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Environmental Toxicology and Chemistry, Vol. 17, No. 12, pp. 2378–2382, 1998q 1998 SETAC

Printed in the USA0730-7268/98 $6.00 1 .00

EFFECT OF WETTING AND DRYING ON THE BIOAVAILABILITY OF ORGANICCOMPOUNDS SEQUESTERED IN SOIL

JASON C. WHITE, ANTONIO QUINONES-RIVERA, and MARTIN ALEXANDER*Institute of Comparative and Environmental Toxicology and Department of Soil, Crop, and Atmospheric Sciences,

Cornell University, Ithaca, New York 14853, USA

(Received 2 December 1997; Accepted 23 April 1998)

Abstract—A study was conducted to determine whether cycles of wetting and drying alter the availability of organic compoundsthat have aged in soil. Subjecting soil to wetting-and-drying cycles during periods of aging ,60 d decreased the biodegradability,extractability, and uptake by earthworms of phenanthrene and reduced the extractability of di(2-ethylhexyl) phthalate (DEHP)sequestered in soil compared with soil aged at constant moisture. The mineralization of sequestered DEHP was greater in soil thatwas wet and dried during a 41-d period of aging than in soil incubated at constant moisture. Wetting and drying soil during periodsof aging of 100 or more days had no effect on the biodegradability or assimilation by Eisenia foetida of sequestered phenanthreneand DEHP. Subjecting soil containing previously sequestered phenanthrene to one, three, or four wetting-and-drying cycles increasedthe biodegradability of the compound. The extractability of sequestered phenanthrene was greater in soil that was wet and driedonce after aging than in soil maintained at constant moisture, but three wetting-and-drying cycles did not affect extractability. Thebiodegradability of sequestered DEHP was unaffected by wetting and drying. We suggest that wetting and drying may be usefulin the remediation of contaminated soils.

Keywords—Biodegradability Bioavailability Aging Sequestration Phenanthrene

INTRODUCTION

The sequestration of organic compounds that have persistedin soil may reduce their bioavailability. For example, chlori-nated insecticides that had persisted in soil were less toxic thanthe freshly added compounds [1,2], and the herbicide napro-pamide became progressively more toxic as it aged in soil [3].More recent investigations have verified the earlier studies andshown that the toxicity of simazine to plants [4], the mortalityof insects caused by dieldrin and 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane (DDT) [5], and the uptake of phenan-threne and atrazine by earthworms [6] are less if the com-pounds have persisted and become sequestered in soil. Simi-larly, the biodegradability of a number of aged compounds isless than if the same chemical is freshly introduced into soil[7,8].

A practical means for increasing the bioavailability of agedtoxicants in soil would be a useful adjunct to current biore-mediation technologies because it would allow for more ex-tensive biodegradation and result in a low level of residualchemical in soil. Conversely, a means for reducing the bio-availability by increasing sequestration would reduce the ex-posure to, and thus the risk from, organic pollutants in soil.In a previous study from this laboratory, preliminary experi-ments suggested that the biodegradability and extractability ofsequestered phenanthrene were reduced if the soils were wetand dried during short periods (,53 d) of aging [9].

Because wetting and drying might offer a simple and in-expensive method to reduce the bioavailability of sequesteredcompounds and because the lack of information regarding thebioavailability of organic compounds sequestered in soil rep-resents a major weakness in estimations of exposure and risk

* To whom correspondence may be addressed(jae2@cornell.edu).

at contaminated sites, a study was conducted to determine theeffect of wetting-and-drying cycles on the bioavailability oforganic compounds sequestered in soil. The test compoundswere phenanthrene and di(2-ethylhexyl) phthalate (DEHP).

MATERIALS AND METHODS

Aging

Samples of Lima loam (pH 7.2, 9.9% organic matter), Mid-dlebury silt loam (pH 6.9, 5.1% organic matter), Chenangogravelly loam (pH 6.7, 6.7% organic matter), and Mount Pleas-ant silt loam (site no. 1, pH 6.2, 8.0% organic matter; site no.2, pH 5.2, 3.2% organic matter), all from locations in NewYork, were air dried, passed through a 2- or 4-mm sieve, andsterilized with 2.5 Mrad of g-irradiation from a 60Co source.Portions (5 or 10 g) of sterile soil were added to 50-ml sterilescrew-cap test tubes, and 1 3 105 dpm of [9-14C]phenanthrene(8.3 mCi/mmol, .98% purity; Sigma Chemical, St. Louis,MO, USA) or [U-ring-14C]di(2-ethylhexyl) phthalate (DEHP;2.7 mCi/mmol, .98% purity; Sigma Chemical) and unlabeledphenanthrene or DEHP were added in 50 ml of dichlorometh-ane to give 1.1, 2.1, or 11.0 mg/g soil. The soil samples weremixed manually 10 times and shaken with a vortex mixer for6 s every 10 min in a 50-min period to allow the dichloro-methane to volatilize and to mix the compound with the soil.Half of the samples were moistened to 21.1 bar, capped withTeflont-lined closures, and stored at 308C. The remaining sam-ples were transferred to sterile 250-ml Erlenmeyer flasks,moistened to 21.1 bar, covered loosely with aluminum foil,and stored at 308C. Phenanthrene is not lost from soil underthese conditions, and 96.4% is recovered [10]. When the lattergroup of samples had dried to constant weight, they wereremoistened to 21.1 bar. Additional samples were prepared inthe same way to give soils in which the test compounds had

Wetting and drying effects Environ. Toxicol. Chem. 17, 1998 2379

been aged for different time periods and subjected to differentnumbers of wet-dry cycles. All treatments were in triplicate,and 0-d values for each were determined.

A DEHP-degrading bacterium was obtained from Lima loamby enrichment culture using a medium containing 0.05% DEHPand 0.2 g KH2PO4, 0.4 g K2HPO4, 0.1 CaCl2·2H2O, 0.05 gMgSO4, and 0.01 g FeCl3 per liter of distilled water. The isolatewas an oxidase- and catalase-positive gram-negative rod. Toobtain large number of cells, the DEHP-degrading organismand Pseudomonas strain R were grown at 308C in a mediumcontaining DEHP (40 ml/L) or phenanthrene (40 mg/L) and theinorganic salts. After 7 to 14 d, the culture of Pseudomonasstrain R was passed through a 40-mm pore-size glass frit toremove phenanthrene crystals. The cells of both bacteria werecollected by centrifugation at 10,400 g for 10 min, and the cellswere resuspended in 0.01 M phosphate buffer (pH 7.0). Thecell density was determined by plating on Trypticase soy agar.

Wetting and drying during short-term aging

Phenanthrene (1.0 mg/g) was added to 10-g samples of soil.Half of the samples of each soil were incubated at constantmoisture. The other half of the samples of Chenango gravellyloam, Middlebury silt loam, and Mount Pleasant silt loamnumber 1 were subjected to one or four wetting-and-dryingcycles during 7 or 28 d of aging (rewet at 7, 14, 21, and 28d), nine wetting-and-drying cycles during 58 d of aging (rewetat 7, 14, 21, 28, 35, 42, 48, 53, and 58 d), or five wetting-and-drying cycles during 44 d of aging (rewet at 5, 12, 21,31, and 44 d), respectively. The samples of the first two soilswere added to 50-ml test tubes and inoculated with 108 to 109

cells of Pseudomonas strain R. The 14CO2 formed from min-eralization was trapped in 2.0 ml of 0.5 M NaOH that wascontained in a 5-ml vial suspended in the tube from a Teflon-taped rubber stopper. The alkali was periodically added toLiquiscint scintillation fluid (National Diagnostics, Somer-ville, NJ, USA) for measurement of radioactivity with a liquidscintillation counter (model LS 7500, Beckman Instruments,Irvine, CA, USA).

The remaining samples of Mount Pleasant silt loam, whichwere used to determine extractability, were added to 50-mlTeflon centrifuge tubes containing 25 ml of an ethanol:water(45:55) solution. The samples were placed on a rotary shakeroperating at 75 rpm, and after 24 h, the quantity of 14C-phe-nanthrene in the extract was measured. The soil samples werethen sequentially extracted with 25 ml of n-butanol for 24 hand 25 ml of hexanes for 24 h. The amount of phenanthrenein each extract was determined by liquid scintillation counting.

Di(2-ethylhexyl)phthalate (11.0 mg) was added to 5-g por-tions of Mount Pleasant silt loam number 1, and half of thesamples were subjected to five wet-and-dry cycles during 41d of aging (rewet at 6, 14, 22, 30, and 41 d). The remainingsamples were aged for 41 d at constant moisture. Some of thesamples that were and were not wet and dried were transferredto 250-ml flasks containing 100 ml of sterile distilled waterand 108 to 109 cells of the DEHP-degrading bacteria. The flaskswere incubated on a rotary shaker operating at 100 rpm at228C. The remaining soil samples were transferred to 50-mlTeflon tubes containing 25 ml of n-butanol. The samples wereshaken vigorously on a vortex mixer for 30 s and centrifugedfor 10 min at 12,000 g. The radioactivity of the supernatantwas determined by liquid scintillation counting. The soil sam-ples then were extracted first by shaking with n-butanol for24 h at 308C and then by shaking with hexanes for 72 h at

308C. The extracts were analyzed for 14C-DEHP by liquid scin-tillation counting.

Wetting and drying during long-term aging

Phenanthrene (11.0 mg) was added to 10-g samples ofMount Pleasant silt loam no. 1 in sterile 50-ml centrifuge tubes.Half of the samples were transferred to 250-ml flasks andsubjected to repeated wetting-and-drying cycles during 100 d(rewet at 6, 16, 26, 39, 49, 60, 71, 82, 91, and 100 d) or 151d (rewet at 7, 15, 23, 32, 39, 46, 56, 65, 75, 85, 96, 107, 119,130, 141, and 151 d) of aging, respectively. The remainingsamples were aged at constant moisture for 100 or 151 d. Soilcontaining unaged substrate was prepared, and all sampleswere inoculated with Pseudomonas strain R.

To determine the effect of wetting-and-drying cycles on theavailability of sequestered phenanthrene to bacteria and earth-worms, 5-g portions of Lima loam were amended with 6.0 mgof phenanthrene, and 10-g portions received 11.0 mg of thecompound. Half of the 5-g samples were subjected to six wet-ting-and-drying cycles during 54 d (rewet at 6, 14, 23, 32, 42,and 54 d), and half of the 10-g samples were wet and dried 6times (rewet at 15, 32, 47, 64, 81, and 102 d) during 102 dof aging. The remaining 5- and 10-g samples were kept atconstant moisture for 54 or 102 d, respectively. Some of thesamples that were and were not wet and dried were inoculatedwith Pseudomonas strain R, and mineralization was measured.The remaining samples were extracted.

After mineralization had stopped, the soil samples wereextracted to determine the residual amount of phenanthrene.The samples were shaken vigorously for 2 min with 25 ml ofn-butanol, and the slurry was passed through Whatman no. 1filter paper. The culture tubes were washed with 10 ml of n-butanol, and this liquid was also filtered. Soil remaining onthe filter was subject to Soxhlet extraction for 3 h with 90 mlof hexanes and 2 ml of n-butanol. The hexanes were removedby evaporation, and the remaining n-butanol was combinedwith the first butanol extract. The combined extracts werepassed through a 0.22-mm Teflon syringe filter and analyzedby high-performance liquid chromatography (HPLC) (Hew-lett-Packard liquid chromatograph series 1050, Hewlett-Pack-ard Co., Avondale, PA, USA) fitted with a Spherosorb ODS-2octadecyl-bonded silica column (5 mm, 250 3 4 mm) usingacetonitrile-water (86:14) as the mobile phase at a flow rateof 0.8 ml/min. Phenanthrene was detected by its absorbanceat 254 nm.

The remaining samples of Lima loam containing phenan-threne aged for 0, 54, or 102 d under conditions of constantmoisture or wetting and drying were used for an earthwormassay. Five 10-g portions and six 5-g portions of soil wereseparately combined in 250-ml glass jars and received five orfour earthworms (Eisenia foetida), respectively. At 8 d, theworms were transferred to petri plates containing Whatmanfilter paper no. 1 for 24 h of depuration. The worms were thenfrozen, ground with a mortar and pestle with Na2SO4, andsubjected to a Soxhlet extraction for 3 h with 90 ml of hexanes.The extract was analyzed by HPLC for phenanthrene.

Wetting and drying after aging

Samples (5, 10, or 10 g) of Mount Pleasant silt loam no.1 were amended with 11.0 or 101 mg of phenanthrene or 101mg of DEHP, respectively. Phenanthrene was aged for 130 din the 10-g sample, but some of the samples were dried at 118d and stored for an additional 10 d. A second set of replicates

2380 Environ. Toxicol. Chem. 17, 1998 J.C. White et al.

Table 1. Effect of aging and wetting-and-drying cycles on the extentof mineralization of phenanthrene and di(2-ethylhexyl)phthalate

(DEHP) in three soils

Soil CompoundAging

time (d)No. of wet-dry cycles

Extent ofmineral-

ization(%)a

Chenango Phenanthrene 07

28

NAb

0104

33.0Cc

35.1C30.6B35.2C24.8A

Middlebury Phenanthrene 058

NA09

36.1C22.1B11.7A

Mount Pleasant(no. 1)

DEHP 041

NA05

63.7B49.8A61.9B

a Values at 38, 50, or 48 d in Chenango, Middlebury, or Mount Pleas-ant soil, respectively.

b NA 5 not applicable.c Within each soil, values followed by the same letter are not signif-

icantly different (p , 0.05).

Table 2. Effect of aging and wetting-and-drying cycles onphenanthrene mineralization in Mount Pleasant silt loam no. 1

Aging time(d)

No.wet-dry cycles

Extent of mineralizationat 46 d (%)

0100

151

NAa

010

016

40.8Bb

21.0A23.1A23.8A22.1A

a NA 5 not applicable.b Values followed by the same letter are not significantly different (p, 0.05).

was removed at 88 d and subjected to four wetting-and-dryingcycles during the remaining 42 d of aging (the rewettingsoccurring at 88, 102, 117, and 130 d). Phenanthrene was agedfor 192 d in the 5-g samples, but some of the samples weresubjected to three wetting-and-drying cycles with drying start-ing at 164 d and rewetting occurring at 171, 181, and 192 d.Di(2-ethylhexyl) phthalate was aged for 297 d, but some ofthe samples were dried at 264 d and then subjected to wetting-and-drying cycles (the samples being remoistened at 275, 286,and 297 d). The soils were inoculated with Pseudomonas strainR or the DEHP-degrading bacterium, and mineralization wasmeasured.

To determine how wetting and drying soil containing se-questered phenanthrene affected the extractability of the com-pound, 10-g samples of Mount Pleasant silt loam no. 2 and 5-gsamples of Mount Pleasant silt loam no. 1 were amended with11.0 mg of phenanthrene, which was aged for 620 or 170 d,respectively. At 596 and 146 d, some of the samples of siltloam no. 1 and 2 were subjected to three wetting-and-dryingcycles during the remaining 24 d of aging (rewet at 8, 16, and24 d). At 612 and 166 d, a second set of samples of each soilwas subjected to one wet-and-dry cycle during the remaining8 d of aging. Soil with unaged substrate was prepared, and allsamples were extracted with 25 ml of ethanol:water (45:55).The added substrate was quantitatively recovered by sequentialextraction with n-butanol and Soxhlet extraction with hexanes.

RESULTS

Wetting and drying during short-term aging

The extents of mineralization of phenanthrene aged for pe-riods up to 58 d are given in Table 1. Aging for 7 or 28 d hadno effect on the extent of mineralization of phenanthrene inChenango gravelly loam, but one or four wetting-and-dryingcycles during the aging period significantly reduced the extentof mineralization compared with the compound aged in soilat constant moisture. In Middlebury silt loam, the extent ofmineralization of phenanthrene aged for 58 d was significantlyless than that of the unaged compound. Subjecting the soil tonine wet-and-dry cycles during the aging period further re-duced the availability of sequestered phenanthrene to bacteria.Aging DEHP for 41 d in Mount Pleasant soil significantly

reduced the extent of its mineralization, but more of the agedcompound was mineralized in soil subjected to five wet-and-dry cycles during aging than in soil maintained at constantmoisture during aging.

The effect of wetting-and-drying cycles on the extracta-bility of aged phenanthrene (1 mg/g) or DEHP (2 mg/g) inMount Pleasant silt loam was tested. The recoveries by ethanol:water (45:55) of unaged phenanthrene and phenanthrene thatwas aged for 44 d with 5 wetting-and-drying cycles (rewet at5, 12, 21, 31, and 41 d) or with no drying were 34.0 (60.3),25.6 (60.7), and 28.2% (61.0), respectively. These values aresignificantly different at p , 0.05. The recoveries by a mildextractant (n-butanol) of unaged DEHP and DEHP that wasaged for 41 d in soil that was wet and dried five times or insoil not subjected to drying were 87.8 (61.9), 64.5 (64.1),and 79.7 (64.8)%, respectively. These values are significantlydifferent at p , 0.05.

Wetting and drying during long-term aging

The extents of mineralization of phenanthrene that was agedfor 0, 100, or 151 d in Mount Pleasant silt loam no. 1 thatwas wet and dried 10 to 16 times or kept at constant moistureduring aging are given in Table 2. Less phenanthrene aged for100 or 151 d than unaged chemical was mineralized, but wet-ting and drying had no effect on the extent of mineralization.

An experiment was conducted to test the effect of six wet-and-dry cycles during 54 or 102 d of aging on the availabilityof phenanthrene to bacteria and earthworms. Aging for 54 or102 d significantly reduced the extent of mineralization andincreased the concentration of phenanthrene remaining afterbiodegradation (Table 3). The extents of mineralization of phe-nanthrene aged for 54 or 102 d were the same in soil at constantmoisture or after wetting-and-drying cycles. However, moreof the compound remained after biodegradation of phenan-threne aged for 54 d in soil that was wet and dried six timesduring aging than in samples incubated at constant moistureduring aging, but no such effect was observed with phenan-threne aged for 102 d.

The percentages of phenanthrene assimilated by earth-worms are also given in Table 3. The values represent thepercentage of compound added to soil that was found in wormtissue. Significantly less phenanthrene aged for 54 or 102 dthan unaged chemical was assimilated (p , 0.05). The uptakeof phenanthrene by earthworms from soil that was wet anddried six times during 54 d of aging was significantly less thanthat from soil in which the chemical was aged at constantmoisture, but no such effect was evident if the compound wasaged for 102 d.

Wetting and drying effects Environ. Toxicol. Chem. 17, 1998 2381

Table 3. Effect of aging and wetting-and-drying cycles on microbialdegradation and uptake by earthworms of phenanthrene in Lima

loam

Agingtime(d)

No.wet-drycycles

Extent ofmineralization

(%)a% Remaining

after biodegradationEarthwormuptake (%)

054

0102

NAb

06

NA06

33.9Bc

14.8A12.4A20.6B10.0A10.9A

53.3A65.0B78.3C63.6A82.7B83.6B

1.9C1.2B0.73A3.0B1.9A1.8A

a Extent determined after 38 or 32 d of biodegradation of phenanthreneaged for 54 or 102 d, respectively.

b NA 5 not applicable.c Within each column and in comparisons of 0 versus 54 and 0 versus

102 d, values followed by the same letter are not significantly dif-ferent (p , 0.05).

Table 4. Effect of aging and wetting-and-drying cycles on the biodegradation of phenanthrene and di(2-ethylhexyl)phthalate (DEHP) in Mount Pleasant silt loam

CompoundAging time

(d)

No.wet-drycycles

Extent ofmineralization

(%)a% Remaining

after biodegradation

Phenanthrene 0130

0

NAb

014

NA

35.7Cc

25.4A32.5B28.2A44.8C

16.2A41.4D30.5B35.2C

2.5A

DEHP

192

0297

03

NA03

18.9A22.2B57.2B31.9A37.8A

30.9C25.5BNDd

NDND

a Extent determined after 85, 51, or 54 d of biodegradation of phenanthrene aged for 130, 192, or 277d, respectively.

b NA 5 not applicable.c Within each column and in comparisons of 0 versus 129, 0 versus 192, and 0 versus 293 d, values

followed by the same letter are not significantly different (p , 0.05).d ND 5 not determined.

Wetting and drying after aging

A study was conducted to determine the effect of wettingand drying soil containing previously sequestered phenan-threne or DEHP on the biodegradability of the chemicals. Theextent of mineralization of phenanthrene aged for 130 or 192d was significantly less (p , 0.05) than that of unaged sub-strate, and more of the aged compound remained after bio-degradation (Table 4). Subjecting soil containing previouslyaged phenanthrene to wetting-and-drying cycles at the end ofaging increased the mineralization of sequestered substrate(statistically significant at p , 0.05 for the one and threecycles). Similarly, the one, three, or four wet-and-dry cyclesafter aging resulted in less of the sequestered phenanthrenebeing present in the soil after biodegradation than in soil in-cubated at constant moisture.

The extents of mineralization of unaged DEHP and DEHPthat was aged for 297 d with or without three wet-and-drycycles at the end of the aging period are also given in Table4. The mineralization of aged DEHP was significantly lessthan that of unaged substrate, but cycles of wetting and dryinghad no effect on the mineralization of sequestered DEHP.

Tests were conducted of the effect of wetting and drying

samples of Mount Pleasant silt loam on the extractability ofsequestered phenanthrene. The recoveries by the mild extrac-tant of phenanthrene aged for 174 or 620 d in both samplesof Mount Pleasant silt loam were significantly less than thatof the unaged compound (Table 5). Subjecting the soil samplesto three wet-and-dry cycles had no effect on recovery by themild extractant, but one wetting-and-drying cycle at the endof aging significantly increased the recovery of sequesteredphenanthrene by ethanol:water compared with soil incubatedat constant moisture. All of the compound was recovered whenthe soils were extracted with the vigorous extractant.

DISCUSSION

The data show that the bioavailability of phenanthrene andDEHP sequestered in soil may be altered by subjecting thesamples to wetting-and-drying cycles during or after aging.Because the recovery of test compounds by vigorous extractionmethods was unaffected by aging or wetting and drying, theuse of such extraction procedures to predict exposure and riskis inappropriate for compounds that become sequestered.

Subjecting soil to wetting-and-drying cycles during agingperiods of ,60 d significantly decreased the biodegradability,extractability, and uptake by earthworms of sequestered phe-nanthrene and extractability of sequestered DEHP comparedwith compounds aged at constant moisture. However, wetting-and-drying cycles during short-term aging increased the min-eralization of sequestered DEHP in soil, and during longerperiods of aging, wetting and drying did not affect the avail-ability of either compound. Subjecting soil containing previ-ously sequestered compounds to wetting-and-drying cycles en-hanced the biodegradability and extractability of phenanthrenebut had no effect on DEHP mineralization. These findingsagree with observations that sorption and toxicity of pesticidesin soil are affected by moisture content [11–13].

The reasons for the changes associated with the additionsand losses of water from soil are uncertain. It has been sug-gested that soil organic matter becomes fragmented and moreporous and the mesh structure disintegrates upon drying. Uponrewetting, the organic matter swells and partially regains itscohesiveness [14]. It has also been observed that periodic wet-ting and drying causes a shrinkage and swelling of the soilstructure and the formation of aggregates, which result in a

2382 Environ. Toxicol. Chem. 17, 1998 J.C. White et al.

Table 5. Effect of aging and wetting and drying cycles on the recoveryof phenanthrene from soil by mild and rigorous extraction

Soil

Agingtime(d)

No.wet–drycycles

% Recovered

Ethanol:water Vigorous

Mount Pleasant no. 1 0174

NAa

013

35.5Cb

24.0A26.7B24.1A

111.598.699.799.3

Mount Pleasant no. 2 0620

NA013

48.2C35.1A39.9B35.8A

100.6102.3103.5101.8

a NA 5 not applicable.b Within each soil type, values followed by the same letter are not

significantly different (p , 0.05).

decrease in surface area [15]. Drying soil may also result inpore shrinkage and distortion as water menisci between porewalls exerted increasing tension as the water evaporates [16].

The effects of wetting and drying on the behavior of organiccompounds in soil remain largely unexplored. Bailey andWhite [11] observed increased sorption of methyl bromide,chloropicrin, and 1,2-dibromoethane to dry soil, and they con-cluded that water molecules outcompete the organic moleculesfor adsorption sites on soil minerals at high moisture levelsbut more of the organic molecules sorb to the solid phase atlow moisture levels. Greater sorption of pesticides to dry thanmoist soil has been observed frequently [17–19].

Wetting and drying soil enhanced phenanthrene sequestra-tion during short periods of time. These findings agree withthose of Shelton and coworkers [12,13], who observed thatcycles of wetting and drying increased the sorption to soil ofatrazine and carbofuran. However, the reason for the lack ofan effect of wetting and drying on biodegradability duringlong-term aging is uncertain. It is possible that wetting-and-drying cycles may enhance the diffusion of phenanthrene intothe soil matrix as a result of a distortion of the pore structure.Shelton and Parkin [12] suggested that carbofuran partitionedinto less accessible binding sites in the soil that were exposedupon drying. The extractability of DEHP also declined withwetting-and-drying cycles, but the mineralization of seques-tered DEHP was greater in soil that was wet and dried thanin soil at constant moisture. Because the octanol/water partitioncoefficient of DEHP is several orders of magnitude greaterthan that of phenanthrene, the phthalate may sorb to organicmatter by a different mechanism than phenanthrene, and thus,changes in soil structure caused by drying and rewetting mayhave a different effect on DEHP availability.

In soil containing previously sequestered phenanthrene,wetting and drying cycles after the aging period enhancedsubstrate availability. This may result from the sequesteredmolecules occupying remote sites within the soil, possiblyassociated with humic materials. The shrinkage and expansionof that organic matter induced by drying and rewetting maylead to a diffusion of the substrate out of these inaccessibleregions. In contrast, the unaged substrate may be concentratedon outer sites of the organic matter, and the alterations in soilstructure caused by drying and rewetting may facilitate move-ment of the phenanthrene into remote regions in the soil andthus enhance sequestration. Ma and Uren [20] observed de-

creased amounts of extractable zinc in soil that was wet anddried, and they suggested that drying and rewetting enhancedthe diffusion of zinc cations into soil micropores where itbecomes less available.

The results show that the bioavailability of two organicchemicals in soil can be altered significantly by simple wettingand drying. Although sufficient study has not yet been madeto determine the extent of change in sequestration as a resultof the wetting and drying or even whether it will consistentlyincrease or decrease sequestration, the simplicity and low costof the procedure suggests that this technique is worthy offurther investigation for the remediation of contaminated soils.

Acknowledgement—Support for this research was provided by theNational Institute of Environmental Health Sciences, training grantES07052, and the Air Force Office of Scientific Research grantF49620-95-1-0336.

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