evaluated these two techniques and noted they were

comparable, but the static headspace had additional advantages, which included

reproducibility, reduced cost, and preparation time. Static headspace analysis was utilized in

this study. Peak heights were used to construct a calibration curve and quantitate the samples.

RESULTS AND DISCUSSION

Volatility studies

The volatility data for MeBr-treated soils incubated at 15 °C, 25 "C, and 35 "C are

shown in Table 1. Methyl bromide was significantly more volatile in soil samples incubated at

35 "C, with no significant difference between the 15 "C and 25 "C soils. The flux of MeBr in

35 °C samples, after 3 h, exceeded the cumulative concentrations (at 72 h) in the cooler soil

samples. Of the total MeBr applied, 32%, 35%, and 54% volatilized m the 15 "C, 25 °C, and

35 °C samples, respectively. Over 86% of the total MeBr flux occurred within the 3 h at all

the three temperatures tested.

Volatility of MeBr significantly increased with increasing soil moisture (Table 1). A

measured 4%, 35%, and 65% of the applied MeBr volatilized fi-om the -300 kPa, -33 kPa, and

-3 kPa soil samples within 72 h. Over 59% of the flux, at all the soil moistures tested,

occurred during the first hour of analysis. Volatility of MeBr fi-om fiimigated soil samples at -

3 kPa was 2% and 16% greater than volatility at -33 kPa and -300 kPa, respectively. Yagi et

al. [9,15] reported 34% and 87 % of field applied MeBr was emitted into the atmosphere.

23

Goss [30,31 ] studied the effects of relative humidity on the sorption of organic vapors on

quartz sand and clay minerals and noted that as the mineral sur&ces become hydrated, the

sorption coeflScients decreased rapidly. Our results are consistent with previous research

which shows that as soil moisture decreases, the adsorptivity of MeBr in the soil increases

[14]. Chisholm and Koblitsky [32] observed a greater adsorption of MeBr in dry soils than

wet soils. As sorption increases there is less volatilization of the chemical from the sur&ce of

the soil uito atmosphere. Thus the increased volatility of MeBr with increased soil moisture

may be a result of competition between water and MeBr molecules for sites.

Degradation Studies

Bromide ion was measured to determine the influence of temperature and moisture on

the degradation of MeBr in fumigated soil samples. MeBr degradation significantly increased

at higher temperatures (Table 3). Samples incubated at 35 "C contained 2 to 7 times more Br"

than soils at 25 °C and 15 °C, respectively. Within 48 h after application, 1%, 3%, and 7% of

applied MeBr degraded to Br" in the 15 °C, 25 "C, and 35 "C soil samples. Yagi et al. [9,15]

reported 19% and 70% of field-applied MeBr decomposed to bromide ion after 7 days. From

this study it was not clear whether the transformation of MeBr was abiotic, biotic, or

combination of the two. Gentile et al. [22] reported a decrease in MeBr half-life, in static-

anaerobic water samples, as the temperature and pH increased. They also observed greater

MeBr hydrolysis in natural fresh water with an increase in temperature from 18 °C to 30 °C.

In the current study, the higher soil temperatures may have increased the rate of MeBr

hydrolysis and microbial activity, therefore resulting in greater degradation of MeBr.

MeBr degradation increased significantly at the highest soil moisture (-3 kPa) (Table

3). There was a significant difference in MeBr degradation between the -33 kPa and -300 kPa

bar soils. Three percent of the applied MeBr in the -33 kPa soils was transformed to Br •

within 48 h. Fumigated soil samples with moisture levels above field capacity contained

Table 3. Degradation of methyl bromide to bromide ion as influenced by soil moisture and soil temperature.

Values are reported as percentage of the initially applied methyl bromide after a

48-h fumigation period (0 h post fiimigation)

Soil Moisture Soil Temperature % Degradation Rate of Transformation

(kPa)« CC) (0 h post fumigation) (^g/g/day)''

Hg/g ± SD** (%r

-33 15 27 ± 1 ( I f 22

-33 25 90 ± 18 (3)B 45

-33 35 191 ±56 ( I f 96

-300 25 56 ± 1 (ir 28

-33 25 86 ± 1 or 43

-3 25 138 + 5 { S f 69

* Moistures are determined by applying suction to the soil therefore, pressure values are reported as negative.

''Data for treatments are means +.standard deviation for 3 to 5 replicates.

*^Means followed by the same letter are not significantly difterent (p < O.OS).

''Rate based on transformation of MeBr to Br ' 48 h after treatment. Calculations were based on

limited samples (n = 3 to S) and assuming a linear relationship.

25

approximately 2 times more Br *. Greater soil moisture typically results in greater

bioavailability of a chemical, as well as increased microbial activity in the soil. As the soil

moisture increases, MeBr may compete with water for sorption sites on the soil and organic

matter. Therefore, less MeBr is adsorbed to the soil making it more readily available for

microbial degradation and dissolution into the soil water where hydrolysis may occur. Greater

soil moisture will favor a fester rate of hydrolysis. Bromide ions are formed as a result of

abiotic and biotic degradation of MeBr [19,22,23]. Demethylation, hydrolysis and

substitution reactions of MeBr with organic matter will form Br *. Yagi et al. [6,15] compared

two field fumigation experiments that measured the flux of MeBr and formation of Br' in soil.

The second study showed an increase in Br' and a decrease in atmospheric MeBr relative to

field one. The authors concluded the results observed in field two were due to a combination

of increased soil moisture and pH, organic matter, and injection depth.

Comparisons of the degradation and volatility data at 48-h after treatment (Time 0 h

from Table I and 3) showed approximately 5 to 20 times and 2 to 10 times more MeBr

volatilized fi-om the soil than was transformed to Br" in the samples with different soil

temperatures and soil moistures, respectively. A similar value can be calculated fi'om numbers

cited in the literature. A comparison of the quantity of MeBr that volatilized fi'om a field in 7

d (87%) [9] with the estimated amount that transformed to Br" in the soil (19%) [15] shows

volatility of MeBr was favored 4 to 5 times more than MeBr transformation to Br".

Microbial toxicity study

Microbial respiration was measured in soils fumigated with MeBr (2,733 ng/g and

350 fig/g) to determine the effect on microorganisms (Fig. 2 and 3). Qualitative differences

between fumigated and control (unfumigated) soils were compared. The 350-ng MeBr/g soil

treatment caused temporary depression in CO, efflux, but it was not significantly different

from the control after 4 days. Soil samples fumigated with 2,733 ng MeBr/g soil sustained

0.009

0.008

0.007

0.006

0.005

0.004

0.003

0.002

0.001

0

s s Q z O

o ® F 5^ < p 2 E 2 2 2 S d P C

•CONTROL

•METHYL BROMIDE

I I I I

-5 5 10 TIME (DAYS)

15 20 25

Figure 2. Microbial respiration in soil ilimigated with 2,733 ^g/g methyl bromide. Data points are the mean ± one standard deviation.

0.02

0.018

0.016

0.014

^ 0012

"bh 3 0.01

g 0.008

0.006

0.004

0.002

0

•CONTROL

-METHYL BROMIDE

3 4 5

TIME (DAYS)

8

Figure 3. Microbial respiration in soil fumigated with 350 ^g/g methyl bromide. Data points are the mean ± one standard deviation.

28

depressed respiration throughout the 24-d experiment. A reduction in soil microbial

respiration suggests a reduction in microbial activity and/or biomass [33-34]. M^r is a

broad-spectrum, nonselective fumigant that kills soil-borne pathogens (Fusarium, Pythium,

m^Rhizoctonia) as well as beneficial microorganisms (pathogen parasites, antagonists,

competitors, and mycorrhizal fungi). Sensitivity to MeBr varies; however, all organisms are

susceptible at high concentrations [35]. Walton et al. [36] reported several chemicals initially

depressed COj efQux, but there was no effect after 6 days. Similar results were observed with

our soil samples fumigated at 350 ^g/g. The microbial population was able to recover from

the MeBr-induced toxicity. A similar or increased recovery might be expected in soils

fumigated at the field rate (392 kg/ha = 132 ng MeBr/g soil). In contrast, the 2,733 |ig/g

fumigation rate appeared to be very toxic to the microbial population. The reduction in soil

respiration was evident im'tially after the equilibration period and continued throughout the

experiment. Oremland et al. [21] reported soils exposed to 10,000 |ig/g MeBr had a

decreased removal rate of MeBr by methanotrophic bacteria. They noted degradation levels

were equivalent to the killed controls which indicated biological degradation did not occur at

this concentration.

Column study

Undisturbed soil columns were used to study the volatility, degradation, movement,

and leaching potential of MeBr. The flux of MeBr from the soil columns are shown in Figure

4. MeBr volatilized rapidly from the soil columns. Most of the MeBr flux (>75%) occurred

within 48 h after the fumigation period. MeBr was not detected in the colunm headspace after

7 d. These results are consistent with the results from our volatility studies and those reported

by Yagi et al. [9].

Leachates fi-om each rain event were analyzed for MeBr and Br'. MeBr was not

detected in any of the soil column leachates throughout the 23 week study. Br • increased

I WEEKLY

•CUMULATIVE

-48

K> O

32 176 344

TIME (HOURS)

512 680

Figure 4. Volatility of methyl bromide in undisturbed soil columns following a 48-h fumigation period. Data points are the mean ± one standard deviation.

30

(from a background of 0.01 ^g/g to 0.4 ^g/g) within the first rain event following fumigation

(Figure 5). Levels of Br* continued to increase, peaked at 3 weeks (4.3 ng/g), and gradually

decreased with subsequent rain events. A total of 28.8 |ag/g Br' leached through the soil

column, which represents > 5% of the MeBr initially applied. Wegmand et al. [23] detected

MeBr and Br* in drainage water fi'om fiimigated glasshouse soUs. The estimated half-life for

MeBr in the drainage ditch was 6.6 h at 11 °C. In addition, th^ observed a sharp increase in

Br* concentration during initial irrigation of the soils, followed by a steady decrease. Cirilli

and Borgioli [37] reported that MeBr degraded in soil at a rate of approximately 14% daily.

In the current study, the absence of MeBr in the leachate headspace indicated MeBr did not

leach through the soil profile of the undisturbed soil column.

After 23 rain events (final leachate was at background level) the soil column was

divided into 5-cm fractions and analyzed for Br'. No bound residual MeBr or Br' were

detected throughout the soil profile. Levels of Br' were similar to control (untreated) soil

samples. The increased quantity of Br' in the leachate and no detection of residual MeBr and

Br' in the soil profile at the completion of the test, imply the remaining MeBr degraded in the

soil. Persistence of MeBr in soil appears to be low, primarily due to its rapid volatilization, as

well as biological and chemical degradation.

Field stu<fy

The field fumigation study indicates that 43% of the field-applied MeBr was

volatilized within 4 d (Fig. 6). During the first 48 h, 18% of the MeBr flux escaped through

the tarp. A rise in flux occurred following the removal of the tarp. An additional 24% MeBr

volatilized fi'om the soil within the next 24 h. Only trace amounts of this fiimigant were

detected 5 d after application. Yagi et al. [9,15] reported a 34% and 87% flux of MeBr

within 7 d fi'om the fumigated fields. They concluded the greater soil moisture.

30

WEEKLY

CUMULATIVE

RAIN EVENT (WEEKS)

Figure 5. Bromide ion breakthrough from an undisturbed soil column treated with methyl bromide. Soil columns leached weekly after the 48-h fumigation period.

•METHYL BROMIDE (g/m2/d) •CUMULATIVE EMISSION

0 2.3 3

TIME (DAYS)

Figure 6. Volatilization of field-applied methyl bromide.

50

45

40

35

30

25

20

15

10

5

0

3.5

I ' ri 2.5 O CA

Pi pq w 1 5 S in

O ' cn S O S

FIELD#!

FIELD #2

FIELD #3

-I I I I I I I I J I I I I L—J

0 1 4 5 6 7 8 9 10 11 12 13 14 15 16

TIME (DAYS)

U> U)

Figure 7. Concentration of gaseous methyl bromide detected in soil from three fumigated fields.

34

organic content, higher soil pH, and deeper injection reduced the levels of MeBr that escaped

into the atmosphere.

Concentrations of MeBr in soil gas were also measured at several time intervals (Fig.

7). MeBr rapidly dissipated with time. Approximately 10% of applied MeBr was detected in

the soil gas phase within 48 h after application. In addition, only trace amounts of MeBr were

observed after IS d. The half-life of MeBr in soil is 0.10 years at 20 °C [20], Yagi et al. [15]

reported negligible quantities of soil gas MeBr after 7 d.

Soil samples from the fumigated field were analyzed for bromide ion to determine the

degradation of M^r. Levels of bromide ion should increase as MeBr degrades.

Concentrations of bromide ion in soil after MeBr fumigation were significantly different than

control soil samples collected prior to fumigation (Fig. 8). Approximately 30% of the field-

applied MeBr had degraded to bromide ion within 2 d. Levels of bromide ion decreased with

time and returned to background level within 24 d. The large increase of bromide ion within

48 h indicates rapid degradation of MeBr. Degradation of MeBr can occur in soil by abiotic

and/or biotic reactions. Abiotic processes include hydrolysis and conjugation. In addition,

MeBr under aerobic conditions, is biotically transformed to formaldehyde and bromide ion.

Anaerobically, MeBr is reductively debrominated to bromide ion and methane Vogel et al.

[20].

Microbial respiration was measured to determine the potential toxicity of 3 SO lb/A

field-applied MeBr to soil microorganisms. Reduction of microbial respiration is indicative of

toxicity and reduced microbial biomass. MeBr applied at a field rate of350 lb/A was

apparently not toxic to the microbial populations since no significant difference was noted in

microbial respiration rate between the control and fumigated soil samples. Previously, we

performed similar laboratory studies using quantities of MeBr that represent structural

fumigation rate and 2.6 times the field application rate (350 ^g/g) (Fig. 2 and Fig. 3). A

temporary depression in CO^ efflux was noted in the 350 ^g/g samples, but it was not

12

10

0 2 2.3 3 4 9 15 24 34 40 51

TIME (DAYS)

Figure 8. Mean concentrations of bromide ion detected in soil from three methyl bromide-fumigated fields.

36

significantly different fi-om control soils after 4 d. Soil samples fimiigated with 594 g/m^

sustained depressed respiration throughout the 24-d experiment.

CONCLUSIONS

The use of methyl bromide as a soil fumigant and its potential role in the distruction of

the ozone necessitates the importance of reducing the emissions of MeBr into the atmosphere.

Previous research has shown significant quantities of MeBr are released into the

atmosphere after field application. Yagi et al. [9,15] observed different quantities of MeBr

volatilized fi'om two field experiments and concluded the differences were related to a

combination of soil moisture, organic matter, pH, and injection depth. In this research the

influence of soil environmental variables on MeBr fate was studied to increase our

understanding of MeBr transformation and movement in soil. Our data show a potential for

reducing the flux of MeBr into the atmosphere fi-om fiimigated soils. Significantly less MeBr

volatilized from the samples with lower soil temperatures and soil moistures. To help reduce

the emission of MeBr into the atmosphere, applicators should apply MeBr on a relatively cool

day or in the morning or evening when lower temperatures occur. This should not effect

eflBcacy since MeBr is still very volatile at lower temperatures (boiling point 4 "C).

Application should also be discouraged after a recent rainfall when the soil moisture is high.

The degradation of MeBr in the soil is positively related to the soil temperature and

moisture. MeBr degraded more rapidly as the soil temperature and moisture increased.

Similar results are expected with other soil types, due to MeBr's low persistence, weak

adsorption, hydrolysis in soil water, and volatility. MeBr does induce toxicity upon contact

with microorganisms. The concentration of MeBr applied to the soil determines whether

microbial communities are able to recover fi'om the chemically induced toxicity. At a typical

field application rate (132 ^g MeBr/g soil), the microbial population should recover and

37

participate in the degradation of residual MeBr in the soil. In our undisturbed soil column

study, MeBr was not detected in the soil column leachate. Based on these results we would

not expect MeBr to contaminate ground water unless preferential flow was involved.

Furthermore, MeBr volatilized readily within the first few days, and the residual MeBr in the

soil appears to degrade or be incorporated into the soil making less MeBr available for

leaching.

ACKNOWLEDGMENT

This research was supported by a grant fi'om the North Central Regional Pesticide

Impact Assessment Program (NAPIAP). We express our thanks to Great Lakes Chemical Co.

for supplying the methyl bromide used in this study. We would like to thank Ellen Kruger for

her expertise and input on the soil column technique. In addition, we express our gratitude to

Pam Rice, Mark Petersen, and Theresa Klubertanz for their help in collecting the soil columns

for this study. Journal paper No. J-16438 of the Iowa Agricultural and Home Economics

Experiment Station Project No. 3187.

REFERENCES

1. National Agricultural Pesticide Impact Assessment Program (NAPIAP), United

States Department of Agriculture. 1993. The Biologic and Economic Assessment of

Methyl Bromide. Technical Report. National Agricultural Pesticide Impact Assessment

Program, United States Department of Agriculture, Washington, DC, USA.

2. California Action Network. 1992. Into the Sunlight: Exposing Methyl Bromide's

Threat to the Ozone Layer. Technical Report. Friends of the Earth, Washington, DC,

USA.

3S

3. Lewis, D. 1994. EPA report on U. S. pesticide use. Horticulture and Home Pest News

ISUExtension Newsletter 23:138.

4. Singh, H.B. and M. Kanaiddou. 1993. An investigation of the atmospheric sources

and sinks of methyl bromide. Geophys. Res. Lett. 20:133-136.

5. Mano, S. and M.O. Andreae. 1994. Emission of methyl bromide from biomass

burning. Science 2<S3:1255-1257.

6. Khalil, M^.K., IL4. Rasmussen and R. Gunawardena. 1993. Atmospheric methyl

bromide:trends and global mass balance. J. Geophys. Res. 9 :2887-2896.

7. Wofsy, S.C., M.B. McEIroy and Y.L. Yung. 1975. The chemistry of atmospheric

bromine. Geophys. Res. Lett. 2:215-219.

8. Yung, Y.L., J.P. Pinto, RT. Watson and S.P. Sander. 1980. Atmospheric bromine

and ozone perturbation in the lower stratosphere. J. Atmos. Sci. 37:339-353.

9. Yagi, K., J. Williams, N.-Y. Wang, and R. J. Cicerone. 1993. Agricultural soil

fumigation as a source of atmospheric methyl bromide. Proc. Nat. Acad. Sci. 90:8420-

8423.

10. Butler, J.H. 1994. The potential role of the ocean in regulating atmospheric CHjBr.

Geophys. Res. Lett. 21:185-188.

11. Lobert, J.M., J.H. Butler, S.A. Montzka, L.S. Geller, R.C. Myer and J.W. Elkins.

1995. A net sink for atmospheric CH^Br in the east Pacific Ocean. Science 267:1002-

1005.

12. Mignard, E. and J.C. Benet. 1989. Difiiision of methyl bromide in soil. J. Soil Sci.

40:151-165.

13. Oremland, R.S., L.G. Miller and EE. Strohmaler. 1994. Degradation of methyl

bromide in anaerobic sediments. Environ. Sci. Technol 28:514-520.

14. Brown, B.D., D.E. Rolston. 1980. Transport and transformation of methyl bromide in

soils. J. Soil Sci. 130:68-75.

39

15. Yagi, K, J. Williams, N.-Y. Wang and R.J. Cicerone. 1995. Atmospheric methyl

bromide (CHjBr) from agricultural soil fumigations. Science 267:1979-1981.

16. Smart, N.A. 1990. Residues in foodstuffs from bromomethane soil fumigation. In J.O.

Nriagu, M.S. Simmons, eds.. Advances in Environmental Science and Technology.

JohnWiley and Sons, New York, NY, USA, pp. 227-255.

17. Howard, P.H. 1989. Methyl bromide. In P.H. Howard, ed.. Handbook of

Environmental Fate and Exposure Data, Vol. 1. Lewis Publishers, Ann Arbor, MI,

USA, pp. 386-393.

18. Shorter, J.H., C.E. Kolb, P.N. Crill, R.A. Kerwin, R.W. Talbot, M.E. Hines and

R.C. Harriss. 1995. Rapid degradation of atmospheric methyl bromide in soils.

Nature(London) 377:717-719.

19. Rasche, M.E., M.R. Hyman and D.J. Arp. 1990. Biodegradation of halogenated

hydrocarbon flimigants by nitrifying bacteria. Appl Environ. Microbiol. 56:2568-

2571.

20. Vogel, T.M., C.S. Criddle and P.L. McCarty. 1987. Transformations of halogenated

aliphatic compounds. £>7v/ro/7. Sci. Technol. 21:722-737,

21. Ormeland, R.S., L.G. Miller, C.W. Culbertson, T.L. Connell and L. Jahnke. 1994.

Degradation of methyl bromide by methanotrophic bacteria in cell suspensions and

soils. Appl. Environ. Microbiol. 60:3640-3646.

22. Gentile, I.A., L. Ferraris and S. Crespi. 1989. The degradation of methyl bromide in

some natural fresh waters. Influence of temperature, pH and light. Pestic. Sci.

25:261-272.

23. Wegman, R.C.C., P.A. Greve, H. De Heer and P.H. Hamaker. 1981. Methyl

bromide and bromide-ion in drainage water after leaching of glasshouse soils. Water

Air SoilPollut. 16:3-11.

40

24. Woodrow, J.E., M.M. McChesney and J.N. Seibcr. 1988. Detemiination of methyl

bromide in air samples by headspace gas chromatography. Ana/. Chem. 60:509-512.

25. Steel, R.G.O. and XH. Torrie. 1980. Principles and Procedures of Statistics A

Biometrical Approach. McGraw-Kll Book Company, New York, NY, USA.

26. Edwards, N.T. 1982. A timesaving technique for measuring respiration rates in

incubated soil samples. Soil Sci. Soc. Am. J. 46:1114-1116.

27. Singh, P. and RS. Kanwar. 1991. Preferential solute transport through macropores

in large undisturbed saturated soil columns. J. Environ. Qual. 20:295-300.

28. Kruger, E.L., L. Somasundaram, RS. Kanwar and J.R Coats. 1993. Movement

and degradation of [' C] atrazine in undisturbed soil columns. Ermron. Toxicol.

Chem. 12:1969-1975.

29. Riga, T.J. and E.T. Lewis. 1995. Static headspace versus purge and trap: is there a

difference for low-level soil analysis? Am. Environ. Lab. 4:14-15.

30. Goss, K-U. 1992. Effects of temperature and relative humidity on the sorption of

organic vapors on quartz sand. Environ. Sci. Technol. 26:2287-2294.

31. Goss, K-U. 1993. Effects of temperature and relative humidity on the sorption of

organic vapors on clay minerals. £>iv//ion. Sci. Technol. 27:2127-2132.

32. Chisholm, RD. and L. Kobiitsky. 1943. Sorption of methyl bromide by soil in a

fumigation chamber. J. Econ. Entomoi 36:549-551.

33. Anderson, J.P.E. 1982. Soil respiration. In A.L. Page, R.H. Miller and D.R. Keeney,

eds., Methods of Soil Analysis Part 2, 2nd ed., Soil Science Society of America, Inc.,

Madison, WI, USA, pp. 831-871.

34. Yeates, G.W., S.S. Bamforth, D.J. Ross, K.R Tate and G.P. Sparling. 1991.

Recolonization of methyl bromide sterilized soils under four different field conditions.

Biol. Fertil. 50/75.11:181-189.

41

35. James, R. L. 1989. Effects of fumigation on soil pathogens and beneficial

microorganisms. General Technical Report. RM-Rocky Mountain Forest and Range

Experiment Station, U.S. Department of Agriculture Forest Service, Fort Collins,

CO. USA.

36. Walton, B.T., TA. Anderson, M.S. Hendricks and S.S. Talmage. 1989.

Physicochemical properties as predictors of organic chemical effects on soil microbial

respiration. J&iv/ro/i. Toxicol. Chem. 8:53-63.

37. Cirilli, L. and A. Borgioli. 1986. Methyl bromide in surface drinking waters. Water.

i?M.20:273-275.

42

CHAPTER 3. EVALUATION OF THE USE OF VEGETATION FOR REDUCING THE ENVIRONMENTAL IMPACT OF DEICING AGENTS

A paper to be submitted to Environmental Toxicology and Chemistry

Patricia J. Rice,t Todd A. Anderson^ and Joel R. Coatsf

Abstract This research project was conducted to evaluate the use of plants for reducing the

environmental impact of aircraft deicers. Significant quantities of ethylene glycol-based deicing

fluids spill to the ground and inadvertently contaminate soil and surface water environments.

Comparisons of the biodegradation of ''*C-ethylene glycol (['^]EG) in rhizosphere soils from

five different plant species, nonvegetated soUs, and autoclaved control soils at various temperatures

(-10 "C, 0 °C, 20 "C) indicate enhanced mineralization production) in the rhizosphere

soils. After 28 days at 0 °C, 60.4%, 49.6%, and 24.4% of applied ['^]EG degraded to ^*€0^ in

the alfalfa (Medicago sativa), Kentucky bluegrass {Poa pratensis), and nonvegetated soils,

respectively. Ethylene glycol mineralization was also enhanced with increased soil temperatures.

Our results provide evidence that plants can enhance the degradation of ethylene glycol in soil.

Vegetation may be a method for reducing the volume of aircraft deicers in the environment and

minimizing ofiTsite movement to surface waters.

Keywords- Ethylene glycol Propylene glycol Phytoremediation

Rhizosphere soil

fPesticide Toxicology Laboratory, Iowa State University, Ames, Iowa 50011

JThe Institute of Wildlife and Environmental Toxicology, Clemson University, Pendleton, SC 29670

43

INTRODUCTION

Under FAA regulation, deicing agents must be used to remove and prevent ice and frost

from accumulating on aircraft and airfield runways. Aviation dddng-fluids used in North America

primarily consist of ethylene glycol (EG) and/or propylene glycol (PG) with a minimal amount

of additives [1]. Vast quantities of glycols enter the environment through deicing of aircraft,

spills, and improper disposal of used antifreeze. Approximately 43 million L/yr of aircraft deicing

products are used nationwide. During severe storms, large planes may require thousands of

gallons of deicing-fluid per deicing event [1]. An estimated 80% of the fluids spill onto the

ground, which may lead to the contamination of soil, surface water, and groundwater [1-3].

Runoff may also be collected in airport storm-sewer systems and directly released (untreated)

into streams, rivers, or on-site retention basins [ 1,2,4,5], Airport runoff and storm-sewer discharge

have been found to contain concentrations of EG ranging from 70 mg/L to > 5,000 mg/L [1].

Hartwell et al. [3] reported 4,800 mg/L EG in a creek which had received drainage from an

airport storage basin. Ethylene glycol has been detected in groundwater at 415 mg/L [1] and

2,100 mg/L [6]. Surface waters contaminated with airport runoff have been shown to be harmful

to aquatic communities [1,4,7]. Fisher and co-workers [8] studied the acute impact of airport

storm-water discharge on aquatic life and reported a 48-h LC50 of 34.3 and 69.3% eflQuent for

Pimephales promelas and Daphnia magna, respectively. The primary concern of untreated

runoff released into surface waters is the high biological oxygen demand produced by the rapid

biodegradation of EG and PG. Even dilute levels of contamination may deplete the available

dissolved oxygen, resulting in asphyxiation [1,2,4,7]. Fish kills have been observed in waters

with direct discharge of airport runoff and waste [1],

Vegetation can enhance the removal of human-made organic compounds and pollutants

in soil environments by microbial degradation in the rhizosphere and plant uptake [9,10]. The

rhizosphere is the region of soil influenced by the roots. Plant roots secrete energy rich exudates

and mucilages which support large and diverse populations of microorganisms [11-14]. Increased

44

diversity and biomass of microbial communities in the rhizosphere render this zone better for

degradation of organic pollutants. Previous research has shown enhanced degradation of industrial

chemicals such as tiichloroetl^lene [15,16], polycyclic aromatic hydrocarbons [17], and petroleum

[18] in riuzosphere soil as compared with root-free soil. addition to enhanced degradation in

the rhizosphere, plants may take up contaminants as part of their transpiration stream [9].

Vegetation may play a vital role in remediating polluted ecosystems and preventing further

contamination by enhancing degradation and uptake into tissues, thereby reducing migration to

surface waters and groundwater aquifers.

Previous research has revealed that microbial degradation of EG can occur in both aerobic

and anaerobic environments. Several genera of bacteria that utilize EG as a carbon and energy

source have been isolated [19-21]. Only recently has the fate of EG been studied in the soil,

despite the widespread use of this compound [5,22]. McGahey and Bouwer [22] studied the

biodegradation of EG in simulated subsurface environments, utilizing inocula from soil,

groundwater, and wastewater. They concluded that naturally occurring microorganisms were

capable of degrading EG and that substrate concentration, soil type, temperature, and quantity

of oxygen affect the rate of biodegradation. In addition, Klecka and co-workep [5] measured

the biodegradation rates of five different aircraft deicing-fluids in soil collected near an airport

runway. Rates of degradation for the deicers ranged from 2.3 to 4.5 mg/kg soil per day and 66.3

to 93.3 mg/kg soil per day for samples at -2 °C and 25 °C, respectively.

Recently, there has been interest in reducing the contamination of glycol-based deicing

agents in the environment, because of their widespread use and adverse effects on aquatic

ecosystems. The purpose of our research was to evaluate the use of plants to enhance the

biodegradation of glycols in soil. In addition, we observed the influence of two potential rate-

limiting factors (soil temperature and substrate concentration) on the mineralization rate of EG

in the rhizosphere and nonvegetated soils.

45

MATERIALS AND METHODS

Chemicals

Ethylene glycol (EG) and propylene glycol (PG) were obtained from Fisher Scientific

(Fair Lawn, NJ) and Sigma Chemical Company (St. Louis, MO). The radiolabeled compounds

ethylene glycoI-1,2-'^ (['^]EG) and uniformly labeled propylene glycol (['^]PG) were

purchased from Aldrich Chemical Company (Milwaukee, WI) and New England Nuclear-Dupont

(Boston, MS). Upon receipt, the ["CIEG and ["^]PG were diluted with ethylene glycol and

propylene glycol to yield a stock solution of0.277 |iCi/^l and 0.247 ^Ci/)il, respectively.

Soil collection

Pesticide-free soil was collected from the Iowa State University Agronomy and

Agricultural Engineering Farm near Ames, (Boone County) Iowa. Ten golf-cup cutter (10.5

cm X 10 cm, Paraide Products Co.) soil samples were randomly removed from the field and

combined for each replicate. Samples were sieved (2.0 mm), placed in polyethylene bags, and

stored in the dark at 4°C until needed. Soils were analyzed by A & L Mid West Laboratories

(Omaha, NE) to determine physical and chemical properties. The sandy loam soil had a measured

pH of 6.6 and consisted of 54% sand, 29% silt, 17% clay, 3.1% organic matter.

In cooperation with the Air Force, soil samples were collected from Offiitt Air Force

Base (Omaha, NE). The sampling sites were adjacent to airport runways or taxiways where

deicing activities had once occurred (Fig. 1). Observations were made on the type of vegetation

located in these areas. The top sue inches of soil was collected with a shovel. Upon return to the

laboratory the soils were sieved, analyzed by A & L Mid West Laboratories (Table 1), and

stored as described above. Three 10-g soil (dry weight) subsamples from each site were analyzed

to determine if ethylene glycol or propylene glycol was present in the soil. Ten grams of soil

(dry weight) were extracted with 30 ml methanol and analyzed on a Varian 3740 gas

chromatograph (Varian Associates, Surmyvale, CA, USA) equipped with a flame ionization

46

Fig. 1. Sampling sites at Ofiiitt Air Force Base, Omaha, NE.

Table 2. Soil characteristics of surface soil (0-10 cm) from three fields professionally fumigated with methyl bromide

Sand Silt Clay O.M" C.E.C.''

Texture (%) (%) (%) (%) (meq/lOOg) pH®

Field 1 Clay loam 42 28 30 3.3 18.4 7.4

Field 2 Sandy clay loam 48 28 24 2.9 14.2 7.2

Field 3 Clay loam 40 32 28 3.4 17.2 7.1

"Organic matter.

^Cation exchange capacity.

*^1:1 (soil'.distilled water).

48

detector and a 31 cm x 2 mm (ID) column containing 5% Carbowax 20M coated on Chromosorb

WHP (Supelco, Inc., Belefonte, PA, USA) [23], The soil extracts were analyzed at column

temperatures ranging from 120 "C to 160 °C.

Rhizosphere soils from several different grass and legume plant species were used in this

study. Plants were grown from seed for 6 to 8 weeks in pesticide-free soil under the same

environmental conditions (25 °C, 14:10 lightrdark cycle). The different plant species consisted

of tall fescue (Festuca arundinacea), perennial rye grass (Lolium perenne L.), Kentucky blue

grass {Poapratensis I.), alfalfa (Medicago sativa), and birdsfoot trefoil {Lotus comiculatus).

These plants were chosen to represent vegetation that may be found adjacent to airport deicing

areas, airport nmways, and leguminous plants capable of fixing atmospheric nitrogen. Rhizosphere

soil was collected from each plant species. Soil that closely adhered to the roots was considered

rhizosphere soil. In addition, a mixed rhizosphere soil was studied. Nfixed rhizosphere soil was

collected from soil that contained the cool season grasses (E arundinacea, P. pratensis), a

legume (M sativa), and L perenne. Soils were sieved (2 nmi), placed in a polyethylene bag,

and stored in the dark at 4 °C for less than 48 h before they were used in the degradation studies.

Degradation stucfy: treatment and incubation

Portions of the ['''C]EG stock solution were diluted with acetone and ethylene glycol to

make a 100 ng/g (0.5 (iCi/0.004 g), 1,000 ng/g (0.5 |iCi/0.04 g), and 10,000 ig/g (0.5 \iC\lQA

g) treating solutions. A measured 1,000 ng/g ['"CJEG and ["*C]PG were applied to rhizosphere

soil, Ofiutt site soil, nonvegetated soil, and autoclaved (autoclaved 3 consecutive d for 1 h) soil.

In addition, 100 ng/g and 10,000 ng/g [''^JEG were added toM sativa rhizosphere soil and

nonvegetated soil determine the effect of substrate concentration on the rate ofEG mineralization.

After the acetone evaporated from the soil, four 10- or 20-g (dry weight) subsamples of the

treated soils were transferred to individual incubation jars, and the soil moistures were adjusted

to 1/3 bar (-33 kPa). One sample from each soil treatment was extracted three times with either

4 9

30 ml 9:1 (v/v) CHjOHiHjO or 30 ml CHjOH to determine the actual quantity ofapplied to

the soil. The extraction efBciencies ranged from 9S% to 103%. The three remaining samples

were the three replicates for each soil treatment. A vial containing 3 ml 2.77 M NaOH was

suspended in the headspace of each incubation jar to trap evolved from the mineralization

of ['^JEG. These traps were replaced every 24 h for the first 3 d, and every 48 h thereafter for

the remainder of the study. The quantity of ['^]EG mineralized to was determined by

radioassaying subsamples of the NaOH on a RackBeta® model 1217 liquid scintillation counter

(Pharmacia LKB Biotechnology, Inc., Gaithersburg, MD). Soils were incubated at -10 °C, 0 "C,

and 20 "C for 30 d (28 to 30 d).

Mineralization is considered the ultimate degradation of an organic compound. The

''^Oj produced during the mineralization of a radiolabeled substrate can be used to determine

the degradation rates of that compound [24], Therefore we calculated the mineralization time

50% (MT50), the estimated time required for 50% of the applied ['^]EG to mineralize, by

using formulas previously used for determining degradation rate constants and half-lives [25,26].

Calculations of MT50s were based on the assumption that the dissipation of ethylene glycol

fi'om the soil by mineralization followed first-order icinetics. Linear regressions of the natural

log of percentage (100% of applied '"'C - % '"'COj evolved) vs. time were used to determine

the MT50 and coefficients of determination (r^). Data points used to calculate these values

include the quantity of '^CO^ produced fi'om the initial treatment of the soil through the log or

exponential phase of the mineralization curve (Fig. 2). The lag phase was accounted for in the

calculations as described by Larson [26], Lag time in this study was defined as the number of

days before '"CO^ exceeded 2% of the applied radiocarbon. The MT50 values compared well

with the actual time required for 50% of the applied "C to mineralize (fiirther discussed in the

results). These calculated MT50s were only used to compare the differences between the different

soil types at -10 °C, 0 °C, and 20 °C, because oversimplification of the actual mineralization rates

5 0

Q. O. ot

100 • BOOWfl ettHd (OC) 100

to • • noaveii staled (20 C) to

60 . 60

40 40

20 • 20

0 0

— • taeae (-IOC) • fiKoe (OC) AftKae (20C)

A ^ * A . • " " • A m

• • • A • * _ • • •

10 20 30 10 20 30

4) 61) 5 s 9i U U V 6

o u

100

10

fiO

40

20

0

100

to

60

40

20

0

• bluepie(-10C) • bluepia(OC) * bhirffiM (20 a

• • m' 10 20 30

• nixed (-IOC) • mixed (OC) »mixed (20C)

Ml : 2

100

10

fiO

40

20

0

100

to

60

40

20

0

• lye (-10C) • lye (OC) Aiye (20C)

' '• 1

1 •

4 4 . 1 > : I ' ' • • •

• A • • ,

10 20 30

10 20 30

•acroa(-io) • Btfoa(0) 4lrefoa(20)

f " •» ' ' 10 20 30

Time (days)

Fig. 2. Mineralization of ['^]ethylene glycol in nonvegetated soils and bluegrass (P. pratensis), fescue (E arundinacea), rye (L. pereme), trefoil (Z. comiculatus), and mixed rhizosphere soils at -10 "C, 0 "C, and 20 "C. Mixed rhizosphere soils were collected from soil that containedM sativa, F. arundinacea, L. perenne, and/! pratensis.

5 1

may have occurred. Analysis of variance and the least squared means were used to test for

significant differences between the different soils at the/K0.05 level of significance [27],

Soil extraction and analyses

At the completion of the study, soils were extracted three times with either 30 ml 9; 1 (v/

v) CHjOHiÔ or 30 ml CHjOH. The extractable was analyzed on a liquid scintillation

counter (Pharmada LKB Biotechnology, Inc., Gaithersbuig, MD). The extracted soils were air

dried then crushed and homogenized in a plastic bag. Subsamples of the soils were made into

pellets (O.S g soil and O.lg hydrolyzed starch) and combusted in a Packard sample oxidizer

(Packard Instrument Co.). The produced firom the soil combustion was trapped in

Permafluor® V and Carbo-Sorb® E. Spec-Chec® "C standard (9.12 x lO^dpm/ml) was used

to determine the trapping efficiency. Three to six soil pellets were combusted for each replicate.

The soil-bound radiocarbon was quantified by liquid scintillation. The data were statistically

analyzed by analysis of variance and least significant differences at S% [27].

RESULTS

Mineralization of p^CJEG in rhizosphere and nonvegetated soils

The mineralization of different [''*C]EG concentrations in nonvegetated and M sativa

rhizosphere soil, incubated at 0 °C, is shown in Figure 3 and Figure 4. An inverse relationship

was evident between the concentration of ['"CJEG applied to the soils and the percentage of

radiocarbon mineralized. Significantly (p<0.05) smaller percentages of the applied ["C]EG was

transformed to '"*€02 as the substrate concentration increased. After 28 days, 55.2%, 20.5%,

and 7.14% of applied '"C evolved as in the nonvegetated soils treated with lOOpig/g, 1,000

^g/g, and 10,000 îg/g ["CJEG, respectively. Comparison of the data in the nonvegetated soils

(Fig. 3) and the M. sativa rhizosphere soil (Fig. 4) indicated significantly (p<0.05) enhanced

mineralization in the rhizosphere soil. Within 8 days after treatment, the production of'"Ôjin

0

100 ug/g

1,000 ug/g

10,000 ug/g

10 15 20

Time (days)

25 30

Fig. 3. Mineralization of 100 |ig/g, 1,000 ^g/g, and 10,000 ^g/g ['X^]ethylene glycol in nonvegetated soils incubated at 0 °C. Data points are the mean of three replicated ± one standard deviation.

7 0

U

60

50

T3 t) a Oi 40 Ph

o 30

(S u 3 20

10

100 ug/g

1,000 ug/g

10,000 ug/g

H-i'

LO Ui

0 10 15 20 25 30

Fig. 4. Mineralization of 100 ^g/g, 1,000 ug/g, and 10,000 ug/g ['^CJethylene glycol inM sativa rhizosphere soils incubated at 0 °C. Data points are the mean of three replicated ± one standard deviation.

5 4

the 100 |ig/g ['^]EG M sativa rhizosphere soils was elevated by 26% compared with the

nonvegetated sample at the same concentration. After 28 days, 62.2%, 49.7%, and 21.2% of the

added was liberated as '^COj in the 100 ng/g, 1,000 ^g/g, and 10,000 ng/g rhizosphere soils,

respectively. Overall, M. sativa rhizosphere soils significantly enhanced the mineralization of

ethylene glycol by 7% to 29% as compared with the nonvegetated soils with similar ['^C]EG

concentrations. Furthermore, the total percentage of applied radiocarbon that evolved as '^O,

from the 1,000 |ig/g nonvegetated soils and the 10,000 |ig/gM sativa rhizosphere soils was not

significantly different.

The effect of vegetation and temperature on the degradation of ['X]EG and ['^]PG in

the soil was studied by comparing the mineralization of 1,000 )ig/g EG and 1,000 ng/g PG in

several rhizosphere soils, nonvegetated soils, and sterile soils, and Ofilitt site soils incubated at

-10 "C, 0 "C, and 20 "C (Table 2-4). Examination of '"CO^ produced after 15 days showed

significantly greater (^0.05) mineralization of [''*C]EG as the temperature increased, except for

the sterile soils (Fig. 5). A average of 2.7%, 12.2%, and 50.3% of applied radiocarbon was

evolved as '"CO^ in the L. perenne rhizosphere soils incubated at -10 "C, 0 °C, and 20 "C,

respectively. L. comiculatus rhizosphere soil produced the greatest quantity of'"CO, within the

initial 15-day incubation period at -10 "C. No significant differences were observed between the

F. arundinacea, L. perenne, and P. pratensis and the mixed rhizosphere soils. A comparison of

the rhizosphere soils, sterile soils, and autoclaved soils at 0 "C and 20''C indicated that the

rhizosphere soils significantly enhanced the mineralization of ethylene glycol. After 15 days, the

greatest quantity of '''CO, produced at 0 °C occurred in the mixed and M sativa rhizosphere

soils. Over 17.3% and 19.3% of the applied radiocarbon was mineralized in the mixed and M

sativa rhizosphere soils compared with 6.73% in the nonvegetated soils. Significant differences

were observed between all the soils studied at 20 °C. The transformation of ['•'C]EG to '"COj in

descending order was F. arundinacea rhizosphere>M sativa rhizosphere>Z. comiculatus

rhizosphere>/'. pratensis rhizosphere>L. perenne rhizosphere>mixture rhizosphere

5 5

Table 2. Calculated MTSOs for ['^]ethylene glycol. MTSOs represent the time estimated for 50% of the applied ['X^jethylene glycol to transform to

Soil sample Temperature (°C) MT50 (r2)«

Sterile -10 >10,000 (r2=0.8I) A Sterile 0 >10,000 (r2=0.81) A Sterile 20 1,523 (r^.99)B

Nonvegetated 0 73 (rM).93) C Nonvegetated 20 43(r2=0.70)D

M. sativa rhizosphere 0 26 (rM).96)E M. sativa rhizosphere 20 6(rM).91)F

F. arundinacea rhizosphere -10 533 (rM).50) G F. arundinacea rhizosphere 0 28 (r2=0.69) E F arundinacea rhizosphere 20 7 (r==0.92) F

L. perenne rhizosphere -10 40 (r2=0.56) D L perenne rhizosphere 0 20 (r2=0.83) E,H L perenne rhizosphere 20 10 (f=0.92) F,H

P. pratensis rhizosphere -10 59 (rM).56) I P. pratensis rhizosphere 0 20 (r^.80) E,H P. pratensis rhizosphere 20 9 (r2=0.96) F

L. comiculatus rhizosphere -10 107 (r^.91) J L. comiculatus rhizosphere 0 103 (rM).95) J L. comiculatus rhizosphere 20 3 (rM).97) F

mixed rhizosphere** -10 27 (rM).71) E mbced rhizosphere** 0 20 (rM).86) E,H mixed rhizosphere*" 20 5 (r2=0.91) F

•Means in each column followed by the same letter are not significantly different (> = 0.05).

•"Samples collected from soils planted with a mixture ofM sativa, F. arundinacea, L. perenne, and P. pratensis.

5 6

Table 3. Calculated MTSOs for P^]propylene glycol. MTSOs represent the time estimated for 50% of the applied ['^]propylene glycol to transform to '**€02

Soil sample Temperature ("C) MT50 (r^'

Sterile 0 >10,000 (rH).90) Sterile 20 630 (rM).59)

Nonvegetated 0 54 (rM).86) Nonvegetated 20 13 (rM).64)

M sativa rhizosphere 0 30 (rM).81) M. sativa rhizosphere 20 9 (rM).91)

F. arundinacea rhizosphere -10 F. arundinacea rhizosphere 0 40 (rM).74) F arundinacea rhizosphere 20 5 (rM).85)

L perenne rhizosphere -10 L perenne rhizosphere 0 18 (r^.85) L perenne rhizosphere 20 10 (rM).92)

P. pratensis rhizosphere -10 P. pratensis rhizosphere 0 34 (f=Q.16) P. pratensis rhizosphere 20 6 (rM).70)

L comiculatus rhizosphere -10 L comiculatus rhizosphere 0 18 (rM).74) L comiculatus rhizosphere 20 5 (rM).55)

mixed rhizosphere** -10 mixed rhizosphere'* 0 28 (r2=0.75) mixed rhizosphere** 20 11 (r2=0.54)

*A problem with the incubation system at -10 °C caused the temperature to rise above 5 "C, therefore the MT50s and rate constants were not correct for -10 °C and were omitted from the table.

''Samples collected from soils planted with a mixture of A/, sativa, F. arundinacea, L. perenne, and P. pratensis.

5 7

Table 4. Calculated MTSOs for site soils collected at Ofilitt Air Fore Base.

Soil sample Temperature ("C) MT50(r2)

Site 1 -10 187 (rM).95) Site 1 0 95 (rM).93) Site 1 20 13 (f=0.72)

Site 2 -10 128 (M.85) Site 2 0 40 (r^.85) Site 2 20 16(rM).61)

Site 3 -10 141 (rM).75) Site 3 0 78 (f=0.97) Site 3 20 17(^=0.768)

Site 4 -10 89 (rM).64) Site 4 0 34 (rM)83) Site 4 20 11 (rM).67)

-p3 kl a> .s a a

Q* O. eQ

«> bA 2 a Q> U U u

Pk

8 0

70

60

^ 50 c/l •5? ^

S. «

40

30

20

10

0

B autoclaved

B nonvegetated

S rhizosphere (M. sativa)

• rhizosphere (F. arundinacea)

B rhizosphere (L. perenne)

• rhizosphere (P. pratensis)

B rhizosphere (L. comiculatus)

B rhizosphere - mixture*

•10

1 1

Temperature (°C)

20

Fig. 5. The effects of vegetation and soil temperature on the mineralization of ['''C]ethylene glycol after a 15 d incubation period. Each bar is the mean of three replicates. Bars followed by the same letter are not significantly different (/J=0.05).

5 9

>nonvegetated>steriIe soils. After 15 days, 65.5%, 50.3%, ZlSPAt, and 0.27% of the applied

radiocarbon mineralized in the F. arundinacea, L. pereme, nonvegetated, and sterile soils,

respectively. Comparisons of'*COj produced after 15 d in ["K2]EGand ['^]PG samples indicate

['^]PG mineralized more rapidly in soil than ['*C]EG (Pig. 6).

One month (28 d to 30 d) after the application of EG, the different rhizosphere soils

continued to enhance the mineralization of ['"^JEG by 1.7 to 2.4 times and 1.2 to 1.6 times

greater than the nonvegetated soils at 0 "C and 20 "C, respectively (Table 5). Our results showed

significantly (^0.05) greater quantities of'KIOj evolved in the soils tested at 20 °C compared

with -10 "C, with the exception of the mixed rhizosphere soils. A measured 52.9%, 56.8%, and

53.9% of the applied parent compound was mineralized in the -10 °C, 0 °C, and 20 "C mixed

rhizosphere soils, respectively. Further examination of the data at 0 "C and 20 "C (Table 5)

revealed no significant differences between the production of COj at 30 days in the L. perenne,

P. pratensis, and mixed rhizosphere soils. After 30 days, the largest quantity of '''CO^ that

evolved at -10 "C, 0 °C, and 20 "C occurred in the mixed rhizosphere soil, P. pratensis and mixed

rhizosphere soils, and theM sativa and F. arundinacea rhizosphere soils, respectively.

At the completion of the degradation study, the percentage of extractable radiocarbon

ranged fi"om 2.4 % to 95.6% (Table 5). Significantly greater quantities of extractable '"'C was

detected in the sterile soil samples compared with the nonvegetated and rhizosphere soils. Over

93% of the applied radiocarbon was detected in the soil extracts of the autoclaved soils incubated

at -10 "C and 0 "C. In addition, extractable was significantly (^0.05) more abundant in the

nonvegetated soils incubated at 0 "C than the rhizosphere soils. With the exception ofZ. perenne

and mixed rhizosphere soils, significantly greater quantities of extractable radiocarbon were

detected in the -10 °C soils compared with the 20 °C soils. The extractable radiocarbon was not

significantly diflferent between the biologically active soils at 20 "C.

The quantity of soil-bound residues detected in the soil samples, ranged fi-om 2.7% to

34.0% of the applied radiocarbon (Table 5). Examination of the data in Table 5 indicated that

90

80

70

60

50

40

30

20

10

0

• -10C ^OC

• 20 C

EG-FA PG-FA EG-T PG-T EG-MIX PG-MIX

Treatment ation of ethylene glycol and propylene glycol in different rhizosphere soils incubated at -10 °C, 0 lols represent the following treatments EG (ethylene glycol), PG (propylene glycol), FA {E arunti , T (L. corniculatus rhizosphere soil), and M (mixed rhizosphere soil).

Table S. The effect of vegetation and soil temperature on the degradation of [''*C]EG after a 30 d incubation period (reported as percentage of applied '^)

Soil sample Temperature (°C) CO/ Extractable* Soil-bound residues^ Mass balance

Sterile -10 0.03 A 95.6 A 3.2 AB 98.8 Sterile 0 0.03 A 93.6 A 2.7 A 96.3 Sterile 20 1.7 AB 78.1 B 4.7 B 84.5

Nonvegetated 0 24.4 0 62.8 0 17.5 CD ICS Nonvegetated 20 42.6 D 5.2 D 29.2 E 77.0

M. sativa rhizosphere 0 49.6 EF 3.9 D 34.0 F 87.5 M sativa rhizosphere 20 71.9 G 4.8 D 26.8 E 104

F. arundinacea rhizosphere -10 22.2 C 24.8 E 23.3 0 70.3 F. arundinacea rhizosphere 0 43.6 D 5.6 D 22.10 71.3 F. arundinacea rhizosphere 20 67.8 G 3.5 D 23.0 0 94.3

L perenne rhizosphere -10 45.2 DF 3.8 D 23.5 G 72.5 L perenne rhizosphere 0 47.1 DFH 3.9 D 17.5 CD 68.5 L perenne rhizosphere 20 52.4 EHI 3.3 D 18.7 0 74.4

P. pratensis rhizosphere -10 32.2 J 26.7 E 24.6 G 83.5 P. pratensis rhizosphere 0 60.4 K 4.2 D 23.4 0 88.0 P. pratensis rhizosphere 20 60.7 K 7.5 D 23.10 91.3

L comiculatus rhizosphere -10 19.5 C 50.2 F 15.5 D 85.2 L comiculatus rhizosphere 0 20.1 C 42.8 G 12.7 H 75.6 L comiculatus rhizosphere 20 62.0 K 2.4 D 11.9 H 76.3

mixed rhizosphere'* -10 52.9 El 4.0 H 23.3 0 80.2

mixed rhizosphere** 0 56.8 DC 3.7 D 19.3 0 79.8

mixed rhizosphere'* 20 53.91 3.0 D 18.0 0 74.9 'Means in each column followed by the same letter are not significantly different ( p = 0.05),

''Samples were collected from soils planted with a mixture of A/, saliva, F. arundinacea, L perenne, and P. pratensis.

6 2

the rhizosphere and nonvegetated soils had significantly (p<0.05) greater quantities of bound

residues than sterile soils.

CalculatedMT50 and mineralization rate of p*C]EG mineralization

Ethylene glycol was mineralized at a faster rate in rhizosphere soils than nonvegetated or

sterile soils (Table 2 and 3). The MT50s were determined for all the different soil types studied

at the various temperatures. Smaller MTSO values represent fiister mineralization rates. The

MT50 for ['T]EG in the sterile soils, nonvegetated soils, and F. anmdinacea rhizosphere soils

incubated at 20 °C was 1523 d, 43 d, and 7 d, respectively. Calculated MTSO values compared

well with the actual time required for 50% of ethylene glycol to mineralize in the soil.

Approximately 50 % of the ethylene glycol applied to P. pratensis and K arundinace rhizosphere

soils at 0 °C and 20 "C was mineralized in 20 d to 21 d and 7 d to 8 d compared with 20 d and

7 d for the calculated MT50s, respectively. Among the soils evaluated at -10 °C, the rate of

ethylene glycol mineralization was greatest to least for mixed rhizosphere>Z. perenne

rhizosphere>P. pratensis rhizosphere>£. corniculatus rhizosphere>7 arundinacea

rhizosphere>nonvegetated>sterile soils. Except for the L corniculatus rhizosphere soils, the

MTSOs were not significantly different between the rhizosphere soils incubated at 0 "C. Based

on theMT50s (Table 2), mixed rhizosphere soils mineralized ethylene glycol approximately 1.5

times to 19.7 times faster than the other rhizosphere soils at the same temperature and 1.6 times

faster than the nonvegetated soils at 20 °C.

Furthermore, the data (Table 2 and 3) indicate the MTSOs significantly (^0.05) decreased

with increased temperatures. The MTSO forarundinacea rhizosphere soil at -10 °C, 0 "C, and

20 °C were 533 d, 28 d, and 7 d, respectively. Increasing the temperature from -10 °C to 20 "C

for F. arundinacea liiizosphere soils enhanced the mineralization rate by a factor of 76. Generally,

a 15 d to 21 d and a 21 d to 27 d lag phase was observed in the 0 "C and -10 "C soil samples,

respectively (Fig. 2). Low quantities of'"COj (<6% of applied "C) were produced during the

6 3

lag phase. Nonvegetated and rhizosphere soils incubated at 20 °C showed no lag phase and

consistently mineralized >45% of the applied radiocarbon within 9 d after treatment.

DISCUSSION

Results obtained from our investigation indicate that vegetation can enhance the

mineralization rate of and ["^]PG in the soil. Significantly (^0.05) greater quantities

of '"COj were consistently produced in the M sativa, F. arundinacea, L. perenne, P. pratensis,

L. comiculatus, and mixed rhizosphere soils than the amount of '''COj produced in both the

sterile control and nonvegetated soils. A comparison of rhizosphere soils and nonvegetated soils

showed a two- to four-fold increase in the transformation of ['^C]EG to The accelerated

mineralization rate observed in these soils may be a result of greater microbial biomass and

activity generally found in rhizosphere soils [11-14]. Previous research has shown enhanced

biodegradation of industrial chemicals [16,17] and pesticides [19-22,24] in rhizosphere soils

compared with nonvegetated soils. In addition, microorganisms that utilize ethylene glycol as a

carbon and energy source have been previously isolated [20,21].

Results fi'om this study provide strong evidence that mineralization was the predominant

factor involved in the dissipation and reduction of ethylene glycol in the soil. Within 30 days,

42.6% to 71.9% of the applied radiocarbon evolved as fi'om the biologically active soils at

20 °C. Ethylene glycol mineralization at 0 °C in the sterile soils was minimal (0.03%) compared

with the nonvegetated (24.4%) and rhizosphere soils (>43.6%) indicating that transformation of

this aircraft deicer was a microbiological process. Several genera of bacteria have been shown

to utilize ethylene glycol as a source of carbon and energy for growth [20,21]. Our results

indicate significantly (^0.05) greater quantities of radiocarbon were detected in the soil-bound

residues of the nonvegetated and rhizosphere soils compared with the autoclaved soils. Previous

research has shown ethylene glycol does not adsorb to soil [23]. Lokke [23] observed the

mobility of ethylene glycol through an anaerobic soil colunm and reported that very little to no

6 4

ethylene glycol adsorbed onto the subhorizon of melt water sand, sandy till, and clayey soils.

Therefore, we conclude that the increased quantity ofsoil-bound residues in the biologically

active soil was a result of ['^]EG mineralization and, thus, portions of the radiocarbon were

incorporated into the cell constituents.

Substrate concentration significantly influenced the mineralization of ethylene glycol in

the soil. Our results showed an increase in [''*C]EG concentration significantly reduced the

percentage of applied radiocarbon that evolved as in both the nonvegetated and iliizosphere

soils. McGahey and Bouwer [22] noted an increase in the time required for 95% of the applied

ethylene glycol to be removed from the samples with increased substrate concentrations.

Comparisons of the various ['''CjEG concentrations in nonvegetated and M. sativa rhizosphere

soils clearly indicate that the rhizosphere soil significantly enhanced the mineralization of EG

compared Avith the nonvegetated soils.

A positive relationship occurred between the soil temperature and the ethylene glycol

mineralization rate. Increasing the temperature from -10 °C to 20 °C in the biologically active

soils resulted in enhanced mineralization rates that were approximately 6 to 7 times faster than

the rates noted in the -10 °C soils. Klecka et al. [5] also noted an increase in the biodegradation

rate of ethylene glycol from the soil with increased temperatures. Temperature has been shown

to greatly effect the enzyme activity and the growth rate of microorganisms [24]. Generally a 10

®C increase approximately doubles the rate of biological reactions [13,14,24], Examination of

our data also indicates that the nonvegetated and rhizosphere soils had significantly (p<0.05)

greater mineralization rates at -10 °C than the autoclaved soils at 20 "C. These results indicate

the microbial communities were able to survive and mineralize ethylene glycol at this cold

temperature. Microorganisms are capable of growing and metabolizing organic compounds at

low temperatures as long as water continues to exist as a liquid [13,14,24], The presence of

ethylene glycol contamination in the soil may have reduced the freezing point of the water within

the soil. Thus, psychrophilic bacteria may have been able to metabolize ethylene glycol at the

6 5

subzero temperature. Lag phases were observed in the soils incubated at the two cooler

temperatures (-10 and 0 °C). This may be due to lower enzyme and biological activity at the

cooler temperature and therefore acclimation time was needed. No lag phase was observed in

the soils incubated at 20 "C. Rather a large evolution ofoccurred within the first few days

after application. Comparisons of the rhizosphere soils and nonvegetated soils at various

temperatures indicate that rhizosphere soils significantly (p<0.05) enhanced the mineralization

of ethylene glycol in the soil.

Rhizosphere soils of different plant species were studied to determine their effect on the

mineralization rate of ethylene glycol. Soils were collected fi'om the root zone of various

grasses (F. arundirtacea, L. perenne, P. pratensis), legumes (M. sativa, L. comiculatus), and a

mixture of these plant species. The mixed rhizosphere soil had the shortest MT50 of the soils

incubated at -10 °C. A comparison of the MTSOs in the soils incubated at 0 °C indicates the

mixed, P. pratensis, and L perenne rhizosphere soils had significantly faster mineralization rates

than theM sativa and F. arundirtacea rhizosphere soils, but they were not significantly different

fi'om each other. No particular rhizosphere soil collected fi'om an individual plant species was

predominately the most efBcient at mineralizing ethylene glycol at all three temperatures. The

rate of transformation to '"CO^ in the mixed rhizosphere soils was unsurpassed at the

cooler temperatures (-10 "C and 0 "C) with the most significant difference noted at -10 "C.

Approximately, 7% and 30% more '"COj was produced at -10 °C in mixed rhizosphere soils

compared with other rhizosphere soils fi'om individual plant species. In addition, the mineralization

rate of ['''C]EG in the mixed rhizosphere soils incubated at -10 °C was 1.6 times faster than the

mineralization rate in the nonvegetated soils incubated at 20 °C. These results suggest that a

mixed culture of plant species would enhance the degradation of aircraft deicers more than a

monoculture. Bachmann and Kinzed [28] studied the rhizosphere soils of six different plant

species and noted the metabolic activity of the soils were variable depending on the species. The

mixed rhizospheres in our study probably had more diverse exudates secreted into the soil fi'om

6 6

the mixed plant culture than the monocultures. The mixed rhizosphere soils may have contained

more diverse and abundant microbial communities that resulted in greater degradation of ethylene

glycol at -10 °C.

The enhanced mineralization of ethylene glycol observed in the rhizosphere soils from

these studies may be underestimated in comparison to rhizosphere soils in the natural environment.

Plant-soil interactions are responsible for maintaining the increased microbial biomass and activity

in the rhizosphere soil. Therefore, by removing the soil from the roots, we may have lost some of

the beneficial rhizosphere properties by the end of the experiment [29]. Additional studies are

needed that include the intact plant.

CONCXUSION

Our resuhs provide evidence that vegetation may be an effective method for remediating

soils contaminated with aircraft deicing fluids. Rhizosphere soils consistently enhanced the

degradation of ethylene glycol compared with the nonvegetated soils, regardless of changes in

the soil temperature and substrate concentration. In addition, mixed rhizosphere soils were the

most prominent (p<0.05) soil type for mineralizing ethylene glycol at subzero temperatures.

Therefore, a mixed culture of cold-tolerant plant species could be planted alongside airport

deicing areas and runways to help enhance the biodegradation of glycol-based deicers that

inadvertently contaminate the soil. Facilitating the biodegradation of these deicers in the soil

will reduce the oSsite migration and minimize the concentration of glycol-based deicers that

reach the surface waters, thus reducing their environmental impact.

Acknowledgment- This research was supported by a grant from the U.S. Air Force Ofl5ce of

Scientific Research. The authors would like to thank Jennifer Anhalt, Karin Tollefson, Brett

Nelson, John Ramsey, and Piset Khuon for their technical support. This journal paper J-XXX of

the Iowa Agriculture and Home Economics Experiment Station, Project 3187.

6 7

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Biometrical Approach. McGraw-Hill Book Company, New York, NY. 137-191pp.

28. Bachmann, G. and H. Kinzel. 1992. Physiological and ecological aspects of the

interactions between plant roots and rhizosphere soil. Soil Biol. Biochem. 24:543-

552.

7 0

29. Walton, B. T., E. A. Guthrie and A. M. Hoylman. 1994. Toxicant degradation in

the iliizosphere. Li T. A. Anderson and J. R. Coats, eds., Bioremediation Through

Rhizosphere Technology, ACS Symposium Series, Vol. 563. American Chemical

Society, Washington DC, USA, pp. 11-26.

7 1

CHAPTER 4. TEtE USE OF AQUATIC PLANTS TO REMEDIATE SURFACE WATERS CONTAMINATED WITH AIRCRAFT DEICING AGENTS


Patricia J. Rice^ Todd A. Andersonj; and Joel R. Coats^

Abstract The purpose of our research was to evaluate the use of aquatic vegetation to

enhance the transformation of ethylene glycol and propylene glycol in contaminated surface

waters. The mineralization of and ['^]PG in sterile control soil, nonvegetated soil,

and soil containing Scirpusfluniatilis, Scirpus acutus, and Scirpus validus were determined.

Elevated levels of'''COj in whole-plant systems indicate accelerated mineralization in the

vegetated treatments compared to the nonvegetated and sterile control soil samples. After a

7-d incubation period, aquatic macrophytes enhanced the mineralization of ['"^JPG by 11% to

19% and ['"CJEG by 6% to 20%. Less than 8% of applied radiocarbon was detected in the

plant tissues, with the majority of the '''C recovered in the roots. Artificial wetland and

shallow storage basins cultured with aquatic macrophytes may be valuable for treating airport

and air base runo£f, thus reducing the biological oxygen demand and glycol concentrations in

receiving waters.

Keywords- Phytoremediation Aircraft deicers Aquatic emergent plants

Ethylene glycol Propylene glycol

I'esticide Toxicology Laboratory, Iowa State University, Ames, lA 50011

{The Institute of Wildlife and Environmental Toxicology, Clemson University, Pendleton, SC

29670

7 2

INTRODUCnON

Over 43 million L/yr of aircraft deicing-agents are used nationwide to remove ice and

snow that accumulate on aircraft and airfield runways. Aviation deicing-fluids used in North

America primarily consist of ethylene glycol (EG) and/or propylene glycol (PG) with a

minimal amount of additives [1]. During a deicing event, the majority (>80%) of the fluid

spills to the ground, ultimately causing on-site pooling, soil infiltration, runo£^ and

contamination of soil, surface water and groundwater aquifers [1-3]. Airport storm-sewer

systems may collect runoff and directly release untreated wastewater into streams and rivers

[1-3], Sills and Blakeslee [1] reported airport runoff and storm-sewer discharge to contain

concentrations of EG ranging fi-om 70 mg/L to > 5,000 mg/L. Ethylene glycol and

propylene glycol contamination of surface waters creates a high biological oxygen demand

(BOD) that can adversely impact aquatic communities. Depletion of available oxygen in

surface waters has resulted in asphyxiation and death in aquatic organisms [1,2].

Wetland plants may also be utilized to remediate contaminated water and soil. Like

terrestrial plants, aquatic macrophytes are capable of taking contaminants up in their tissues

and of enhancing biodegradation in the rhizosphere. Aquatic plants have an adaptation that

enables eflBcient translocation of oxygen from the shoots to the roots, thereby forming

oxidized microzones in a saturated anaerobic environment [4,5], The rhizosphere of an

emergent aquatic macrophyte is more conducive for microbial growth (aerobes and facultative

anaerobes) and activity than saturated root-fi-ee soil, thus creating a better environment for

enhance biodegradation. Within the past fifteen years, aquatic macrophytes have been utilized

for wastewater treatment. Wetland plants have been shown to reduce nutrients, organic

contaminants and BOD fi-om industrial, municipal, and agricultural wastewater [5-10].

Gersber et al. [7] observed that aquatic emergent macrophytes, bulrush (Scirpus validus),

common reed (Phragmites communis), and cattail (Typha latifola) reduced the BOD and

ammonia levels in primary effluents. The artificial wetland beds cultured with S. validus were

7 3

superior to the other vegetated and nonvegetated beds. S. validus had reduced the BOD

level in the primary wastewater inflow from 118 mg/L to S.3 mg/L. Reddy et al. [6] also

noted emergent and floating aquatic macrophytes were able to improve sewage effluent by

decreasing the BOD and increasing the concentration of dissolved oxygen. The purpose of

our research was to evaluate the use of aquatic vegetation to enhance the transformation of

ethylene glycol and propylene glycol in contaminated sur&ce waters.


Chemicals

Ethylene glycol (EG) and propylene glycol (PG) were obtained from Fisher Scientific

(Fair Lawn, NJ) and Sigma Chemical Company (St. Louis, MO). The radiolabeled compounds

ethylene glycol-1,2-"C (['"CJEG) and uniformly labeled propylene glycol (['""CjPG) were

purchased from Aldrich Chemical Company (Milwaukee, WI) and New England Nuclear-Dupont

(Boston, MS). Upon receipt, the ['"^JEG and ['"'CJPG were diluted with ethylene glycol and

propylene glycol to yield a stock solution of0.277 nCi/|xl and 0.247 nCi/^l, respectively.

Plants and soil

Pesticide-free soil used in this investigation was collected from the Iowa State

University Agronomy and Agricultural Engineering Farm near Ames, (Boone County) Iowa.

The soil was randomly removed from the field with a golf-cup cutter (10.5 cm x 10 cm,

Paraide Products Co.). Ten samples were combined for each replicate. Each of the three

soil replicates were sieved (2.0 mm) and analyzed (A&L Mid West Laboratories, Omaha,

NE) to determine the physical and chemical properties. The soils were stored in

polyurethane bags in the dark at 4 °C until needed.

Roots of aquatic emergent plants were purchased from V & J Seed Farms

(Woodstock, IL), and some plants were collected from a small lake and shallow ditch located

7 4

in Story County, Iowa. The three plant species utilized in this study were hard-stem bulrush

{Scirpus acutus), soft-stem bulrush (Sdrpus validus), and river bulrush (Scirpusfluniatilis).

Upon arrival, the roots were separated by species and planted in glass aquaria containing

pesticide-free soil. These plants were grown in saturated soils and maintained in a greenhouse

at 25 °C ± 2 °C with a 16:8 lightrdark diurnal cycle. Several months later, after the plants had

developed healthy root systems, snudl root masses or rhizomes from each plant species were

individually planted into 2S0-ml glass jars with pesticide-free soil corresponding to 100 g of

dry weight. Each jar was covered with tape to eliminate light in an attempt to deter algal

growth and photodegradation of the ['"^Jethylene glycol and ['^Jpropylene glycol in the

saturated soils. Nonvegetated control samples were set up identically to the vegetated soil

and were maintained under the same environmental conditions. After six weeks, the

vegetated and nonvegetated samples were placed in the exposure chamber (described below)

and acclimated for 48 h. Often more than one plant emerged from a rhizome. When this

occurred, the healthiest plant under 17 cm in length was chosen and the remaining shoots

were cut below the water surface. Sterile control soils were autoclaved (1 h on 3 consecutive

days) no more than three days prior to the treatment.

Soil-plant systems

Special incubation flasks were used to monitor the fate of ["C]EG and ['"CJPG in the

aquatic macrophyte whole-plant system (Fig. 1). The apparatus for this system was modified

from Anderson and Walton [11] and Federle and Schwab [12]. Just prior to the glycol

treatment, water was removed from each incubation flask to adjust the water level to 1 cm

above the soil surface. A 3-ml plastic vial, containing 2 ml 0.01 M NaOH for trapping '''COj,

was suspended inside each jar. After application of either 1,000 |ig/g ["C]EG or 1,000 ng/g

['"C] PG to the water layer, the soil-plant systems were sealed around the aquatic

macrophytes by using split rubber stoppers and RTV sealant [11]. Each stopper contained

7 5

Babndi

NiCHtnp

km water

Fig. 1. Apparatus used to measure the fate of ["^]ethylene glycol and ["K:]propylene glycol

in the aquatic emergent whole-plant system.

7 6

two i^HHitinnal openings from which the tr^ was changed and sterile water was added

to replace the moisture lost through transpiration. These openings were closed with two

smaller rubber stoppers. Deionized water (100 g), in a similar apparatus, was also treated

with 1,000 ng/g [''H3]EG or 1,000 ng/g ['^] PG. Between three and five replicates of each

soil treatment (water, sterile control soil, nonvegetated soil, and vegetated soils) was

included in the [**C]EG and ['^]PG experiments. Both the ['^]EG and ['^]PG studies

were conducted for 7 d.

After every 24-h interval, the traps were changed to prevent saturation of the

0.01 MNaOH and to maintain aerobic conditions within the soil-plant system. Each

vegetated incubation jar was weighed to determine if water (sterile) was need to replace

moisture lost through transpiration. The full content of each trap was radioassayed

on a RackBeta model 1217 liquid scintillation counter (Pharmacia LKB Biotechnology, Inc.,

Gaithersburg, MD).

Exposure chamber

All the incubation flasks were placed in a glass exposure chamber that was modified

fi-om Anderson and Walton [11] (Fig. 2). Air within the glass chamber was constantly

replaced by two pumps located on either side of the chamber. Each pump was set on

alternating 15-min. cycles. Air evacuated fi'om the chamber was bubbled through a 100ml

0.1 NNaOH trap and a 100-ml Ultima Gold scintillation cocktail trap (Packard Instrument

Co., Downer's Grove, IL). The 0.1 N NaOH and scintillation cocktail were used to capture

any '"COj and volatile '^-glycol or '"C-metabolites that were released into the chamber with

the evapotranspiration stream. The glass exposure chamber was contained in an

environmentally controlled room at 25 "C ± 1 "C with a 14.10 lightrdark cycle. The

temperature within the chamber was maintained at 26 °C ± 1 "C. At the completion of the

I

Enviionmental chamber

Pump

Fig. 2. Glass exposure chamber used to collect radiocarbon released by the plants.

7 8

study, subsamples of the 0.1 NNaOH and scintillation cocktail traps were radioassayed on a

liquid scintillation counter.

Soil andplant tissue analysis for "C

Upon completion of the 7-day study, the soil was extracted 3 times with 30 ml

methanol. Subsamples of the soil extracts were analyzed on a liquid scintillation counter to

determine the percent of extractable remaining in the soil. Subsamples of crushed and

homogenized air-dried extracted-soils were combusted using a Packard sample oxidizer.

Radiolabeled carbon dioxide from the combusted soils was trapped in Carbo-Sorb E and

Permafluor V (Packard) and radioassayed on a liquid scintillation counter to determine the

amount of'^-soil bound residue. Roots and shoots were analyzed separately.

Plant tissues were combusted on a Packard sample oxidizer (Packard Instrument Co.)

and radioassayed on a liquid scintillation counter (Pharmacia LKB biotechnology. Inc.,

Githersburg, MD) to determine the quantity ofassociated with the plants. The mass

balance for the soil-plant system ("CO^, ''*C-extractabIe organics, ''*C-soil-bound organics, and

'''C in the plant tissues) was determined for each sample (Table 1). Analysis of variance and

LSD (5%) were used to determine the significant differences between the treatments [13].

RESULTS

Mineralization of p*C]EG and P*C]PG

Analysis of the '"'COj traps from the aquatic emergent whole-plant degradation studies

indicate significantly (p<0.05) greater quantities ofwas evolved from the vegetated soils

compared to either the sterile control or the nonvegetated soils (Fig. 3 and Fig. 3). After a 7-

d incubation period, 45.6%, 32.6%, and 32.3% of applied [''*C]EG mineralized in the S.

validus, S. acutus, and S. flmiatilis soil-plant systems (Fig. 3). Production of'''COj was

elevated by approximately 6% to 19% in the vegetated soil. Significantly (p<0.05) greater

Table 1. Distribution of ''*C in the ['"CJethylene glycol and ['"Clpropylene glycol soil-plant systems.

Treatment

Percentage of total '"'C*

Treatment Compound '"CO, Extractable Soil-bound Plant uptake^* Total recovery

Sterile control EG 15.7 a 98.2 a 9.59 a na 123

Nonvegetated EG 25.9 b 10.0 b 27.4 b na 63.3

S. fluniatilis EG 32.3 c 10.3 b 20.4 c 7.08 a 70,1

S. acuUis EG 32 J c 20.2 b 19.2 c 4.58 b,c 76.6

S. validus EG 45.6 d 15.0 b 19.3 c 5.46 a,b 85.6

Sterile control PG 14.6 a 78.0 c 6.00 d na 98.6

Nonvegetated PG 43.0 d 10.3 b 22.5 e na 75.8

S. fluniatilis PG 61.7e 9.76 b 13.6 f 6,09 a,c 85.1

S. acutus PG 53.8 f 12.0 b 12.5 f,g 3.61 b 81.9

S. vaiidus PG 43.0 d 12.1 b 11.4g 3.72 b 70.3

*Means in each column followed by the same letter are not significantly different (p = 0.05). •"na = not applicable.

70

U

-n w 'p3 a a, 09

60

50

40

0

—•— Water —•— Sterile Control • ^ " Nonvegetated —X—S. fluniaiilis

* S. acutus —• — S. validus « a

20

Time (days) Fig. 3. Mineralization of ["<:]ethylene glycol in nonvegetated soil, sterile soil, and soil that contained either Scirpus flunlatilis, Scirpus acutus, or Scirpus validus. Data points (cumulative '^O,) followed by the same letter are not significantly different (p=0.05).

70

60

50

40

30

20

10

0

I ]

•Water • Sterile Control Nonvegetated

•S. fluniatilis 'S. aculus

— validus

00

Time (days) neralization of ['X]propylene glycol in nonvegetated soil, sterile soil, and soil that contained either Scirpus actHus, or Scirpus validtts. Data points (cumulative' W,) followed by the same letter are not y different <p=0.05).

82

mineralization was also noted in the nonvegetated soil compared to the sterile control soil.

Minimal quantities of'X^02 was evolved from the water samples. Biologically active soils

transformed 3% to 14% of the applied to within the first 24 h.

Enhanced mineralization was also observed in the vegetated soils treated with

propylene glycol (Fig. 4). Significantly (^0.05) greater quantities of ['^]PG was

transformed to 'TO, in soil containing S. fluniatilis and S. acutus than in either the

nonvegetated and or the sterile control soil. production was elevated 10.8% to 18.7%

i n t h e s e v e g e t a t e d s o i l s c o m p a r e d t o n o n v e g e t a t e d s o i l . C o m p a r a b l e a m o u n t s o f w a s

evolved from the S. validus and nonvegetated soil samples. A comparison of ['TJPG and

["C]EG mineralization in identical soil-plant systems indicate increased (^0.05) production

of'"COj in the nonvegetated, the S. acutus, and the S. fluniatilis soil samples treated with

['"CIPG compared to ['*C]EG (Table 1). Transformation of ['^]PG and ['^]EG to '"COj

was comparable in the sterile control and S. validus soil.

Soil analysis for "C

Analysis of soil from each ['^C]EG and ['"CIPG whole-plant study indicates

significantly (p<0.05) greater quantities of soil-bound '"C was detected in the biologically

active soil than the sterile control soil (Table 1). In sterile soil, 6.0% and 9.6% of the added

['•'CjPG and [''*C]EG were bound, compared to 20.4% and 13.6% "C bound in S. fluniatilis

soil, respectively. Nonvegetated soils were also observed to contain increased levels of bound

radiocarbon. Among the five ['*C]EG soil treatments evaluated, apparent detection of'"C soil

residues was greatest to least for nonvegetated>5. fluniatilis=S. acuius=S. validus>sXen!ie

control soil. In contrast, significantly (p<0.05) larger quantities of extractable '"'C were

observed in the sterile control soils; approximately 98% and 77% of applied ['"CJEG and

['"CjPG was detected in the methanol soil extracts, respectively. No significant difference was

83

observed between the or ['*C]PG nonvegetated and vegetated soil extracts. Less

than 21% of applied ['^]EG was detected in the extractable portions in the vegetated soils.

Uptake of *C into plant tissue

Recovery of applied radiocarbon in the three plant tissues ranged from 4.58% to

7.08% for EG and 3.61% to 6.09% for PG in the tested soil-plant systems (Table 1). Plant

roots consistently contained more '"C than plant shoots (Fig. 5). Greater than 70% of the

recovered radiocarbon from ['"K^IEG was detected in association with the roots. Air

evacuated from the test chamber contained small quantities of radiocarbon. A comparison of

plant species in the ['"CJEG and ['"CIPG studies indicated significantly (^0.05) greater

quantities ofin the tissues of S. fluniatilis than S. acutus. Percentages of radiocarbon

recovered in plant tissues of the ["C]EG whole-plant studies were elevated, but not

significantly different than in the ['"CJPG treated samples. The form or identity of '"C within

the plant tissue was not determined.

DISCUSSION

Our results clearly indicate aquatic emergent plants significantly (p<0.05) enhanced

mineralization of aircraft deicers (EG and PG) in surface water systems. Soils containing S.

fluniatilis, S. acutus, and S. vaiidus increased (p<0.05) the transformation of ['''C]EGto

'"•COj compared to nonvegetated soils. Enhanced degradation of ['''C]PG also occurred in the

S. fluniatilis and S. acutus soil samples, but '"CO, produaion was comparable in the S.

vaiidus and nonvegetated soils.

Dissipation of ethylene glycol and propylene glycol from surface water was primarily a

result of mineralization rather than uptake into plant tissues (Table I). At the completion of

the studies, 43% to 61% of applied ['''C]PG mineralized from the soil-plant surface water

system as compared to less than 7% of applied radiocarbon taken up into plant tissues.

100

• S. fluniatilis

MS. acutus

• 5*. validus

EG-Shoot EG-Root PG-Shoot PG-Root

Treatment

Fig. 5. The distribution of recovered '"C in the plant shoots and roots. The total quantity of applied '^C detected the plant tissues was less than 8% of the radiocarbon applied.

85

Aquatic emergent plants can enhance microbial remediation of contaminated water and soil by

enhancing degradation in the rhizosphere soil and taking contaminants up in their tissues. In

addition, these aquatic macrophytes create a more conducive environment for microbial

growth and activity than saturated root-free soil, due to the translocation of oxygen from the

shoots to the roots by aerechyma cells [4,5], Previous research has shown that artificial

wetlands containing S. validus and S. acutus significantly reduced BOD, NHj-N, and NO^-N

[7,14]

The majority of recovered in the plant tissues was associated with the roots.

R a diocarbon detected in plant roots may be a result of uptake or adsorption ofto the

roots. We believe '''C was primarily due to uptake since EG and PG are small polar

compounds, and '''C was detected in shoots and evacuated air from the test chamber,

indicating that radiocarbon was translocated from the roots to the shoots.

In addition, significantly greater (^0.05) quantities of '''C soil-bound residues were

found in the biologically active soils compared to the sterile control soils. This increased

radiocarbon was a result of EG and PG mineralization. Lokke [15] reported ethylene glycol

will not adsorb to soil. He observed very little to no adsorption of this glycol in saturated soil

columns that contained subhorizons of clayey till, sandy till, and melt water sand. Therefore,

glycol-based deicers in surface water and soil water are more bioavailable for microbial

degradation and plant uptake. Several genera of bacteria have been reported to utilize

ethylene glycol as a carbon and energy source [16,17], As microorganisms metabolize

ethylene glycol and propylene glycol, they incorporate a portion of the '"C into their cell

constituents. Therefore, significantly lower levels of soil-bound radiocarbon in the sterile soils

were a result of decreased microbial activity.

86

CONCLUSION

Results from this study cleaiiy indicate aquatic emergent plants enhanced the

mineralization of glycol-based deicers in surface waters. Artificial wetland and shallow

storage basins cultured with aquatic macrophytes may be usefiil for treating airport runo£^

thus reducing the BOD and glycol concentrations in receiving waters. In addition this

management approach is beneficial due to the low cost and easy maintenance of the s3rstem.

Remediating airport wastewaters prior to its discharge into nearby surface waters will reduce

the environmental impact of deicers on aquatic ecosystems.

Acknowledgment- This research was supported by a grant from the U.S. Air Force Office of

Scientific Research. The authors would like to thank Jennifer Anhalt, Karin ToUefson, Brett

Nelson, John Rams^, and Piset Khuon for their technical support. In addition, we would like

to express our thanks to Ellen Kruger, Pamela Rice, and Tracy Michaels for their assistance in

collecting, maintenance, and pest control of the aquatic emergent plants. Journal paper

J-XXX of the Iowa Agricultural and Home Economics Experiment Station Project 3187.

REFERENCES

1. Sills, R. D., and P. A. Blakeslee. 1992. The environmental impact of deicers in

airport stormwater mnofif. In F. M. DTtri, ed., Chemical Deicers end the


2. PiUard, D. A. 1995. Comparative toxicity of formulated glycol deicers and pure





87

4. Davis, L. C., L. E., Erickson, E. Lee, J. F. Shimp and J. C. Tracy. 1993.

Modeling the efifects of plants on the bioremediation of contaminated soil and

groundwater. Environ. Prog. 12:67-75.

5. Brix, H. 1987. Treatment of wastewater in the rhizosphere of wetland plants-the root-

zone method. Water Set. Technol. 19:107-118.

6. Reddy, K. R., E. M. D' Angelo and T. A. DeBusk. 1989. Oxygen transport through

aquatic macrophytes: the role in wastewater treatment. J. Environ. Qual. 19:261-267.

7. Gersberg, R. M., B. V. Elldns, S. R. Lyon and C. R Goldman. 1986. Role of

aquatic plants in wastewater treatment by artificial wetlands. Water Res. 20:363-

368.

8. Zhenbin, W., X. Yicheng, D. Jiaqi and K. Qijun. 1993. Studies on wastewater

treatment by means of integrated biological pond system: design and function of

macrophytes. ffiiterSci. Technol. 27:97-105.

9. Moore, J. A., S. M. Skarda and R Sherwood. Wetland treatment of pulp mill

wastewater. Water Sci. Technol. 29:241-247.

10. Yeoh, B. G. 1993. Use of water hyacinth (Eichomia crassipes) in upgrading small

agroindustril wastewater treatment plants. Miter Sci. Technol 28:207-213.

11. Anderson, T. A., and B. T. Walton. 1995. Comparative fate of ["*C]trichloroethyIene

in the root zone of plants from a former solvent disposal site. Environ. Toxicol.

Chem. 14:2041-2047.

12. Federie, T. W., and B. S. Schwab. 1989. Mineralization of surfactants by microbiota

of aquatic plants. Appl. Environ. Microbiol. 55:2092-2094.

13. Mclntyre, B. D. and S. J. Riha. 1991. Hydraulic conductivity and nitrogen removal

in an artificial wetland system. J. Environ. Qual. 20:259-263.


Pollut. 22:373-387.

88

15. Wiegant, W. M., and J. A. N. De Bont 1980. A new route for ethylene glycol

metabolism vnMycobacteritan E44. J. Gen. Microbiol 120:325-331.


Clostriditm gfycolicum SP. N. J. Bacterial. 8:356-362.

89

GENERAL CONCXUSION

Within the last few years methyl bromide ^eBr), ethylene glycol (EG), and propylene

glycol (PG) have become environmental concerns due to their adverse impact on the

environment. MeBr is a biocidal fumigant used to control a broad spectnun of pests and

diseases including nematodes insects, weed seeds, viruses, and fungi [1], By volume, it is the

fifth most widely used pesticide in U.S. agriculture with more than 55 million pounds used per

year [1-4], Large quantities of field-applied MeBr (>80%) volatilize fi'om soil. MeBr in the

atmosphere is believed to be the primary source of atmospheric bromine radicals that

catalytically destroy ozone [5,9], In addition, large quantities of EG- and PG-based deicers

spill to the ground after a deicing event and inadvertently contaminate soil and water

environments [4,36,41], Often airports collect this runoff in storm-sewers and directly release

this wastewater into nearby surface waters. Ethylene glycol and propylene glycol

contamination of surface waters creates a high biological oxygen demand (BOD) that can

adversely impact aquatic communities [4-6], The purpose of our research was to study each

compound within the fi-ameworlc of where and how they are environmental concerns by

investigating 1) the influence of soil environmental variables on the degradation and volatility

of methyl bromide in soil and 2) evaluate the use of vegetation to remediate soil and surface

waters contaminated with aircraft deicing agents and therefore reducing its environmental

impact.

Influence of soil environmental variables on the fate of methyl bromide in soil

Incubation studies were conducted to determine the influence of soil environmental

variables on the degradation and volatility of MeBr in soil. In addition, large undisturbed soil

colunms were utilized to asses the movement, degradation, and leaching potential of MeBr

under controlled laboratory conditions. Volatility and degradation of field-applied MeBr were

90

also studied and compared with laboratory results. MeBr rapidly volatilized from field-

applied and laboratory-fumigated soils. Significantly less M^r volatilized from the samples

at lower soil temperatures (35 °C > 25 "C > 15 °C) and soil moistures (-3 IcPa > -33 kPa >

-300 kPa). The degradation of this fumigant in the soil was positively related to soil

temperature and moisture. Approximately 30 % of field applied MeBr was degraded after

2 d. Methyl bromide was not detected in the soil column leachate, therefore we would not

expect MeBr to contaminate groundwater under the soil used, unless preferential flow was

involved. In addition, residual MeBr in the soil was degraded or incorporated into the soil,

making less MeBr available for leaching. These studies provide valuable information for

assessing the fate of MeBr in soil and reducing the flux of MeBr into the atmosphere. To help

reduce the emission of this flimigant, applicators should apply MeBr on a relatively cool day

or in the morning or evening when lower temperatures occur, addition, application should

be discouraged after a recent rainfall when the soil moisture is high.

Use of vegetation to reduce the environmental impact of deicing agents

Our research evaluated the use of plants for reducing the environmental impact of

deicing agents. Rhizosphere soils from various plant species significantly enhanced the

mineralization of ethylene glycol (EG) and propylene glycol (PG) compared to nonvegetated

soils. After 28 days at 0 °C, 60%, 50%, and 24% of applied [''*C]EG degraded to '^O^ in the

alfalfa QAedicago sativa), Kentucky bluegrass (Poa pratensis) and nonvegetated soils,

respectively. Glycol mineralization was also enhanced with increased soil temperatures. Our

results provide evidence that plants can enhance the degradation of ethylene glycol in soil.

Vegetation may be a method for reducing the volume of aircraft deicers in the environment

and minimizing offsite movement to surface waters. In addition, Scirpusflmiatilis, Scirpus,

acutus, and Scirpus validus were also evaluated to determine if emergent aquatic plants could

remediate surface waters contaminated with aircraft deicing agents. After a 7-d incubation

91

period, aquatic macrophytes enhanced the mineralization of ['^]PG and ['^]EG by 11 to

19% and 6 to 20%, respectively. Less than 8% of applied radiocarbon was detected in the

plant tissues, with majority of the 'X: recovered in the roots. Furthermore, artificial wetland

and shallow storage basins cultured with aquatic macrophytes may be useful for treating

airport runofi^ thus reducing the BOD and glycol concentration in receiving waters.

92

APPENDED TBE INFLUENCE OF VEGETATION ON THE MOBILITY OF PROPYLENE GLYCOL THROUGH THE SOIL PROFILE


Patricia J. Rice,t Todd A. Anderson,^ and Joel R. Coatsf

Abstract - The purpose of this investigations was to evaluate the influence of vegetation on

the mobility of aircraft deicing-fluids through the soil profile and their potential to leach to

groundwater. Undisturbed soil columns were planted with alfalfa (Meei'cago sativi) or rye

grass (Lolium perenne L) to assess their potential to reduce the infiltration of propylene

glycol (PG). Propylene glycol was applied to the surface of the nonvegetated, M. sativa, and

L perenne soil column and leached daily with deionized water. Leachates were collected at

the bottom of the columns and analyzed to determine the quantity of propylene glycol and

potential degradation products that leached through the soil. The vegetated soil columns

reduced the infiltration of propylene glycol through the soil profile. These resuhs suggest

plants can reduce the mobility of glycol-based deicing fluids in the soil and minimize its

potential to leach and contaminate groundwater.

Keywords - Propylene glycol Vegetation Infiltration

fPesticide Toxicology Laboratory, Iowa State University, Ames, lA 50011

JThe Institute of Wildlife and Environmental Toxicology, Clemson University, Pendleton, SC 29670

93

INTRODUCTION

There is a growing concern about the quantity of aircraft deicing agents that migrate

offsite and inadvertently contaminate the soil and water environments. Propylene glycol (1,2-

propanediol) is widely used in aircraft deicing agents and vehicular antifreeze. Type I deicers

that are commonly used in North America consist of a minimum of 80% glycol by weight,

primarily ethylene glycol (EG) or propylene glycol (PG). Under FAA regulation, deicing

agents must be used to remove and prevent ice and frost from accumulating on aircraft and

airfield runways. As a result, over 43 million liters of aircraft deicing products are used each

year nationwide [1]. During severe storms, large planes may require thousands of gallons of

deicing-fluids per deicing event. An estimated 80% of the fluids spill onto the ground,

ultimately causing on-site pooling, soil infiltration, ninofl^ and contamination of soil, surface

water and groundwater aquifers [1-3]. Ethylene glycol has been detected in groundwater at

415 mg/L [1] and 2,100 mg/L [4].

Vegetation can enhance the removal of man-made organic compounds and pollutants

in soil environments by microbial degradation in the rhizosphere and plant uptake [5,6].

Increased diversity and biomass of microbial conununities in the rhizosphere render this zone

better for degradation of organic pollutants. Previous research has shown enhanced

degradation of industrial chemicals (trichloroethylene [7], polycyclic aromatic hydrocarbons

[8]), and petroleum [9] in rhizosphere soil compared to root-free soil. Anderson and Walton

[10] studied the fate of ['^]TCE in soil-plant systems collected from a contaminated site.

They reported 1 to 21% of the recovered radiocarbon (depending on the plant species) was

detected in the plant tissues, particularly in the roots. Vegetation may play a vital role in

reclaiming polluted ecosystems and preventing further contamination by enhancing

degradation and uptake into tissues, thereby reducing migration to surface waters and

groundwater aquifers.

94

The purpose of this research was to evaluate the influence of vegetation on the

movement and leaching potential of propylene glycol through the soil profile. High

concentrations of propylene glycol were applied to nonvegetated and vegetated undisturbed

soil columns to mimic the quantities of glycols detected in airport runoff.


Column siucfy

Undisturbed soil columns (15 cm diameter x 38 cm length) were obtained from an

agricultural field site (no pesticide history) near Ames, lA. The procedures for collection and

removal of the columns were previously described by Singh and Kanwar [11]. Columns were

stored in the dark at 4 "C until needed. Additional soil samples were collected at the same

depths as the column, and soil physical and chemical properties were determined (Table 1).

Soil columns were prepared for laboratory studies as described by Singh and Kanwar

[11]. Four soil columns (2 each) were planted with alfalfa (Medicago safiva) or rye grass

(Loliumperenne L.) (Fig. 1). Nonvegetated and vegetated colunms were maintained in a

greenhouse (25 "C, 16:8 lightrdaric) for 4 months to allow suflBcient growth of the plants.

Water was added to the columns as needed. Roots of M. saiiva and L. perenne were

observed through the clear perforated plexiglass bottom of the columns. This indicates M

saliva and L. perenne roots were established through the length of the columns. Following

the four month growth period, soil columns were saturated with 0.005 M CaSO^ [11] then

drained to field capacity. Two 200-mL quantities of deionized water were leached through

the columns and analyzed on a gas chromatograph equipped with a flame ionization detector

(GC-FID). These leachates were considered to be background samples. A KBr tracer was

applied to the soil surface and leached through the soil columns with deionized water.

Breakthrough curves were determined for each column by analyzing the quantity of bromide

ion with a bromide-specific electrode.

Table 1. Soil characteristics of the undisturbed soil columns

Depth Sand Silt Clay O.M.'' C.E.C« (cm) Texture (%) (%) (%) PH* (%) (meq/lOOg)

0 Sandy clay loam 52 26 22 5.3 2.3 12.5

15 Loam 54 24 22 5.5 3.0 12.0

45 Sandy clay loam 42 34 24 5.9 2.5 13.8

60 Sandy clay loam 44 30 26 6.3 1.8 13.2

°1:1 (soil;distiUed water).

^Organic matter.

'^Cation exchange capacity.

96

PVC Pipe

Paraffin Wax

Perforated Plexiglas™

Soil

Aluminum collar

38 cm.

Spacers Wire Screen

Funnel For Leachate Collection

Fig. 1. Vegetated undisturbed soil column used to study the influence of plants on the

mobility of aircraft deicers through the soil profile.

97

Soil columns were moved to an incubator set at 25 °C, and the temperature was slowly

decreased (approximately 3 °C/24h) to 10 °C to represent spring conditions. Soil columns

were maintained at 10 °C with a 16:8 light:dark cycle for 96 h prior to the treatment to ensure

the plants had acclimated to this temperature. During this 96 h time period, soil columns were

leached with 400 ml deionized water.

Propylene glycol (Fisher Scientific, Fair Lawn, NJ) solution (1.76 ml PG/ 364 ml

water) was applied to the soil surface. Twenty-four hours after the treatment, soil columns

were leached with 400 ml deionized water daily. Water was applied to the columns in four

100-ml increments. This caused a temporary pooling of water each time, which was meant to

represent a runoff situation. Leachates were collected at the bottom of the columns and

analyzed on a GC-FID[12]. Soil columns were leached daily until the level of PG was below

the detection limit. Peak heights were used to construct a calibration curve and quantitate the

samples. The data will be statistically analyzed by using analysis of variance and least

significant differences at 5% [13].

Analysis of Br' with a bromide-specific electrode

Following the addition of the KBr tracer, columns were leached with deionized water

and leachates were collected and measured for Br' using a bromide-specific electrode attached

to a pH meter (Fisher Scientific, Pittsburgh, PA). Br" standards were prepared with KBr,

deionized water, and 5 M NaNOj (ionic strength buffer). Calibration curves were

constructed fi-om the standards and used to determine the sample concentrations.

RESULTS AND DISCUSSION

Propylene glycol was detected in the leachates of all the soil columns studied (Fig. 2).

The greatest PG concentrations occurred within the first two leaching events then gradually

decreased until PG was not detectable after 18 d. Approximately 51 to 71% of the recovered

00

R n' —

7 8 9 10 11

Time (days)

12 14 16 17 18

Fig. 2. Concentration of propylene glycol detected in the leachate of vegetated and nonvegetated soil columns.

99

PG was detected in the leachates within S d. Movement ofPG through the soil profile

depends on its properties and adsorptive characteristics, soil characteristics, soil temperature,

and the quantity and fi-equent^ of runoff or precipitation [14]. Ethylene glycol, a compound

similar in structure to propylene glycol, does not adsorb to soil. Lokke [12] reported little or

no adsorption of EG to sandy till, muddy till, or clayey soils. Propylene glycol is water

soluble and appears to be mobile within the 38 cm soil profile. Our results indicate vegetation

reduced the quantity of propylene glycol that moved through the soil profile (Fig. 3).

Measured quantities of 93.3 mg, 75.4 mg, and 75.6 mg of propylene glycol were detected in

leachates of nonvegetated, M saiiva, and L. perenne soil columns, respectively. Plants can

decrease the concentration of PG in soil and reduce its movement through the soil profile to

groundwater by plant uptake, enhanced degradation in root-associated soils, and reducing the

soil water status [5]. Our results firom previous studies (Chapter 2 of the thesis) have shown

enhanced degradation of PG in the M sativa and L. perenne rhizosphere soils compared to

nonvegetated soil. Results fi'om the current investigation showed a 4 to 8% decrease in the

quantity of water that leached through vegetated soil columns relative to the nonvegetated soil

column. Overall, vegetation can reduce the downward movement and leaching of propylene

glycol through the soil profile. This implies vegetation can be planted alongside deicing areas

and runways to help minimize the quantity of aircraft deicing agents that reach the

groundwater.

Acknowledgment- This research was supported by a grant fi-om the U.S. Air Force Office of

Scientific Research. The authors would like to thank Jennifer Anhalt, Karin Tollefson, Brett

Nelson, John Ramsey, and Piset Khuon for their technical support. In addition we would like

to express to express our thanks to Ellen Kruger, Pamela Rice, Teresa Klubertanz, and Mark

Peterson for their assistance in collecting the undisturbed soil columns. Journal paper J-XXX

of the Iowa Agricultural and Home Economics Experiment Station Project 3187.

100

90

80

70

60

50

40

30

20

10

0

Nonvegetated

Alfalfa

Rye grass

2 4 6 8 10 12 14 16 18

Time (days)

of propylene glycol detected in the leachate of vegetated and nonvegetated soil columns.

101

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the fate of methyl bromide, ethylene glycol, and propylene

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