temporal changes in kerosene content and composition in field soil as a result of leaching

Ž .Journal of Contaminant Hydrology 48 2001 305–323www.elsevier.comrlocaterjconhyd

Temporal changes in kerosene content andcomposition in field soil as a result of leaching

Ishai Dror), Zev Gerstl, Bruno YaronInstitute of Soil, Water and EnÕironmental Sciences, ARO, The Volcani Center, Bet Dagan 50250, Israel

Received 20 September 1999; received in revised form 10 March 2000; received in revised form 19 June2000; accepted 25 October 2000

Abstract

A field experiment was designed to determine the combined effect of leaching and naturalattenuation on the redistribution dynamics of kerosene—a volatile petroleum hydrocarbon mixtureŽ .VPHM —and of its selected individual components in the soil subsurface. Variables included thecomposition of contaminant spilled, the soil water content before contamination and the leachingpattern. Temporal changes in the residual kerosene concentration and composition in the soilsubsurface of the experimental field during 39 days and leaching by 500 mm of irrigation waterwere determined to a depth of 100 cm.

The main processes controlling contaminant attenuation were volatilization and redistributionwith depth. Soil hydration status was found to affect the attenuation, redistribution and composi-tion of VPHM in the porous media. An initial relative increase of n-alkanes in the subsurfacecompared with the total VPHM in the first leaching period was a result of the volatilization of lowvapor pressure compounds. The redistribution of individual components in the soil profile duringleaching was in accordance with their physico-chemical properties. q 2001 Elsevier Science B.V.All rights reserved.

Keywords: Kerosene; VPHM; Natural attenuation; Volatilization; Transport; Field experiment

1. Introduction

The pollution of soils and the subsurface environment by petroleum product spills is amajor concern in the industrial world. Petroleum spills may persist in the soil as a source

) Corresponding author. Present address: Clemson Institute of Environmental Toxicology, Clemson Univer-sity, 509 Westinghouse Drive, P.O. Box 709, Lendleton, SC 29670, USA.

Ž .E-mail address: [email protected] I. Dror .

0169-7722r01r$ - see front matter q2001 Elsevier Science B.V. All rights reserved.Ž .PII: S0169-7722 00 00183-2

( )I. Dror et al.rJournal of Contaminant Hydrology 48 2001 305–323306

Ž .of hazardous hydrocarbons for a long time e.g., months or years because of the lowsolubility and the moderate to low volatility of these compounds. Contaminations of thiskind is common because of storage tank and piping leaks, spills on land surfaces, and

Žimproper disposal practices e.g., Essaid et al., 1993; Ostendorf, 1990; Wang et al.,.1998a; Thorn and Aiken, 1998 . Many investigations have been carried out on the

redistribution and natural attenuation of volatile petroleum hydrocarbon mixturesŽ . ŽVPHM over time e.g., Eiceman et al., 1986; Page, 1989; Mercer and Cohen, 1990;Ostendorf, 1990; Durnford et al., 1991; Johnson and Perrott, 1991; Dean-Ross et al.,

.1992; Rubin et al., 1994, Chen et al., 1998; MacLeod and Mackay, 1999 . Thesemixtures migrate in the soil profile until a balance is achieved among pressure, gravityand capillary forces, and are affected during their transport by volatilization anddegradation of the various components of the mixtures. The physical and chemicalproperties of each component of the organic hydrocarbon mixture influence its migrationrate and fate. The diversity of the compounds in the organic mixture can lead to a

Žcontinuing change in the composition of the contaminant mixture e.g., Hayden et al.,.1994; Wang et al., 1998b and, hence, in its behavior in the subsurface environment.

The prevailing environmental conditions affect the behavior of contaminants in the soil,and influence the partitioning of the organic compounds between the different phases

Žand their degradation rate e.g., Imhoff et al., 1997; Abriola and Bradford, 1998; Chen.and Wu, 1998 . Existing information on post-spill behavior of VPHMs in general, and of

kerosene especially, in the unsaturated zone is based on field surveys which haveŽ .followed uncontrolled accidents e.g., Cozzaraelli et al., 1994 and, to our knowledge,

only few field studies were carried out to provide experimental data on the impact ofvarious factors on the fate of petroleum products under natural conditions.

The reported study was carried out following a series of laboratory and pilot studiesŽe.g., Yaron et al., 1980, 1989, 1998; Galin et al., 1990; Fine and Yaron, 1993; Gerstl et

.al., 1994; Jarsjo et al., 1997 . The experiments were designed to determine, under fieldŽ .conditions: a the combined effect of leaching and natural attenuation on the dynamics

Ž .of redistribution of residual kerosene a volatile petroleum hydrocarbon mixture, VPHMŽ .and b the fate of selected individual components in the soil subsurface. These were

studied when the amount and the composition of contaminant spilled, the soil watercontent before contamination and the leaching pattern were controlled. The dataobtained could be used for validation of VPHM transport models through the unsatu-rated zone and for designing renovation procedures for petroleum-polluted soil.

2. Materials and methods

2.1. Materials

Ž .The experiment was performed on a sandy loam Mediterranean Red Soil Rhodoxeralflocated at the Volcani Center in the coastal area of Israel. Relevant properties of aselected soil profile are presented in Table 1. Kerosene—an industrial petroleum product

Ž .—was chosen as a volatile petroleum hydrocarbon mixture VPHM for our studies.

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Table 1Properties of a selected soil profile from the experimental field

Depth Silt Sand Clay Organic pHŽ . Ž . Ž . Ž . Ž .cm % % % carbon %

0–10 6.3 77.5 16.2 1.2 7.710–30 5.0 82.5 12.5 1.2 7.530–50 3.8 80.0 16.2 0.8 7.850–70 8.1 63.1 28.8 0.6 7.970–90 7.5 64.4 28.1 0.7 8.090–110 12.5 52.5 35.0 0.6 8.1110–130 13.8 56.2 30.0 0.7 8.2

Two qualities of kerosene were used: one was neat and the other reflected a compositionafter a 12% weight loss via volatilization. This degree of volatilization was selectedbased on previous experiments that showed a significant change in the mixture composi-tion after 12% weight loss. Fig. 1 shows the chromatograms of the two kerosenes, andthe selected components identified. Relevant properties of some kerosene componentsare given in Table 2. At the time of application, the neat kerosene contained more than100 hydrocarbons, with carbon numbers ranging between C and C . Loss of volatile7 16

Ž .components from the neat kerosene 12% weight loss resulted in changes in both itsphysical and chemical properties. The viscosity of the liquid increased from 1.32=10y3

Pa s for neat kerosene to 1.40=10y3 Pa s for the volatilized kerosene. The total peakarea of components characterized by C-11 was 32.1% of the overall total for the neatkerosene and 22.5% for the volatilized material.

2.2. Field experiment

Twelve double-ring sampling plots of 0.385 m2 each were selected within the 120 m2

Žexperimental field 10 for the experiment and two for verifying kerosene application. 2 Ž .distribution . Kerosene was applied to each plot at a rate of 5.2 lrm 2 l per plot . This

Ž .amount was selected based on a previous study Fine and Yaron, 1993 that quantifiedŽthe relationship between kerosene residual capacity residual kerosene concentration is

.used by analogy with water field capacity in field soils as a factor of their clay content,organic matter and moisture content. In addition, it was found that 2 l of kerosene perplot would prevent the kerosene from moving below 20 cm depth. The kerosene was

Ž .applied by a back mounted sprayer with a spray radius of 10 cm to each plot in 15min. The application distribution was checked on two additional plots in the same field.Ten petri dishes were randomly placed in each plot prior to kerosene application andcollected immediately after spraying ceased. The kerosene content in each dish wasdetermined and a standard deviation of 20% was found per plot.

Kerosene was applied on the experimental field and subsequently leached using amini-sprinkler irrigation system with 500 mm water during the dry summer season toenable control of the leaching pattern. The average annual rainfall in this area is 500mm.


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Table 2Properties of major components of kerosene

b bCompound Solubility M.W. D b.p Vapor Log K Log K C no.10 ow 10 oca,b cŽ . Ž . Ž . Ž . Ž .mgrl grmol grml 8C pressure Pa

p-Xylene 216 107 0.861 137 1498.5 3.18 2.97 8Trimethyl benzene 92 120 0.894 176 408.0 3.42 3.21 9n-Propybezene 521 120 0.862 159 621.3 3.69 3.48 9n-Nonane 0.5 128 0.718 150 825.2 5.63 5.42 9n-Decane 0.14 142 0.730 173 318.6 6.17 5.96 10Naphthalene 31 128 solid 218 18.7 3.41 3.20 10Tetramethylbenzene 25 134 0.891 198 78.2 4.00 3.79 10n-Undecane 0.05 156 0.740 195 74.7 6.64 6.43 11Methylnaphthalene 27 142 1.001 242 3.87 3.66 11n-Dodecane 0.015 170 0.748 215 30.7 7.18 6.97 12n-Tridecane 0.0076 184 0.755 235 1.3 7.49 7.28 13n-Tetradecane 0.0023 198 0.762 253 0.4 8.01 7.80 14n-Pentadecane 0.0007 212 0.769 270 0.1 8.53 8.32 15

a Ž .Kan and Tomson 1996 .b Ž .Mott 1995 .c Ž .Weast 1974 .

Ž .The field was divided into three irrigation treatments: 1 a pulse irrigation treatmentŽin which the 500 mm was applied in 10 pulses of 50 mm each a pulse of 50 mm

. Ž .leaching is a common event during the rainy season , 2 a no-irrigation treatment—noŽ .water applied, and 3 a pulse irrigation treatment in which the 500 mm was applied in

two 250 mm pulses. In the 50-mm pulse treatment, the effect of the VPHM compositionand initial soil water content was measured. The treatments applied consisted of two

Ž Ž . .qualities of kerosene neat and after 12% w volatilization applied on air dry soil, andŽ Ž .neat kerosene applied on two initial soil moisture contents air dry, 2% w ; and field

Ž .. Ž . Ž .capacity, 17% w . In treatments 2 no irrigation and 3 pulses of 250 mm , neatkerosene was applied on air dry soil only to check the effect of leaching regime underfield conditions. Water was applied in the 50 mm pulse zone every 3–4 days and in the250 mm pulses zone 4 and 24 days following kerosene application. Fig. 2 shows thesetup of the experimental field.

The plots were sampled in increments of 10 cm to a depth of 60 cm and then in 20Ž .cm increments to final depth of 100 cm. Two sampling profiles diameter—4.2 cm one

in the center and one by the edge, were taken from each plot. A soil sample of about 500g was obtained from each depth. The sample was stirred immediately and a sub-sampleof about 8 g of the stirred soil was then transferred to an extraction vial prepared inadvance with the extraction solvents. The extraction of kerosene from the soil into asolvent mixture of 8 ml water and 6 ml dichloromethane thus began in the fieldimmediately after sampling. The vials were pre-weighted and exact weight of the soilsample was determined after extraction. The sampling holes were filled in with cleansoil from an adjacent field and marked to avoid sampling in the same location twice. Atthe end of the experiment, there were 10 filled sample holes with 4.2 cm diameter ineach plot with 70 cm diameter which allowed enough space between the sampling pointsto prevent one sampling point from affecting another.


Fig. 2. Diagram of the experimental field-location of the sampling plots and leaching device. Treatments: plotsŽ .1,2,9-12-neat kerosene applied on air dry soil; 3,6-neat kerosene applied on pre-wet field capacity soil; 7,8

Ž .12% wrw kerosene applied on air dry soil.

2.3. Analytical procedure

The soil–solvent mixture was mechanically shaken overnight after which the soil wasŽ .separated from the solvents by centrifugation 10 min–2500 rpm . The organic phase

was then separated and dried with sodium and stored at y188C until analysis.Phenanthrene was added to the organic phase extracts as an internal standard, prior togas chromatography analysis. The recovery of kerosene from the soil samples wasdetermined by two methods. In the first method, 100 ml of kerosene was mixed in an

Ž .8-g soil sample from the soil surface followed by extraction according to the describedŽ .protocol five replicates . The average recovery was found to be 98%. In the second

method, 10 vials were randomly selected during the first sampling of the field experi-ment and extracted twice. The comparison of the first and the second extraction showed93% recovery after the first extraction, indicating high extraction efficiencies for fieldsamples.

Ž .The spiked samples were analyzed on a Varian gas chromatograph. Model 3400Ž .equipped with a Fill detector and DB 1 capillary column 30 m, 0.25 mm, 0.25 J tm . A

Finnigen MAT Magnum GCMS provided compound identification. In both instruments,the temperature program was 408C for 2 min, ramp of 38Crmin to 1808C for 2 min, andanother ramp of 208Crmin to 2508C, for 1 min.

3. Results and discussions

3.1. Changes in kerosene content

The combined processes of degradation, volatilization andror leaching affect theredistribution of the VPHM in the soil profile with time. The soil water content atapplication, the amount of leaching and the leaching pattern may also affect theredistribution dynamics.


Ž . Ž . ŽApplication of kerosene on air dry soil Fig. 3a and on wet field capacity soil Fig..3c showed the effect of the soil moisture content at kerosene application on its

redistribution under leaching. After 5 days, a much higher concentration of kerosene wasŽ 6 . Ž .found in the upper 10-cm layer 9.4=10 mgrkg of the wet field capacity soil than

Ž 6 .in the corresponding layer of the air dry soil 5.3=10 mgrkg . Infiltration of keroseneŽ .into the plots containing water at field capacity moisture content 17% w , which

hereafter we will call the pre-wet soil, was lower than in the air dry soil after 5 days.This may be due to less free pore volume available for the transport of kerosene in thepre-wet field below 20 cm. The high concentration of kerosene near the surface of the

Ž 6 .pre-wet soil was reduced strongly to 2.7=10 mgrkg in the next 14 days, volatiliza-tion being suspected as the major natural attenuation process as no significant downwardmovement of kerosene was observed. In addition, the kerosene retained near the soilsurface was highly affected by evaporation to the atmosphere, a larger fraction being lostfrom the pre-wet soil treatment in which more kerosene was retained in the upper soillayer.

The effect of leaching management on kerosene redistribution is shown in Fig. 3a andb. After 39 days, the irrigation treatment with 500 mm of water applied in two pulses of250 mm had little or no effect on kerosene redistribution compared to irrigationtreatment with pulses of 50 mm. The reduction in the kerosene concentration in the

Ž .upper soil layer 0–40 cm may be explained as a combined effect of volatilization,degradation and leaching. The concentration of kerosene in the 40–100 cm depthsuggests that once the VPHM is transported to the deeper layers, the changes inconcentration are very small. At that depth, volatilization is reduced as a result of thesmaller vapor pressure gradient in the subsurface between the kerosene and theatmosphere and the greater path length for hydrocarbon transport. The gas phase throughwhich the VPHM is partitioning is limited to the volume of pores that can becomesaturated with the hydrocarbon vapors, whereas the kerosene near the surface evaporatesto the unlimited volume of the atmosphere where the vapor pressure gradient is higher.

The redistribution of VPHM in the dry, non-irrigated zone is shown in Fig. 3d.Ž .During this time 0–39 days , the soil moisture content in the soil profile was less than

Ž .1.5% w . The only mode of transport for the kerosene to the deeper soil layers underŽ .this treatment no irrigation is in the gaseous phase once free drainage of the kerosene

Ž .ceases. The concentration after 5 days first sampling for the non-irrigated field isŽsignificantly lower than that for the irrigated field in the upper 40 cm 75% and 58% for

Ž . .the air dry and wet field capacity soil, respectively . From 0 to 20 cm depth, a decreasein concentration was observed between 5 and 17 days for the non-irrigated field. Thekerosene concentration for this treatment does not change significantly between 0 and 40cm for the samplings after 17, 25 and 39 days. The kerosene concentration in thenon-irrigated field, for example, changed from an average of 1.25=106

mgrkg soil inthe 0–20 cm layer to about 1.33=106

mgrkg in the 20–30 cm soil layer after 17 days.This indicates that the major effect of the volatilization occurred during the first 17 daysof the experiment. The kerosene concentration in the non-irrigated field after 39 days inthe 30–40 cm layer was 1.05=106. The kerosene concentration in the same layer at the

Ž .same sampling date 30–40 cm, 39 days that was irrigated with 50 mm pulses was5 5 Ž2.3=10 and 1.5=10 for application of kerosene on air dry and pre-wet field

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Ž .Fig. 3. Redistribution of neat and volatilized kerosene in the field soil during 39 days from surface application: a 10 pulses of 50 mm irrigation after application onŽ . Ž . Ž .air dry soil; b two pulses of 250 mm irrigation after application on air dry soil; c 10 pulses of 50 mm irrigation after application on wet soil; d non-leached after

application on air dry soil.


. Ž . 5capacity soil, respectively, and for 250 mm pulses on air dry soil was 2.9=10 . Thisdifference is due to the effect of the water content in the soil on adsorption since athigher water contents the kerosene had to partition to water before sorbing on mineralsurfaces, whereas in the air dry soil, kerosene could directly contact with soil surface to

Žallow greater adsorption. In Fig. 4, we present the redistribution average and standard. Ž . Ž .deviation of neat and 12% volatilized kerosene after leaching with 50 a , 200 b , 300

Ž . Ž .c and 500 d mm of water applied in irrigation pulses of 50 mm. After the first 50 mmof irrigation, the average concentration of kerosene in the plots that received thevolatilized kerosene was slightly higher in the upper soil profile and slightly lower in all

Žthe deeper layers to a depth of 100 cm. After 200 and 300 mm of irrigation 17 and 25.days , no significant difference in VPHM concentration distribution through the soil

profile was observed between the neat and volatilized kerosene. The distribution ofŽ .volatile and neat kerosene after 500 mm of irrigation Fig. 4d shows that in the upper

30 cm there was no difference in the kerosene concentration. In the 40–100 cm depth, asshown in the sub Fig. 4d, the concentration of kerosene hydrocarbons was greater for theneat kerosene than for the volatilized kerosene. This is most likely due to the presence ofthe more soluble and volatile compounds in the neat kerosene and their leaching to thedeeper soil layers.

Kerosene attenuation can be seen to have occurred from the GC chromatograms ofŽthe extracts from the upper 10-cm soil layer of neat and 12% volatilized kerosene Fig.

.5 . The role of volatilization as a major attenuation process can be seen in the shift ofŽ .the center of mass of the chromatogram after 5 days Fig. 5aqc to higher retention

Ž .times, where the heavier compounds elute, after 40 days Fig. 5bqd . The preferredevaporation of the light compounds, especially alkylated benzenes, from the volatilized

Ž .kerosene can be seen by comparing some of the peaks retention time 5–15 min in theŽ . Ž .chromatogram of the neat Fig. 5a and volatilized Fig. 5c kerosene. Looking at

Ž .specific compounds we, can see that for neat kerosene Fig. 5a the biggest peak after 5Ž . Ž .days was that of n-undecane 5 and the third largest was n-decane 3 , while after 39

Ž . Ž . Ž .days Fig. 5b , the biggest peak was that of n-dodecane 6 , and n-decane 3 was onlyŽ . Ž .the fifth largest. n-Nonane 1 and trimethylbenzene 2 in both neat and volatilized

Ž .kerosene were reduced to residual concentrations at longer times Fig. 5aqc vs. bqdŽ .indicating enhanced volatilization of the high-vapor-pressure compounds Table 2 . The

Ž .preferred degradation of n-alkanes is indicated by the ratio of the peak for n-alkane 3Ž .to a branched alkane with the same C number 4 . The ratio of the peaks after 5 days

Ž . Ž .4.82:1 was much higher than that after 39 days 2.03:1 . A similar example can beŽ .found for the ratio between peaks for n-dodecane 6 and an alkene with 12 carbon

Ž .atoms 7 where the rate was 5.36:1 at 5 days vs. 3.15:1 at 39 days.The natural attenuation patterns shown in Figs. 3, 4 and 5 may be explained by the

fact that in the leached zone the water-to-air ratio is more favorable to biological activityŽ .in the irrigated plots than in the non-irrigated plots soil moisture content -2%wrw

Ž .Alexander, 1980; Keeney, 1983; Paul and Clark, 1989; Yaron et al., 1996 . In the caseof neat kerosene, the light fraction, which includes aromatic compounds such asalkylated benzenes, was drastically reduced soon after application, but the compositionof the residual mass exhibited little or no change after 25 days. The volatilized kerosene

Žmixture was enriched in aliphatic compounds by the preferential evaporation of the

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Fig. 4. Effect of VPHM initial composition on its redistribution in soil profile: neat and 12% volatilized kerosene after 50, 200, 300 and 500 mm water irrigation.

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Ž . Ž .Fig. 5. Chromatogram of VPHMs in the upper 10 cm layer: a and c neat kerosene after 5 and 39 days, respectively; b and d 12% wrw volatilized kerosene after 5and 39 days, respectively. The peaks identified are: 1-n-nonane, 2- trimethylbenzene; 3-n-decane 4-branched alkane C1o; 5-n-undecane; 6- n-dodecane; 7-alkene C ;12

Ž .8-phenanthrene int. std. .


.light aromatic compounds prior to application and exhibited a slower decrease inconcentration than the neat kerosene during the first 25 days. We propose that naturalattenuation of kerosene during the first 25 days following application was controlled byvolatilization and that biological degradation became a significant attenuation processonly after the volatilization of the light compounds. This behavior is in accordance with

Ž .existing knowledge e.g., Douglas et al., 1992; Lee and Levy, 1989 , showing that therate of biodegradation follows the descending order n-alkanes)aromatics)branched-alkanes.

3.2. Changes in kerosene composition during redistribution

The various compounds comprising the VPHM behave differently during the redistri-bution along the soil profile according to their chemical and physical properties.

Fig. 6 shows the change in the ratio between the areas of the n-alkanes peaks and theŽtotal hydrocarbons peaks in the soil profile during leaching with 10 pulses of 50-mm 39

. Ž .days of irrigation water and their standard deviations . The n-alkanes are distributedacross the entire spectrum of the kerosene, starting with light compounds like n-nonaneŽ . Ž . Ž .C9 up to the heavy n-hexadecane C16 . In the upper soil layer Fig. 6a , where mostof the kerosene was retained, the n-alkanes comprised about 50% of the total hydrocar-

Ž .bon content THC of the kerosene mixture. The kerosene in this layer was influencedŽmainly by volatilization throughout the experiment. A relative increase of 10% in the

.n-alkanerTHC ratio to 54.7% after the first 10 days of the experiment can be explainedby further volatilization of the light aromatics and branched alkanes which are character-

Ž .ized by high vapor pressures. The small decrease in the n-alkanerTHC ratio 3.6%between 25 and 39 days following application may have been due to preferential

Žbiodegradation of n-alkanes over branched alkanes and aromatic hydrocarbons. e.g.,.Douglas et al., 1992; Lee and Levy, 1989 .

Ž .In the 30–40 cm layer Fig. 6b , the n-alkanerTHC ratio 5 days after application wassimilar to that in the upper layer. The ratio increased to 65% after 10 days and thenslowly decreased until 25 days; between 25 and 39 days the fraction of alkanesdecreased more rapidly to about 20%. Because of the much lower concentration of theVPHM in this layer, small effects or changes, which would not be noticed among thehigher VPHM concentration in the surface layer, became important. There are threesimultaneous processes that may affectVPHM composition in the 10–25-day period.One process is rapid volatilization of high vapor pressure compounds soon afterapplication. Another process is biodegradation of n-alkanes and the third process isenhanced solubilization of benzenes. The rapid volatilization of the high vapor pressurecompounds will cause the mole fraction of the n-alkanes to become greater, especiallyduring the initial periods. At later times, there will be an increase in volatilization of then-alkanes because their larger mole fraction according to Raoult’s Law. Biodegradation

Ž .would preferably effect the n-alkanes Lee and Levy, 1989; Douglas et al., 1992 .Enhanced solubilization of alkylated benzenes, which are usually much more solubleŽ .more than two orders of magnitude , will lead to their transport to deeper soil layerswith the water phase.

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Ž . Ž . Ž . Ž .Fig. 6. Ratio of n-alkanes peak area to the total peak area in soil profile during 39 days and 500 mm water leaching. a 0–10 cm, b 30–40 cm, c 60–80 cm, d80–100 cm depth.


Ž . Ž .In the deeper layers, at 60–80 cm Fig. 6c and 80–100 cm Fig. 6d , the ration-alkanerTHC after 5 days indicates a slower transport of n-alkanes. This resulted fromthe fact that at this stage the redistribution of kerosene to this depth in the soil profilewas primarily in the gas phase; therefore, the less volatile n-alkanes were present inlower concentrations. After 10 days, the ratio n-alkanesrTHC increased to about 65% inthe 60–80 cm layer and to about 80% in the 80–100 cm layer. This concentration ratio

Ž .cannot be explained by transport in the aqueous as solute or gas phase. The onlyexplanation for the transport of the n-alkanes is preferential flow of the VPHM phase

Ž .itself with composition like in the upper layers about 50% , followed by volatilizationand dissolution of the light residual hydrocarbons in those layers, enriching the residualVPHM in n-alkanes.

The sharply decreasing pattern observed in the 30–40 cm layer between 25 and 39days was repeated in the deeper layers, and the ratio n-alkanesrTHC after 39 days wasabout 20% in all cases. This behavior can also be explained by biodegradation of then-alkanes.

ŽIn Fig. 7, the redistribution of four selected compounds in the soil profile nonane,. Žtrimethylbenzene, dodecane and tetradecane is shown during irrigation with 10 pulses

.of 50 mm . All the compounds exhibit an initial decrease in concentration with depth.Ž .For the first several irrigations 300 mm , the decreases in concentration were greater for

n-nonane and trimethylbenzene than for n-dodecane and n-tetradecane. For n-nonaneand trimethylbenzene, with vapor pressures of 825 and 408 Pa, respectively, the greatest

Ž .decreases in concentration were observed between 5 and 17 days 50–200 mm leaching ,Ž .with no significant change after the next 100 mm of irrigation to 25 days . This

behavior can be explained in terms of rapid volatilization of the compounds with highervapor pressures, leaving a residual capacity after 17 days and almost no furtherevaporation in the next 8 days. A large decrease in the residual concentration between

Ž .25 and 39 days 300–500 mm of irrigation was observed for trimethylbenzene, whichŽmay be explained by biodegradation. Trimethylbenzene is more soluble then nonane the

. Ž .most soluble n-alkane in the mixture by more than two orders of magnitude Table 2Ž .and is, therefore, more accessible and available for biodegradation Carberry, 1998 .

Ž .After 500 mm of irrigation, nonane, with a high vapor pressure 825 Pa and asolubility of 0.5 ppm, showed a decrease in concentration in the upper soil layer and anincrease in the next 30 cm layer, with no significant change in the mass balance in thesoil profile: 79,878 mgrkg soil and 68,584 mgrkg, respectively. Transport in the gasphase and as solute can explain this redistribution of the residuals.

n-Dodecane showed behavior that resembled that of n-nonane, and the differencebetween the two compounds was due to the differences in their physical properties.

Ž .n-Dodecane is less volatile and soluble than n-nonane Table 2 , therefore, its volatiliza-tion is less and its degradation and solubilization are also slower. Because of the lowsolubility of n-dodecane, only part of the transport after 500 mm of irrigation can be

Ž .attributed to dissolution and microemulsion transport Abdul et al., 1990 . Therefore,flow of the VPHM as a separate phase must explain most of it. A mass balance showsalmost no loss of n-dodecane between 300 and 500 mm of irrigation, which supports thehypothesis that non-dissolved transport is a major process because of the preferredbiodegradation of the dissolved VPHM.

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319Fig. 7. Redistribution of kerosene selected compounds in the soil profile after 500 mm leaching with 50 mm pulse irrigation on initially air dry soil.


Tetradecane exhibited the same pattern of redistribution to the deeper soil layers asthe n-alkanes between 300 and 500 mm of irrigation. n-Tetradecane has a very low

Ž .vapor pressure and a low solubility Table 2 and, therefore, is expected to redistributeto the deeper layers very slowly, probably by microemulsion and flow as a separatephase of weathered VPHM.

Redistribution of selected compounds in leached and non-irrigated field soils 39 daysafter application is presented in Table 3. The concentrations of all the compoundsdecreased compared with the amounts applied, but different behavior patterns can beobserved for the different components. The concentrations of the lighter compounds,Ži.e., xylene, n-nonane, trimethylbenzene, n-decane and C9-alkylated cyclic hydrocar-

.bons in the 20–30 cm layer were the same as or slightly greater than those in the 0–10cm layer for the irrigated field treatment. This can be explained in terms of acombination of volatilization of the light compounds from the surface to the atmosphere,and transporting the water phase. The heavier compounds showed a decrease inconcentration with depth. The fact that the heavier, less soluble and less volatilecompounds were found in the 60–80 cm layer in the same relative concentrations as the

Ž .light components 0.2% for the heavier and 0.1% for the light compounds indicates thatVPHM transport to the deeper layers was by microemulsion or as a separate phase. Inthe non-irrigated field, there was a constant decrease in concentration of all thecompounds with increasing depth and, in general, lower concentrations than in the

Žirrigated soil. For the four heavy compounds i.e., n-dodecane, C alkene, n-tridecane12.and tetradecane , the residual concentration in the non-irrigated field was similar to or

greater than that in the irrigated field. This may be attributed to volatilization andbiodegradation. These compounds are less volatile and, therefore, their loss by evapora-tion would be minimal. The greater biodegradation of the heavy components in theirrigated field, where the environmental condition favor biological growth, may accountfor their higher concentrations in the non-irrigated field.

Table 3Ž .Major components of kerosene determined in soil at application and after 39 days mgrg dry soil

Components At application After 39 daysŽ .calculated Ž .Leached 500 mm Dry

Ž .Soil depth cm 0–10 0–10 20–30 60–80 0–10 20–30 60–80

Xylene 132 2.77 3.54 0.00 0.06 0.00 0.00Nonane 485 12.95 25.52 0.29 1.66 1.03 0.00Trimethylbenzene 146 3.19 3.52 0.05 4.95 3.47 0.00Decane 1623 75.47 109.78 1.59 50.35 34.61 0.28C Alkane 370 43.26 41.19 0.64 7.96 0.29 0.129

Undecane 1630 150.89 153.30 2.80 138.93 93.60 1.08C alkane 332 40.55 32.09 0.63 31.18 21.11 0.2110

Dodecane 1271 156.95 129.23 2.83 158.95 106.59 2.12Alkene C 108 28.19 17.91 0.54 39.75 26.69 0.6812

Tridecane 1134 157.80 110.89 2.76 166.86 109.29 2.91Tetradecane 687 101.53 66.08 1.91 118.25 73.72 2.72Total 15,990 1993.90 1626.51 195.49 1584.23 1053.00 36.18


4. Summary and conclusions

Temporal changes in the residual kerosene concentration and composition in the soilsubsurface of the experimental field during leaching by 500 mm of irrigation water weredetermined to a depth of 100 cm. The main processes controlling contaminant naturalattenuation were volatilization and redistribution with depth. The effect of transport of

Žthe contaminant was minor in relation to its volatilization, since the more soluble and.therefor, mobile fraction of the mixture, volatilized immediately after application. As a

consequence, the composition of the residual VPHM in the soil subsurface was affectedby the disappearance of the volatile compounds and a relative increase of the heavyconstituents.

Soil hydration status at the time of VPHM application affects behavior of VPHM inthe porous media. A high soil moisture content prior to VPHM application causes adelay in its infiltration to the deeper soil layers and thus enhanced evaporation oflow-vapor-pressure hydrocarbons from the field surface to the atmosphere. Leaching ofthe contaminated plot, on the other hand, led to a decrease in natural attenuation byvolatilization due to the reduction of free pore volume available for evaporation in theunsaturated zone. The initial increase of the n-alkane group in the subsurface, comparedwith the total VPHM in the first leaching period was a result of the volatilization oflow-vapor-pressure compounds. The subsequent decrease of n-alkanertotal VPHM ratiomay be explained by biodegradation. The presence of the VPHM non-soluble fractionsin the deeper layer of the soil profile suggests their transport as microemulsion or byseparate phase flow. The redistribution of individual soluble components was inaccordance with their physico-chemical properties, leaving the heavier compounds in theupper subsurface layers.

Acknowledgements

The research was supported by grants from the Israeli Ministry of the Environment,the Water Research Institute of the Technion, and the Belfer Foundation for EnergyResearch. Thanks are due to Dr. V. Glezer for GC-MS determinations. We would like toacknowledge the reviewers for their constructive comments.

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temporal changes in kerosene content and composition in field soil as a result of leaching

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