EFFECT OF SOIL CONTAMINANT EXTRACTION METHOD IN DETERMINING TOXICITY USING THE MICROTOX ASSAY

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1 Environmentl Toxicology nd Chemistry, Vol. 19, No. 2, pp , SETAC Printed in the USA /00 $ EFFECT OF SOIL CONTAMINANT EXTRACTION METHOD IN DETERMINING TOXICITY USING THE MICROTOX ASSAY GAIL A. HARKEY* nd THOMAS M. YOUNG Institute of Gs Technology, 1700 South Mount Prospect Rod, Des Plines, Illinois 60018, USA Deprtment of Civil nd Environmentl Engineering, University of Cliforni, Dvis, Cliforni 9616, USA (Received 1 December 1998; Accepted 18 My 1999) Abstrct This project exmined the influence of different extrction methods on the mesured toxicity of contminted soils collected from mnufctured gs plnt (MGP) sites differing in soil composition nd contminnt concentrtion. Aged soils from number of MGP sites were extrcted using sline solution, supercriticl fluid extrction (SFE), nd Soxhlet extrction. Toxicity ws ssessed using two forms of Microtox tests: cute queous tests on sline nd SFE soil extrcts nd solid-phse tests (SPTs) on soil prticles. Microtox SPTs were performed on soils before nd fter SFE to determine resulting toxicity reduction. Three hypotheses were tested: (1) Toxicity of soil extrcts is relted to contminnt concentrtions of the extrcts, (2) mesured toxicity significntly decreses with less vigorous methods of extrction, nd () supercriticl fluid extrctbility correltes with mesured toxicity. The EC0s for SPTs performed before nd fter SFE were not different for some soils but were significntly greter fter extrction for other soils tested. The most significnt toxicity reductions were observed for soils exhibiting the highest toxicity in both preextrction SPTs nd cute queous tests. tests performed on SFE extrcts showed significntly lower EC0s thn those reported from sline-bsed extrction procedures. Toxicity of the soils mesured by Microtox SPTs ws strongly correlted with both SFE efficiency nd mesures of contminnt ging. Dt from this project provide evidence of sequestrtion nd reduced vilbility of polycyclic romtic hydrocrbons (PAHs) from soils extrcted vi physiologiclly bsed procedures compred to vigorous physicl extrction protocols. Keywords Extrction Microtox Soil EC0 Toxicity Polycyclic romtic hydrocrbons INTRODUCTION Numerous studies conducted on soils contminted with orgnic compounds hve shown tht s the contminnts persist in soil, they become incresingly resistnt to desorption nd biologicl uptke [1 ]. Sorption of orgnic contminnts to soils often entils n initilly rpid nd reversible process followed by period of slow sorption over weeks, months, or yers. This slow sorption leds to chemicl frction tht then resists desorption []. The result of this process in ged soils is substntil reduction in the rte of desorption, which correltes with net reduction in biovilbility of the contminnt [2,6,7]. Conventionl protocols use vigorous methods, such s Soxhlet or soniction extrctions, to remove the soluble orgnic contminnts from soil prticles. These methods my remove both biovilble contminnts nd contminnts normlly resistnt to desorption in biologicl or environmentl systems. Hydrocrbon-contminted soils collected from mnufctured gs plnt (MGP) sites re good mterils for studying contminnt desorption nd biovilbility. The MGPs commonly operted in the United Sttes from the mid-1800s to the lte 190s or erly 190s [8]. Contminnts ssocited with MGP sites re typiclly orgnic compounds, including polynucler romtic hydrocrbons (PAHs) tht hve been in contct with soil prticles for decdes. Risk ssessments nd determintion of clenup levels of contminnts t MGP sites re generlly bsed on chemicl concentrtions mesured by exhustive extrction (e.g., Soxhlet extrction) of soil smples collected from the site. However, chemicl dt lone pper * To whom correspondence my be ddressed (gnnhrkey@ol.com). to provide little direct indiction of the potentil risks nd effects of site contminnts. Consequently, concentrtions of totl contminnts determined by these extrctions cn overestimte the mount of contminnt vilble to biot in the environment [2,6]. Evidence of decresed contminnt biovilbility with soil ging hs creted the need for simple nd cost-effective screening ssys to estimte biovilbility of soil contminnts. Tests tht determine the toxicity of historiclly contminted soils re bsed on the vilbility of the contminnts to be tken up by indictor species, such s bcteri, invertebrtes, nd vertebrtes. If the contminnt is vilble, uptke will occur to point where it my produce n effect. However, if portion of the contminnt is sequestered, very little of this frction will likely be vilble for uptke by the orgnisms. Toxicity tests designed to rpidly screen environmentl medi typiclly involve extrction into n queous solution prior to orgnism exposure. The extrction protocol employed nd the type of solvent used will likely determine the extent of contminnt extrction nd subsequent biovilbility to the indictor species. In this study, the reltionship between mesured cute toxicity of ged contminnts ssocited with MGP site soils nd the concentrtion nd extrctbility of the chemicls ws exmined. The study objective ws to use the Microtox test with both solid-phse nd queous extrction protocols to determine toxicity vritions resulting from vigorous nd mild extrction methods. Three hypotheses were tested: (1) toxicity of soil extrcts is relted to contminnt concentrtions of the extrcts, (2) mesured toxicity significntly decreses with less vigorous methods of extrction, nd () supercriticl fluid extrctbility correltes with mesured toxicity. Such corre- 276

2 Soil extrction methods nd Microtox toxicity Environ. Toxicol. Chem. 19, Tble 1. Chrcteristics of mnufctured gs plnt soils used in the study SBO 28 SoCl % Snd % Silt % Cly Soil type Lomy snd Sndy lom Snd Sndy lom Lomy snd % Totl orgnic crbon ph ltion between the cute toxicity of soil extrct using prticulr extrction method nd soil contminnt concentrtion would be vluble in determining cceptble endpoints for site remedition or mesures for risk ssessment of contminnts in ged soils. MATERIALS AND METHODS Soil collection Five rchived soils collected from vrious MGP sites nd stored in the drk in glss continers t to 10 C were used. The soils were originlly collected for contminnt tretbility studies in 1996 nd 1997 nd were chrcterized for ph, prticle size distribution, totl orgnic crbon, nd PAH concentrtions within 2 months of collection. The properties of the soils re presented in Tble 1. Soil ph ws mesured in 1: 1 (w/w) soil wter suspension, the prticle-size distribution ws by hydrometer method [9], nd orgnic crbon ws determined by combustion t 1,000 C. Soil ws collected from the bottom of gs holder in erly Soil SBO28 ws tken from soil boring in 1997 where MGP residuls hd been lndfilled into former creek chnnel. The nd SoCl were lmpblck-bsed soils collected from pproximtely the sme re in 1996 nd 1997, respectively. The soil ws collected in Additionl informtion regrding these soils nd ssocited tretbility studies re included in project reports [10,11]. Soil extrction Supercriticl crbon dioxide (crbon dioxide bove its criticl temperture nd pressure of 1.1 C nd 72.8 tm, respectively) ws used to extrct PAHs from ech soil smple. This pproch ws selected becuse it fetures high mss trnsfer rtes nd tunble solvent strength, chrcteristics tht offer promise for developing rpid chemiclly bsed biovilbility ssys. Supercriticl fluid extrctions followed the procedures of the U.S. Environmentl Protection Agency Method 61 with two importnt exceptions. First, dimethyl sulfoxide (DMSO) ws used s trp rinse solvent for comptibility with subsequent Microtox testing [12]. The second modifiction ws tht soil prticles were not ground prior to extrction. This modifiction ws intended to prevent extrction of contminnts from loctions deeper within the soil mtrix, where they re less likely to be biovilble. Method 61 is designed to recover two frctions of extrcts. The more voltile PAHs re extrcted nd recovered in the first frction (frction A), wheres the lesser voltile PAHs re removed in the second frction (frction B). A finl trp rinse (frction C) ws dded to the method to ensure tht the DMSO recovered ll the nlyte from the trp. Two to four grms (dry weight) soil smples were extrcted using Suprex Flexprep supercriticl fluid extrctor with n Accutrp smple collector nd MP-1 modifier pump (Isco, Lincoln, NE, USA). A finl volume of pprox. 0.8 ml smple in DMSO ws recovered for ech frction. Actul smple volumes for clculting PAH concentrtions nd EC0 equivlents were determined from solvent weights in the individul smple vils. Ech extrction ws conducted in duplicte. In ddition to SFE, ech soil ws Soxhlet extrcted for 16 h nd nlyzed vi gs chromtogrphy mss spectrometry (GC-MS) using U.S. Environmentl Protection Agency SW-86 Method SW0 nd Method SW 8270C [12]. Supercriticl fluid extrct nlysis All SFE extrcts were nlyzed without ny dditionl smple preprtion, except for the ddition of internl stndrds (2- fluorobiphenyl nd d 1 p-terphenyl) prior to GC-MS selected ion monitoring (MS-SIM). Extrcts were nlyzed for 1 individul PAH compounds nd one compound group: nphthlene, cenphthylene, cenphthene, fluorene, phennthrene, nthrcene, fluornthene, pyrene, benzo[]nthrcene, chrysene/ triphenylene, benzo[b]fluornthene, benzo[k]fluornthene, benzo[]pyrene, indeno[1,2,-cd]pyrene, dibenz[,h]nthrcene, nd benzo[g,h,i]perylene. Quntifiction of individul PAHs ws performed using Hewlett-Pckrd 890 II gs chromtogrph/971 mss selective detector (Hewlett-Pckrd, Avondle, PA, USA). The GC nlysis employed 0-m 0.2- mm-i.d. Restek XTI- column (Restek, Bellefonte, PA, USA) with n oven temperture profile of 0 C for min, 0 to 00 C t 10 C/min min, nd isotherml t 00 C for 20 min. Recovery of surrogte stndrds verged 97.78% 7.0 stndrd devitions (SD). Extrcts from frctions A, B, nd C were nlyzed individully. Totl SFE-derived PAH concentrtions were clculted s the sum of these frctions for ech soil smple tested. Microtox testing Both Microtox solid-phse tests (SPTs) nd Microtox cute toxicity bsic tests (cute) were used in this study nd followed the mnufcturer s protocols [1,1]. Microtox is commercil toxicity biossy bsed on the reduction in bioluminescence of the mrine bcterium Vibrio fischerii following exposure to toxicnts [1]. In the cute bsic test, soil contminnts re extrcted into n queous medium prior to testing. Alterntively, the SPT ws developed s stndrd procedure to test complex solid smples tht llows soil prticles to come into direct contct with bcteri for fixed period of time. To test the hypothesis tht mesured toxicity decreses with less vigorous extrction methods, Microtox SPTs were run in triplicte on soils before nd fter SFE. The SPTs were performed by combining soil smples with Microtox solid-phse diluent in 1: (w/v) rtio nd mixing for 10 min. For ech soil smple, 11 seril 1:1 dilutions, in duplicte, were prepred nd the bcteri exposed to these dilutions for 20 min. Microorgnisms were then seprted from the soil suspensions with filter column for nlysis using the Microtox nlyzer (PBR Lbortories, Edmonton, AB, Cnd). To test the hypothesis tht toxicity of soil extrcts is relted to their contminnt concentrtions, cute Microtox tests were performed in triplicte on soil extrcts. Soil extrcts were prepred following sline-bsed extrction procedures outlined in the Americn Society for Testing nd Mterils Designtion D [16]. Briefly, 2gdrysoil were combined with 20 ml Microtox diluent (2% NCl solution) in 0-ml Ok Ridge Teflon tubes (Nlgene, Rochester, NY, USA). The mixture ws mixed on n orbitl rottor t 200 rpm for 16 h. The

3 278 Environ. Toxicol. Chem. 19, 2000 G.A. Hrkey nd T.M. Young Tble 2. Comprison of polycylic romtic hydrocrbon (PAH) concentrtions in soils extrcted vi supercriticl fluid extrction (SFE) nd Soxhlet t the time of collection nd t the time of the present study Soxhlet method (t collection) SFE method Soxhlet method (present) b Two- or threering PAHS Four- to sixring PAHs c Two- or threering PAHS Four- to sixring PAHs b Two- or threering PAHS Four- to sixring PAHs SBO-28 SoCl 21, ,96 1,8 19, , , 1,2 2, , , , ,1 2,09 1, , ,892 Vlues for SFEs represent verges of two replicte smples. b re reported s the sum of 16 compounds extrcted vi Soxhlet nd nlyzed vi GC-MS. c re reported s the sum of 16 compounds extrcted vi SFE nd nlyzed vi GC-MS. smples were centrifuged t 2000 g for 20 min t C nd the superntnt decnted nd subjected to cute testing. Acute tests were lso performed on frctions A, B, nd C fter SFE. Smple preprtion for frctions A, B, nd C ws s follows: 10 l of the DMSO extrct ws dded to 10 ml of Microtox diluent to mke up 1.% (v/v) DMSO dilution. Seril 1:1 dilutions were prepred from this initil dilution. Reconstituted cells of V. fischerii were exposed in duplicte to eight different dilutions of DMSO extrct. A Microtox control blnk of 1.% pure DMSO to diluent ws prepred to offset ny effect from the DMSO nd to set the light mesurement of the Microtox nlyzer. The resulting decrese in bioluminescence ws mesured fter 1 min t constnt temperture of 1 C. A zinc sulfte reference toxicnt ws run s dily internl qulity control procedure whenever cute tests were performed. All results from the zinc sulfte toxicnt were within 2 SD. Dt nlysis All Microtox test dt were expressed in EC0s, defined s the effective concentrtion of test smple tht cuses 0% decrese in light output [17,18]; the lower the EC0, the greter the toxicity. To compre toxicity mong test methods, ll smple concentrtions were reported in units of milligrms dry-weight soil per milliliter of extrction diluent. This represents the mss of dry soil tht, if extrcted into 1 ml of queous solvent, would result in 0% reduction in bioluminescence. The EC0 vlues were log trnsformed nd compred with PAH concentrtions using Person product moment correltion coefficients (r) [19]. Comprisons between EC0 vlues generted with the sme soils nd extrction methods were determined vi Student s t tests using the Microsoft Excel Anlyticl ToolPk softwre pckge (Microsoft, Redmond, WA, USA). RESULTS Soxhlet extrction nd subsequent GC MS SIM nlyses of the five soils used in this study reveled totl PAH concentrtions rnging from 209 to,76 mg/kg, with four- to six-ring compounds mking up the lrgest frction of PAHs in ll smples except (Tble 2). The PAH concentrtions declined significntly for smples SBO-28 nd during storge t to 10 C in the drk, especilly for two- or threering PAHs, suggesting tht either biologicl degrdtion or voltile losses occurred from these smples. Extrction efficiencies with SFE rnged from 27.7 to.% of the present Soxhlet-derived vlues. Efficiencies were generlly below 0% throughout the rnge of the 16 PAHs nlyzed. Although the method of SFE extrction ws intended to isolte the lowermoleculr-weight compounds in frction A nd the higherweight compounds in frction B, this ws not the cse. In ll soils, most of the two- or three-ring nd four- to six-ring PAHs were found in frction B. Dt from Microtox cute tests run on SFE frctions showed tht toxicity of the soil extrcts ws not directly relted to contminnt concentrtions of the extrcts (Tble ). Although toxicity ws generlly gretest in the frction tht contined the gretest concentrtion of contminnts (frction B), liner regression between log EC0s obtined from the three SFE frctions nd their totl PAH concentrtions resulted in wek reltionship (r 2 0.1; n 1). Microtox cute tests on queous extrcts from the soils (tests following the Americn Society for Testing nd Mterils [ASTM] method of Tble. Comprison of cute Microtox EC0 vlues prior to nd fter supercriticl fluid extrction (SFE) with totl polycyclic romtic hydrocrbon (PAH) concentrtions in frctions A, B, nd C queous extrct EC0 b SFE frction A EC0 frction A c SFE frction B EC0 frction B SFE frction C EC0 frction C SBO-28 SoCl.7 (0.01) 18. (.61) 0.9 (0.79) 2.9 (1.1) 20.2 (18.68) 6.81 (0.02).2 (2.91) (0.02) 1. (1.9) 10.2 (2.11) 1.79 (0.27) 0.6 (0.) 2, (1.98) 2. (0.0) 7.2 (9.6) 2.1 (0.1) EC0 vlues re men 1 SD of duplicte or triplicte smples. b Microtox protocol followed ASTM designtion D EC0s re in mg/ml extrctnt. c re reported s the sum of 16 compounds extrcted vi SFE nd nlyzed vi GC-MS, n 2. d EC0 ws greter thn the highest dilution tested.

4 Soil extrction methods nd Microtox toxicity Environ. Toxicol. Chem. 19, Tble. Comprison of EC0 vlues obtined with Microtox solidphse tests (SPT) before nd fter supercriticl fluid extrction (SFE) SBO-28 SoCl Microtox EC0 SPT before SFE (mg/ml extrctnt) 0.06 (0.0) 0.7 (0.10) 1.9 (.2) 22.2 (0.7) (.9) EC0 vlues re men 1 SD of triplicte smples. Microtox EC0 SPT fter SFE (mg/ml extrctnt) 2. (0.72) 9.0 (1.90) 1.96 (19.12) 2.97 (.87).7 (8.20) extrction) showed substntil reduction in toxicity compred to SFE frctions A nd B (t test, p 0.0; Tble ). For three of the soils (, SoCl, nd ), cute EC0s obtined vi the ASTM extrction method were greter thn the highest dilution tested. The EC0 vlues compred between pre- nd post-sfe for Microtox SPTs were not sttisticlly different for three of the smples tested:, SoCl, nd (t test, p 0.0; Tble ). This indictes tht soil toxicity did not chnge significntly fter vigorous extrction with SFE, even with postextrction PAH concentrtions up to 712 mg/kg (Tbles 2 nd ). The other two soils showed sttisticlly significnt decrese in Microtox SPT toxicity fter SFE (p 0.02 nd 0.0 for nd SBO28, respectively). Solid-phse test EC0s showed 0- nd 19-fold reductions in toxicity for SBO- 28 nd, respectively. These smples lso demonstrted the highest cute EC0s with the ASTM extrction method, indicting tht some PAHs were vilble for desorption with mild extrction procedures. To determine ny reltionships between EC0 vlues from the Microtox tests run on soils or soil extrcts nd concentrtions of individul PAH compounds, correltion nlyses were completed. These nlyses compred the logrithm of the EC0 vlues in milligrms dry soil per milliliter extrctnt with PAH compound concentrtions obtined from SFEs of the combined nd individul frctions A, B, nd C. Results from the Microtox cute method using sline extrcts (ASTM method) could not be similrly nlyzed becuse of low toxicity of the smples. When log EC0s were compred with totl PAH concentrtions, correltion coefficients (r) greter thn 0.7 (p 0.01) were seen with ll toxicity test methods except Microtox cute tests on frction C. Greter r vlues were obtined for some compounds when individul PAH compounds were compred with EC0s (Tble ). Correltion ( 70%) between log EC0s nd three-ring PAHs ws seen for ll tests except for frction C. Frction B log EC0s showed the best correltion with individul PAH concentrtions for the three- to five-ring compounds, with r significnt t the 9% or greter level for 11 of the PAHs exmined. However, correltion between log EC0 vlues nd concentrtions of the higher-moleculr-weight compounds ws generlly not sttisticlly significnt (p 0.0) with the exception of benzo[]pyrene (Tble ). DISCUSSION In this study, we showed tht the toxicity of soil smples, mesured either by Microtox cute tests on queous extrcts or by direct solid-phse testing, ws not well correlted with totl contminnt concentrtions in the soils. The correltion coefficient between totl PAH concentrtion mesured by Soxhlet extrction nd the log EC0 from the initil SPTs ws 0.6 (p 0.2). Only two queous extrcts, SBO-28 nd, exhibited significnt toxicity, nd these smples hd the highest nd the second-lowest totl PAH concentrtions, respectively. Previous studies of contminted soils nd sediments hve lso shown no consistent correltion between totl chemicl concentrtions nd Microtox toxicity [20 22] or with toxicity determined with fish nd invertebrte ssys[2,2]. Vigorous physicl nd chemicl methods of contminnt extrction pper to hve the bility to remove contminnts from loctions or binding sites tht would not normlly be ccessed Tble. Correltion coefficients (r) between mesured individul polycyclic romtic hydrocrbon (PAH) compound concentrtions nd log EC0 toxicity prmeters of the mnufctured gs plnt site soils studied PAH compound No. rings in compound Microtox SPT EC0, pre-sfe b Microtox SPT EC0, post-sfe Microtox Acute EC0, SFE frction A c Microtox Acute EC0, SFE frction B Microtox Acute EC0, SFE frction C Nphthlene Acenphthylene Acenphthene Fluorene Phennthrene Anthrcene Fluornthene Pyrene Benzo[]nthrcene Chrysene Triphenylene Benzo[b]fluornthene Benzo[k]fluornthene Benzo[]pyrene Indeno[1,2,-cd]pyrene Dibenz[,h]nthrcene Benzo[g,h,i]perylene d 0.8 d d d d d d d d d d 0.88 d d d 0.81 d d d d d d d 0.88 d 0.80 d d d d d 0.78 d 0.69 d d d d 0.76 d d d 0.77 d 0.7 d 0.78 d 0.81 d 0.80 d d 0.7 d d 0.9 d Vlues re clculted from three replicte smples in ech ctegory. b EC0 vlues prior to supercriticl fluid extrction. c EC0 vlues fter supercriticl fluid extrction. d Sttisticlly significnt t the 9% or greter level [].

5 280 Environ. Toxicol. Chem. 19, 2000 G.A. Hrkey nd T.M. Young under nturl conditions. Extrction procedures nd chemicl nlyses tht rely on more nturl methods (i.e., physiologiclly bsed sline, fluids mimicking gstrointestinl fluids) hve been suggested s more ccurte mens of estimting risk to humn helth nd the environment []. Comprisons between chemicl nlyses nd soil toxicity biossys cn be complex. Soils from typicl MGP sites cn be contminted with hundreds of individul compounds in ddition to PAHs. Further, contminnts my exist in concentrtions below instrument detection limits but my still elicit toxicity. Interctions mong chemicl compounds my ffect toxicity tests in synergetic or ntgonistic wys. For exmple, metbolic biodegrdtion of lipophilic contminnts generlly produces reduction of toxicity. However, oxidtion rections mong chemicls my cuse inhibition or induction of enzymes tht my further lter toxic effects of contminnt mixtures [2]. Of the detectble contminnts for which nlyses re routinely conducted, PAHs were the clss of compounds found to be most significnt in the MGP site soils used in this study. Toxicity seen in these soils ws ssumed to be due to the PAHs contined in them but might be relted to other constituents. Despite the uncertinty of contminnt species or concentrtions contined in the soils used in this study, the wek correltion between totl contminnt concentrtions nd toxicity suggests underlying differences in biovilbility. Soil ging, prticle size distribution, ph, nd mount nd type of orgnic crbon re fctors known to be responsible for differentil biovilbility mong soils nd sediments []. No trends between toxicity nd ny of these fctors were redily pprent for the soils exmined in this study. The persistence of reltively voltile, soluble, nd biodegrdble PAH frctions in soils contminted with col tr over periods of decdes hs been ttributed to combintion of oxygen limittions nd chemicl sequestrtion within soil ggregtes [26]. Regrdless of the initil col tr composition, the ging process is expected to led to n enrichment of hevier PAHs (four- to six-ring) reltive to lighter two- or three-ring compounds [27]. This effect cn be qulittively exmined by compring the Soxhlet extrction results t the time the smples were collected ( ) to the results t the time of the toxicity ssys (1998). Smples nd SBO- 28 contined 7.9 nd 10.2 times s mny two- or three-ring PAHs s four- to six-ring PAHs t the time of collection, suggesting tht they my not hve been subjected to extensive environmentl trnsformtion (Tble 2). These smples exhibited 61 to 91% loss of two- or three-ring compounds during storge. This loss is presumbly due to voltiliztion nd, to lesser extent, to biodegrdtion stimulted by oxygention of the smples during collection. This supposition is consistent with tretbility studies on these smples tht indicted the presence of sufficient microbil popultions nd trce nutrients [10,11]. Similrly, Mdsen et l. [26] found tht removing oxygen limittions resulted in rpid degrdtion of nphthlene from soils contminted with col tr without ny nutrient or microbil mendments. Both the initil composition nd the storge losses in nd SBO-28 re in mrked contrst to those for the lmpblck-bsed,, nd SoCl smples. These smples initilly contined between.7 nd 7.7 times s mny four- to six-ring compounds s two- or three-ring compounds, nd these rtios were not significntly ltered during storge. To summrize these observtions in prmeter tht could be used in correltion nlysis, we chose to crete n vilbility rtio defined s PAH vilbility rtio X/Y originl soxhlet X/Ynew soxhlet where X (concentrtion of 2 nd ring PAHs) nd Y (concentrtion of to 6 ring PAHs). This rtio will increse for smples tht show the gretest loss in lighter PAH compounds during storge or environmentl modifiction. It is expressed s rtio of component concentrtions to ccount for ny vribility introduced by subsmple heterogeneity or interlbortory bises. For our MGP smples stored for between 1 nd 2 yers t to 10 C in the drk, this rtio rnges from 0.9 (for SoCl ) to 1 (for SBO-28). The vilbility rtio proves to be n excellent predictor of smple toxicity for these MGP soil smples. The correltion coefficient between the vilbility rtio nd the logrithm of the pre-sfe SPT ws 0.97 (p 0.01) nd with the post- SFE SPT ws 0.99 (p 0.01). Although only two queous extrcts gve mesurble toxicity, the vilbility rtio ws qulittively relted to those mesurements s well. The two smples with vilbility rtios bove 6 ( nd SBO-28) exhibited toxicity in the cute queous test, but the SoCl soil smples, ech with vilbility rtios below 1.2, were not toxic (EC0 100) t the lowest dilution. These results offer dditionl support for the hypothesis tht the vilbility of PAHs in contminted soils rther thn their totl concentrtion controls toxicity. Supercriticl fluid extrctions following modified U.S. EPA Method 61 protocol were not quntittive compred to Soxhlet extrctions for ny of the MGP smples studied. The primry reson for the filure to extrct ll PAHs probbly reltes to the lrger prticle sizes in our experiments compred to the crushed smples used in vlidting U.S. EPA method 61. Other resons my relte to differences in the composition, ge, nd mount of contminnts on the MGP soils tested. Method 61 ws vlidted on PAH-contminted certified reference mterils (two sediment smples nd one soil smple). These smples differed from those exmined here in two importnt respects. First, the reference mterils contined higher rtios of two-or three-ring PAHs to four- to six-ring PAHs thn ny of the other MGP smples except. The method vlidtion results lso revel tht efficiencies re lower for the higher-moleculr-weight PAH compounds. Second, none of the smples in the vlidtion testing ws col tr bsed. Previous studies hve shown tht fr higher quntities of supercriticl crbon dioxide (SC CO 2 ) re required to extrct col tr thn other forms of PAH contmintion. Only % of PAHs spiked onto se snd were recovered in -h extrction t condensed SFE flow rte of 60 ml/h [28]. Flow rtes of 00 to 800 ml/min for 0 to 90 min were required for quntittive removl of PAHs from MGP soil, nd the highestmoleculr-weight compounds were still not completely removed [29]. By comprison, Method 61 uses totl of 180 ml of SC CO 2 [12]. Differences in SFE efficiencies cross soils proved to be well correlted with smple toxicity nd the PAH vilbility rtio. The correltion coefficient between totl PAH removl efficiency nd the logrithm of the pre-sfe SPT ws 0.89 (p 0.0), nd the correltion coefficient with the vilbility rtio ws 0.97 (p 0.01). These results provide some preliminry indiction tht SFE on uncrushed smples my be useful (nd rpid) chemicl ssy for estimting the biovilble nd potentilly toxic frction of orgnic chemicls in soil. Chemicl ssys bsed on liquid solvents hve shown similr promise in previous reserch [2,6].

6 Soil extrction methods nd Microtox toxicity Environ. Toxicol. Chem. 19, Toxicity of soils extrcted ccording to ASTM procedures ws significntly less thn tht seen in the sme soils extrcted vi SFE. This confirms our hypothesis tht toxicity significntly decreses with less vigorous extrction methods. A detiled exmintion of the toxicity produced by vrious frctions of the extrctnt fter SFE showed tht toxicity ws relted to the mount of PAHs extrcted, lthough this ws not strong liner reltionship. Further testing on the combined frctions A, B, nd C from SFE would hve likely shown toxicity to be much greter thn tht extrcted following ASTM protocols. In ddition, the effect(s) tht SFE exhibited on the soil prticles is not known. The possibility exists tht receptor sites on soil prticles my hve been chnged during the extrction process. Such chnges my influence the movement of PAHs into the surrounding pore wter (the contminnt exposure route) in the SPT. The centrl question tht this study ws intended to ddress is how best to mesure the toxicity of soil smples using the Microtox method or similr biossy pproches. A previous study conducted with sediments reported EC0s from Microtox SPTs by percentge dry weight of the smple [0]. Smple concentrtions greter thn 2% were regrded s negtive response, concentrtions between 1 nd 2% were doubtful or suggestive of toxicity, nd those below 1% were likely to be toxic. More recently, SPT EC0 vlues hve been reported in milligrms equivlent soil or sediment weight per milliliter of extrct [1,2]. Johnson nd Long used n EC0 of.2 mg equivlent sediment extrct per millimeter s reference point for determining sediment toxicity. Smple EC0s significntly less thn this vlue were considered toxic. To normlize EC0 vlues for both cute tests nd SPTs in this study, we reported toxicity in units of milligrms equivlent soil weight per milliliter of extrct. If clculted by percentge dry weight of the smple, SPT EC0 results re ll 1% for both pre- nd post- SFE smples in,, nd SoCl soils. Using the criteri for toxicity suggested by Dy et l. [0], toxicity cn be considered significnt in soils SBO-28 nd, where EC0s for SPTs rnged from 0.0 to 1.0% dry smple weight. Using the criteri of Johnson nd Long [2], toxicity cn lso be considered significnt in SBO-28 nd soils, except for the post-sfe SPT for, for which the men EC0 ws reported s 9.0 (Tble ). The two types of Microtox tests used in this study employ different methods of orgnism exposure. Acute tests were performed on queous extrcts of soils, wheres SPTs llowed for the bcteri to come into direct contct with soil prticles during the test, incresing the probbility for the mesurement of responses to prticle-bound nd mrginlly soluble toxicnts. The method my lso increse the mss trnsfer rtes to the solution phse, s the bcteri my ct s PAH sink, thereby suppressing queous-phse PAH concentrtions nd stimulting desorption. Comprison of SPTs nd cute tests performed prior to SFE showed substntilly greter toxicity produced in the SPT (Tbles nd ). This indictes tht lrge portion of the PAHs present in the MGP site soils ws not prtitioned into the sline-bsed medi used in the cute test during the contct period. However, if combined with ctul soil prticles s performed in the SPT, PAH vilbility to bcteri incresed, resulting in incresed toxicity. To determine the pproprite type of Microtox test to employ in given sitution, one must know the potentil routes of exposure from which estimtes of risk will be importnt. This will differ for ech contminted site under considertion nd will thus depend on site-specific fctors. 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