WSRC-MS--90-346

DE92 009906

GROUND WATER AND SOIL REMEDIATION:IN SITU AIR STRIPPING USING HORIZONTAL WELLS (U)

by

D. S. Kaback, B. B. Looney, C. A. Eddy, and T. C. Hazen

WestinghouSe Savannah River Company i_i:'_i,_!Savannah River Site '

Aiken, South Carolina 29808 i_/1/_i_ii '; '_ .......'

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A paper proposed for presentation and publication at theNational Water Well Association Outdoor Action ConferenceLas Vegas, NevadaMay 13-16, 1991

s

: DISCLAIMER

This report was prepared as an account of work sponsored by an agency of the United StatesGovernment. Neither the United States Government nor any agency thereof, nor any of their

employees, makes any warranty, express or implied, or assumes any legal liability or responsi-bility for the accuracy, completeness, or usefulness of any information, apparatus, product, or

process disclosed, or represents that its use would not infringe privately owned rights. Refer-ence herein to any specific commercial product, process, or service by trade name, trademark,manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recom-

mendation, or favoring by the United States Government or any agency thereof, The view_

and opinions of aathors expressed herein do not nece_arily state or reflect those of theUnited State,s Government or any agency thereof.

This paper was prepared in connection with work done under Contract No DE-AC09-89SR18035with the U.S. Department of Energy. By acceptance of this paper, the publisher and/or recipientacknowledges the U.S. Government's right to retain a nonexclusive, royalty-free license in and toany copyright covering this paper, along with the right to reproduce and to authorize others toreproduce ali or part of the copyrighted paper.

.............. • '.', , .,o _t_t, tlnll:N ! I_

WSRC-MS-90-346

GROUND WATER AND SOIL REMEDIATION:IN SITU AIR STRIPPING USING HORIZONTAL WELLS

Dawn S. Kaback, Brian B. Looney, Carol A. Eddy, and Terry C. Hazen

Westinghouse Savannah River CompanySavannah River Site

Aiken, South Carolina 29808

ABSTRACT

An innovative environmental restoration technology, in situ air stripping, has been demonstrated atthe U.S. Department of Energy (DOE) Savannah River Site (SRS) in South Carolina. Thisprocess, using horizontal wells, is designed to concurrently remediate unsaturated-zone soils andground water containing Volatile Organic Compounds (VOC). In situ technologies have thepotential to substantially reduce costs and time required for remediation as well as improveeffectiveness of remediation. Horizontal wells were selectei'l to deliver and extract fluids from thesubsurface because their geometry can maximize the efficiency of a remediation system and theyhave great potential for remediating cont_uninant sources under existing facilities.

The in situ air stripping concept utilizes two parallel horizontal wells: one below the water tableand one in the unsaturated zone. The deeper well is used as a delivery system for the air injection.VOCs are stripped from the ground water into the injected vapor phase and are removed from thesubsurface by drawing a vacuum on the shallower well in the vadose zone.

The first demonstration of this new technology was conducted for a period of twenty weeks. Avacuumwas first drawn on the vadose zone well until a steady-state removal of VOCs wasobtained. Air was then injected at three different rates and at two different temperatures. Anextensive characterization program was conducted at the site and an extensive monitoring networkwas installed prior to initiation of the test. Significant quantifies of VOCs have been removed fromthe subsurface (equivalent to an eleven-weil, 500-gpm, pump-and-treat system at the same site).Concentrations of VOCs in the ground water have been significantly reduced in a number of themonitoring wells. In addition, the activity of indigenous micro-organisms was increased as muchas two orders of magnitude during the air injection.

OBJECTIVE

The objective of this project was to demonstrate a new in situ remediation technology that utilizedhorizontal wells as delivery and extraction systems. In situ air stripping, involving both airinjection below the water table and vapor extraction above the water table, was to be demonstratedas a new technology for concurrently rernediating both ground water and soils contarninated withvolatile organic compounds. New characterization and monitoring technologies were to bedemonstrated as part of the DOE first integrated demonstration project, which involves scientistsfrom many DOE laboratories, other government agencies, universities, and the industrial sector.

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BACKGROUND

The Savannah River Site is a 300-square-mile facility near Aiken, South Carolina, owned by DOE(Figure 1). The environmental restoration research program at SRS involves investigation of newtechnologies for in situ remediation of ground water and soils. Technologies such as vaperextraction, vitrification, deep soil mixing, air stripping, and biotechnology are currently beingdeveloped to remediate contaminated soils and groundwater in place. Traditional methods involvepumping and above-ground treatmentof ground water and excavation andremoval or treatment ofsoils. In situ methods offer the potential to substantially reduce costs, as well as to improve theeffectiveness and efficiency of remediations.

Subsurface contamination with VOCs is a common problem across the United States. Thesesolvent materials have been used as metal degreasers at numerous industrial facilities for a numberof years. Contamination of ground water with these solvents has created large plumes that migrateboth vertically and laterally fairly rapidly.

The most common technology for these ground waters is to pump them to the surface and treatwith an air stripper. According to Henry's Law, these compounds will partition into the vaporphase from the aqueous phase. The in situ air stripping process contacts the two phases in theground rather than in ata above+ground unit.

At the SRS trichlorethylene (TCE) and tetrachloroethylene (PCE) were used as metal degreasingsolvents for a number of years. A ground water plume containing elevated levels of thesecompounds exists over an area greater than one square mile. A traditional ground water extractionand treatment system has been in operation since 1984 ancl has removed approximately 230,000 ::pounds of solvents from the ground water. However, additional solvents have continued to leachinto the ground water from the vadose zone.

The demonstration site was selected along an abandoned process sewer line that carried wastes to aseepage basin operated between 1958 and 1985. The sewer line acted as a source of VOCs as weknew it had leaked at numerous locations along its length. Because the source of contaminationwas linear at this particular location within the overall plume, horizontal wells were selected as theinjection/extraction system.

GENERAL SITE GEOLOGY AND HYDROLOGY

The Savannah River Site is located on the upper Atlantic Coastal Plain. It is underlain by a thickwedge (approximately 1000 feet thick) of unconsolidated Tertiary and Cretaceous sediments thatoverlay the basement, which consists of PreCambrian and Paleozoic metamorphic rocks andconsolidated red Triassic sediments (siltstones and sandstones). The sedimentary section consistspredominantly of sands, clayey sands, andsandy clays.

Groundwater flow at the site is controlled by hydrologic boundaries. Flow at and immediatelybelow the water table is to local tributaries, whereas flow in the lower Tertiary aquifer is to theSavannah River or one of its major tributaries. Flow in the Cretaceous aquifers is towards theSavannah River. Flow in the shallow aquifers in the immediate vicinity of the test site is highlyinfluenced by the eleven-well recovery network (pump-and-treat).

TEST SITE DESCRIPTION

The test site is located within the large VOC plume in the M-Area of SRS. Two horizontal wellswere successfully installed, borrowing technology from the petroleum industry1, along the processsewer line that carded wastes from the M-Area facilities to the M-Area Settling Basin (Figure 2).

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One horizontal well was installed bclow the water table ata depth of 150 to 175 feet along 300 feetof horizontal. This well was designed to be the air injection system (Figure 3). The secondhorizontal well was installed in the vadose zone in a permeable sand at approximately 75 feet for alength of 200 feet. lt was designed to be used for vapor _xtraction (Figure 3).

The water table is located at a depth of approximately 135 feet. Groundwater in the vicinity of theprocess sewer line contains elc rated concentrations of TCE and PCE to depths greater than 180feet within the local Tertiary aquifer. A detailed geologic and hydrologic model of the test locationwas developed using the voluminous site characterization and monitoring data collected as part ofthis project.

SITE CHARACTERIZATION AND MONITORING!

Extensive c_taracterization data were collected to increase our understanding of the site geology,hydrology,, character of the contaminant plume, and distribution of indigenous microbialpopulation. Characterization and monitoring points were concentrated along the length of theinjection well (Figure 4). Fifteen boreholes were continuously cored. Ten were cored to a 190-foot depth and geophysically logged. Five, ,ere cored to approximately 130 feet. Geologic crosssections showing the continuity of lithologic units have, been prepared.

Samples for chemical and microbiological analyses were collected ft'ore the core at five-to-ten-footintervals or at depths where the lithology changed. Volatile organic compounds in the sedimentsare generally concentrated within the clay-rich intervals at depths of approximately 35 to 40 m_d85to 110 feet.

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Vertical depth sampling of groundwater was also performed in a number of boreholes. Problems

i with advancing the sampling tool at depths greater than 140 feet prevented collection of a completesample grid. However, these samples did provide insight into the vertical distribution of

contaminants in the plume. Generally, the highest concentrations of TCE and PCE in the

groundwater were found at depths greater than 180 feet, below the zone of injection.

The predominant direction of groundwater flow in the upper part of the Tertiary section,

immediately below the water table, is downward. A potentiometric map for the lower Tertiary

, aquifer demonstrated that the groundwater flow gradient is very flat. Flow is toward the west andis likely influenced by the M-Area groundwater recovery well system. "

Monitoring wells were installed as clusters (ten), one screened at the water table and one in the

upper sand in the local Tertiary aquifer where the injection well is located for approximately thefirst 150 feet. The horizontal well then drops to the lower sand in the local Tertiary aquifer at adepth of approximately 175 feet. No monitoring wells were screened at this depth. Ground water

ii sample.s were collected from the wells weekly for VOC analyses and biweekly for microbialanalyses during the in situ air stripping test. Ground water monitoring on a biweekly basis- continued after the conclusion of the test.

IN SITU AIR STRIPPING TEST DESIGN

The in situ air stripping concept involved installation of two parallel horizontal wells to maximizethe process efficiency. Horizontal wells were selected because they provide more surface area forinjection of reactants and extraction of contaminants. Injection into and extraction of fluids fromthe subsurface has been used to enhance recovery of petroleum hydrocarbons from oil fields for anumber of years. Because many water-bearing formations are deposited as relatively thin butareally extensive units and contaminant plumes often mimic the geology, the use of horizontalwells may improve the efficiency of the injection/extraction system. Also, horizontal wells

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can be used along linear sources of contamination and under existing facilities such as buildings,waste sites, or landfills.

The deep well was used for injection of air, whereas the shallow weil was used for extraction of airby drawing a vacuum at the wellhead (Figure 4). Both wells were operated concurrently to removeVOCs from both the ground water and vadose-zone soils. Tubes of varying lengths were installedin both horizontal wells to monitor pressure and concentrations along their entire length. First, avacuum was drawn on the shallow well for a period of two weeks. Concentration and temperatureof the extracted vapors were measured at least three times a day.

Air injection was then added at three different rates and at two different temperatures. Each of theoperating regimes was operated for a minimum of two weeks. Helium tracer tests were conductedto learn more about vapor flow paths and the heterogeneity of the system between the two wells.At the end of the injection period, the vacuum alone was continued for a period of three weeks.The entire test ran for a period of 20 weeks, five of which were for extraction only.

Two-dimensional modeling of the air injection process is underway by scientists at the LawrenceLivermore National Laboratory. These models will be used to design future demonstrations of thisprocess.

RESULTS

Almost 16,000 pounds of solvents were removed during the test (Figure 5). Rates during vaporextraction averaged only 110 pounds of VOCs per day (Figure 6), The extraction flow rate wasconstant at approximately 580 scfm during the entire length of the test. During the air injection ..::-,periods with medium (170 scfm) and high (270 scfm) rates, approximately 130 pounds of VOCs "were removed daily (Figure 6). The bundle tubes demonstrated that air was entering the screenalong the entire length of the horizontal sections in both horizontal wells. However, because of thenatural heterogeneity of the sediments, some sections were more productive than others.

Concentrations of chlorinated solvents removed during the vapor-extraction-only regime decreasedrapidly during the first two days of operation. Initial concentrations were as high as 5000 ppm butstabilized at 200 to 300 ppm. When the medium air injection rate was initiated, the ratio of TCE toPCE changed significantly (Figure 7). Several explanations for this behavior have been proposed.Post-test characterization data will provide additional information to assist in explaining thisphenomenon,

Concentrations of VOCs in the groundwater were significantly reduced in several of the monitoringwells. For example, two wells showed changes from 1600 and 1800 ug/l to 10 to 30 ug/l.However, several of the wells showed no significant change, and three wells actually hadconcentrations increases. One explanation for increases in concentrations of VOCs in groundwateris that more contaminated water at depth (as characterized with deptk sampling of ground water)was being forced upward as a result of the air injection. Two of the wells that had increases werescreened approximately twenty feet above the air injection.

A helium tracer test demonstrated connectivity between the two wells; however, recovery rates forthe helium were slow, indicating significant dispersiom The test was run for a period of 46 days,after which approximately 45 percent of the injection helium was recovered by the extraction well.

The activity of indigenous micro-organisms was found to increase at least on an order ofmagnitude during the air injection period (medium and high rates). This activity then decreasedwhen the air injection was terminated. It is possible that simple injection of air stimulatedindigenous micro-organisms that have the potential to degrade TCE. A mass balance calculationwill be performed after post-test coring and chemical analyses have been completed.

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Vadose-zone piezometers demonstrated the effect of the vacuum, both horizontally and verticallyover the test area. The zone of capture in the vadose zone extended across the entire demonstrationsite (approximately 200 by 300 feet). The effect of the vacuum remained stable during thedemonstrations.

Injection of air heated to 155 degrees F appeared to have no effect on the amount of contaminant ortemperature of the gas extracted from the shallow weil. However, sliglat increases in activity of theindigenous micro-organisms were detected.

CONCLUSIONS

Vapor extraction rates of VOCs in only one horizontal well averaged 110 pounds of solvents perday as compared to approximately 25 pounds per day in one vertical well durifig a vapor extractiontest completed in 1986. The extraction rate was increased by approximately 20 percent by additionof air injection below the water table. In addition, a significant mass of VOCs was removed fromground water during the air injection phase, as demonstrated by mapping the TCE concentrationsin the groundwater over time.

PLANS

Installation of additional horizontal wells are planned to demonstrate several different technologiesfor application in environmental problems. The ultimate goal is to demonstrate that horizontalwells can cost effectively maximize the efficiency of environmental restoration activities.

-," .

The existing horizontal well system will be used to demonstrate an in situ bioremediation that willconsist of injection of a methane/air mixture to stimulate methanotrophic micro-organisms known

to be capable of degrading TCE to carbon dioxide, water, and hydrogen chloride in an aerobicsystem. Plans include demonstration of other remediation technologies _,uchas steam stripping inthe new horizontal wells. Other aspects of the program include new air abatement technologiestests to be used in conjunction with vapor extraction systems and new chemical sensors tests.

ACKNOWLEDGEMENT

The information contained in this paper was developed during the course of work under'Contract: No. DE-AC09-89SRI8035 with the U.S. Department of Energy.

REFERENCES

1. D.S. Kaback, B. M. Looney, J. C. Corey, L. M. Wright, III, and J. L. Steele.Horizontal Wells for In Situ Remediation of Groundwater and Soil6.. NWWA ThirdOutdoor Action Conference proceedings, 121-135 (1989).

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Figure 4. Ali Monitoring Wells at Horizontal Well Site

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