Sorption of 1,2,4-trichlorobenzene and tetrachloroethene within an authigenic soil profile: Changes in Koc with soil depth

Journal of Contaminant Hydrology 29 Ž1998. 347–377

Sorption of 1,2,4-trichlorobenzene and tetrachloroethene within an authigenic soil profile: Changes in K oc with soil depth Bernard N. Kimani Njoroge a,1, William P. Ball Cherry b,2 a

a,)

, Robert S.

Dept. of CiÕil and EnÕironmental Engineering, Duke UniÕersity, Durham, NC, USA b Center for Biochemical Engineering, Duke UniÕersity, Durham, NC, USA Received 31 July 1995; revised 15 April 1997; accepted 15 April 1997

Abstract The sorption of 1,2,4-trichlorobenzene and tetrachloroethene was investigated in a series of well-controlled batch experiments, using authigenic soil materials from a profile extending to 2.5 m below ground surface. Batch experiment techniques were verified by study with both pulverized and unpulverized soil at different times of equilibration, using two widely different soil:water ratios, and at a wide range of aqueous concentration. Sorption isotherms were approximately linear, with sorption distribution coefficients Ž K d . found to decrease roughly 100-fold down the soil profile. K d decreased with depth to an extent greater than could be predicted on the basis of the only 10-fold decrease in natural solid organic matter ŽSOM. content and despite significantly higher specific surface area in the lower horizons. All base-extractable SOM in these deeper soil horizons was operationally defined as fulvic acid ŽFA., although there was also a significant fraction that was not extracted by the standard base technique. The lower K d of the deeper soil horizons is believed to reflect a complex combination of Ž1. lower SOM content; Ž2. a more hydrophilic form of SOM; and Ž3. a more intimate association of the SOM with the mineral fraction, affecting its accessibility, sorptivity, or both. For the deeper horizons, an increase in overall K d by more than 4-fold was observed on solids treated by either base extraction or H 2 O 2 treatment, demonstrating that sorption to remaining soil components could be dramatically increased by fractional SOM removal andror chemical alteration of the soil. A simple regression model that divides SOM into only two types Žshallow and deep SOM. provides a reasonably good

)

Corresponding author. Dept. of Geography and Environmental Engineering, John Hopkins University, Baltimore, MD. Fax: q1-410-516-8996. 1 Current address: Dept. of Civil Engineering, University of Nairobi, Nairobi, Kenya. 2 Current address: Idaho National Engineering Laboratory, Idaho Falls, ID. 0169-7722r98r$17.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 7 7 2 2 Ž 9 7 . 0 0 0 3 9 - 9

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explanation of sorption in all seven horizons and suggests an order-of-magnitude variability in K oc among surface soil and deeper horizons. q 1998 Elsevier Science B.V.

1. Introduction The association of pollutants with solid phases will affect both the transport and remediation of organic pollutants in subsurface environments and is therefore important to understand for purposes of risk assessment or remediation design. Such solid-phase association has been generically referred to as sorption, and may occur through processes of either adsorption to solid surfaces or absorption, including both absorption into immobile pore volumes or partitioning into organic phases ŽKarickhoff, 1984; Schwarzenbach et al., 1993.. For hydrophobic organic chemicals ŽHOCs., most work to date demonstrates that natural soil organic matter ŽSOM. plays a critically important role in determining the extent of sorption with natural soil solids, sediments, and aquifer materials ŽKarickhoff, 1984; Schwarzenbach et al., 1993.. Partitioning into SOM has been hypothesized as a dominant sorption mechanism for neutral HOCs, and a common approach for estimating a chemical’s sorption distribution coefficient has been to take the product of an organic-carbon normalized partition coefficient and the mass fraction of organic carbon in the soil ŽChiou et al., 1983; Karickhoff, 1984; Karickhoff et al., 1979; Piwoni and Banerjee, 1989; Rao and Davidson, 1980; Schwarzenbach et al., 1993; Schwarzenbach and Westall, 1981.. For natural solids where organic carbon content is low Ži.e., less than 0.1% by mass., researchers have also suggested that adsorption to mineral surfaces may contribute significantly to the overall uptake of HOCs by soils and sediments ŽKarickhoff, 1984; Schwarzenbach et al., 1993.. In recognition of this, several authors have suggested the need for additive models of the following general form ŽBackhus, 1990; Curtis et al., 1986; Karickhoff, 1984; McCarty et al., 1981; Rebhun et al., 1992; Schwarzenbach et al., 1993.: K d s f oc K oc q SSA K SSA

Ž 1.

where K d s sorption distribution coefficient wŽmol HOC.rŽg sorbent.xrwmol HOC.rŽml water.x; f oc s mass fraction of organic carbon wŽg organic carbon.rŽg sorbent.x; K oc s distribution coefficient for partitioning into SOM, normalized on the basis of measurable organic carbon wŽmol HOC.rŽg organic carbon.xrwmol HOC.rŽml water.x; SSA s specific surface area wŽm2 mineral surface area.rŽg sorbent.x; and K SSA s distribution coefficient for adsorption to mineral surfaces, normalized on the basis of measurable surface area wŽmol HOC.rŽm2 mineral surface area.xrwmol HOC.rŽml water.x. Numerous correlations have been published for the estimation of K oc and correlations for the estimation of K SSA have also been recently proposed. These correlations have been reviewed elsewhere ŽBall and Roberts, 1991a; Lyman, 1982; Schwarzenbach et al., 1993.. Although Eq. Ž1. is convenient for making rough approximations in the absence of other information, its limitations must be recognized. In particular, laboratory data show many instances where there are important deviations between the sorption predicted on

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the basis of existing correlations and that determined experimentally. Some particular concerns include the following facts. SOM composition and morphology may vary from one type of soil to another, such that, for any given sorbate, variation in K oc is to be expected among soils of different types Že.g., Garbarini and Lion, 1986; Grathwohl, 1990; Kile et al., 1995; Rutherford et al., 1992; Schlautman and Morgan, 1993a.. HOC adsorption to mineral surfaces is still not well understood, especially with regard to naturally weathered and coated surfaces ŽKarickhoff, 1984; Schwarzenbach et al., 1993.. Association of SOM with minerals may affect the SOM’s ability to solvate or adsorb HOCs, owing either to conformational changes to the SOM Že.g., Garbarini and Lion, 1986; Schlautman and Morgan, 1993a. or decreased accessibility to the solution phase Že.g., Holmen and Gschwend, 1997; Schlautman and Morgan, 1993a.. Heterogeneity of SOM composition andror surface adsorption may lead to concentration dependencies of the distribution coefficients Ži.e., nonlinear sorption isotherms Že.g., Spurlock and Biggar, 1994a; Weber et al., 1992... Even in cases where sorption is dominated by ‘simple’ linear hydrophobic partitioning with SOM, variability in the chemical properties of the SOM will influence sorbed-phase activity coefficients of HOCs and thus affect K oc ŽGarbarini and Lion, 1986; Gauthier et al., 1987; Grathwohl, 1990; Rutherford et al., 1992; Schlautman and Morgan, 1993a. In terms of the most common operationally defined fractions of extractable soil organic matter Žfulvic acid, FA; humic acid, HA; and humin, Hu., FA is generally recognized as being the more polar, aliphatic, and hydrophilic component ŽHayes and Bolt, 1986; Hayes and Swift, 1990.. In this regard, K oc has been observed to vary by up to an order-of-magnitude for sorption experiments with a given HOC and different fractions of humic substances. For example, studies with trichloroethene ŽGarbarini and Lion, 1986., toluene ŽGarbarini and Lion, 1986; Tell and Uchrin, 1991., pyrene ŽGauthier et al., 1987. and perylene ŽSchlautman and Morgan, 1993a. have shown the K oc for humic acid to be between 6 and 13 times higher than that estimated for fulvic acid extracted from the same source. Variations in K oc among different types of natural SOM have been shown to correlate reasonably well with decreases in the fraction of organic carbon associated with oxygen-containing functional groups, as reflected, for example, by increasing C:O ŽGarbarini and Lion, 1986., increasing H:O ŽGrathwohl, 1990., or increasing C:ŽN q O. ŽRutherford et al., 1992.. Other investigators ŽGauthier et al., 1987. have proposed that the sorption of unsaturated HOCs will be stronger with SOM having a more polarizable bond structure, and that sorption of many HOCs will therefore be stronger with humic substances of greater aromaticity. More recently, Kile et al. Ž1995. have shown important differences between HOC sorption with soils and river sediments, with higher sorption to the latter attributed to a more reduced and hydrophobic character of SOM. These latter authors did not specifically investigate differences among different types of soils, instead using what they refer to as ‘normal’ surface soils for their comparative study against sediments. In comparing HOC sorption with dissolved and adsorbed organic matter, several investigators have suggested that conformational changes of the SOM may also affect K oc ŽGarbarini and Lion, 1985; Murphy et al., 1990; Murphy et al., 1994; Schlautman

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and Morgan, 1993a., and that aqueous conditions ŽpH, ionic strength. may also have an effect ŽMurphy et al., 1990; Murphy et al., 1992; Schlautman and Morgan, 1993b.. For example, the adsorption of humic substances onto mineral surfaces has been shown to lead to reduced sorption of HOCs relative to partitioning into the dissolved macromolecules ŽGarbarini and Lion, 1985; Laird et al., 1994; Schlautman and Morgan, 1993a.. Some investigators have found that adsorption of humic materials onto mineral surfaces can lead to nonlinear HOC sorption isotherms ŽMurphy et al., 1990; Murphy et al., 1994.. Other investigators have noted that, although hydrophobic partitioning to mineral surfaces is likely to be linear, the surfaces of certain types of ‘hard’ organic matter such as kerogens and shales are likely to adsorb HOCs in a nonlinear manner ŽWeber et al., 1992; Young and Weber, 1995.. Finally, Spurlock and Biggar ŽSpurlock and Biggar, 1994a,b,c. argue that partitioning processes can also be nonlinear under some circumstances. The study reported here was undertaken to provide additional insight into how a soil’s genesis, composition, and formative history can affect HOC sorption, with an ultimate goal of helping to guide the further refinement of predictive models. While not attempting to provide a level of characterization sufficient to address some of the mechanistic issues alluded to above, our study provides a demonstration of the magnitude of effect that may be associated with such issues in a natural soil setting. In particular, our work examines the variability of K d and K oc in subsurface soils where organic matter content and composition, as well as soil mineralogy, are known to vary in accordance with reasonably well-understood processes of soil genesis and weathering. We have studied sorption on material from seven soil horizons taken from various depths at a single site, at a location where the soil horizons are known to have developed authigenically Ži.e., were formed in place, Bates and Jackson, 1984. and where clay content, f oc , and humic acid to fulvic acid ratios Žas operationally defined, using commonly applied techniques. all change systematically with depth. The major soil components of all horizons are believed to derive from the same underlying rock sources and the same surficial organic matter inputs. This has allowed us to assume that any observed variability in SOM is not the result of differences in the organic carbon’s geographic origin, but rather the result of natural processes of soil development. We have also carefully tested and controlled a number of experimental variables that can affect the measurement of K d . This work amply illustrates that natural variations in the composition and mineralogical association of SOM in different soil horizons can lead to order-of-magnitude or greater changes in the extent to which SOM can solvate or adsorb hydrophobic contaminants.

2. Materials and methods 2.1. Sorbents The soil material used in this research was obtained from a pre-excavated 3-m deep pit in the Piedmont of North Carolina, USA, at a site in Duke University Forest ŽOrange, NC.. The pit had been freshly excavated by back-hoe less than 2 months prior to the

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taking of samples reported below. The site area is presently forested with planted pine trees that are about 70 years old. For roughly 150 years prior to the establishment of this forest, the area was used for the cultivation of tobacco, wheat and corn. The soil at the pit has been classified as a kaolinitic, thermic Typic Kanhapludult of the ultisol soil order ŽU.S. Soil Conservation Service, 1977.. The soil is generally low in organic carbon content and was formed in the residuum weathered from acidic igneous and metamorphic rock. In this context, the A and E horizons represent eluvial zones in active states of weathering and microbial transformation of the organic constituents, losing weathering products to the horizons below. The B horizons represent illuvial zones which receive organic matter and clay particles from the overlying layers. EB and BE horizons are transitional zones. Transport to and through these zones is complex. A major mechanism of iron, aluminum, and clay transport in a soil profile is through complexation with dissolved organic macromolecules, such as humic acid, fulvic acid and other even more hydrophilic organic acids ŽSchlesinger, 1991; Stevenson, 1985; Thurman, 1985.. Organo-metal, organo-metal oxide, and organo-clay complexes are thus leached to the underlying B horizons, where dissolved species are adsorbed and colloidal phases deposited. This process, known as podzolization, is found throughout the world but is particularly intense in areas overlain by coniferous forests ŽSchlesinger, 1991.. Although a defining characteristic of spodisol soils Žwith clear spodic horizons concentrated in leached organic matter., such processes of organic acid and mineral leaching are still highly relevant for other soils with agrillic horizons but less intense podzolization ŽSchlesinger, 1991., such as the Duke Forest soil. This soil is an ultisol, characterized by a low pH agrillic B horizon with iron oxide coloration and low base saturation Ži.e., low saturation of ion exchange sites by cations other than protons, U.S. Soil Conservation Service, 1975.. The soil profile has declining organic carbon content with depth, as described subsequently. At the Duke Forest site, seven subsurface soil horizons were identified in the exposed vertical profile. These were assigned the following designations Žfrom shallowest to deepest., based on common soil taxonomy nomenclature: A, E, EB, BE, B1, B2 and B3, with depth intervals for each horizon as shown in Table 1. An undisturbed 3-ft wide vertical section on one side of the pit was selected as the source of soil materials. The O-horizon at the soil surface Ža thin layer of surface soil composed of decaying forest litter. was removed carefully, and the underlying seven soil horizons were clearly identified and marked. The exposed vertical surface was cleaned of the outermost inch of material and 10 to 20 kg of soil materials was excavated from each of the seven soil horizons, using hand trowels. The soil from each horizon was spread on a polyethylene tarp, homogenized, split into smaller portions of about 4 kg each and transported to the laboratory in 4-l polyethylene bags ŽNjoroge, 1994.. In the laboratory, the soil samples were stored in a refrigerator at 48C until air-drying was conducted under loose aluminum foil cover at 22–238C. After air drying, the soil solids were sieved through a No. 10 Ž2 mm. brass sieve. The clay-sized materials of the B horizons tended to aggregate during drying. For these horizons, materials retained in the No. 10 sieve were crushed by hand under mild pressure Žusing the flat end of an aluminum rod. until all the material passed through the No. 10 sieve size. The sieved

352

Table 1 Characterization of soil horizons at the Duke Forest site a Depth interval bgs b Žcm.

pH

A

0–28

E

28–41

EB

41–61

BE

61–84

B1

84–134

B2

134–184

B3

184–234

3.9, 3.9 Ž2. 4.6, 5.0 Ž2. 4.4, 5.4 Ž2. 4.4, 5.4 Ž2. 5.2, 5.4 Ž2. 4.4, 5.0 Ž2. 4.6, 5.0 Ž2.

a

Organic carbon

Soil texture

TOC Ž% C.

Humic acid Ž% C.

Fulvic acid Ž% C.

Huminc Ž% C.

Sand Ž%.

Silt d Ž%.

Clay Ž%.

0.536 w0.04x Ž4. 0.191 w0.02x Ž4. 0.098 w0.006x Ž5. 0.127 w0.009x Ž4. 0.099 w0.004x Ž4. 0.079 w0.004x Ž5. 0.063 w0.004x Ž4.

0.136

0.164

0.063

0.025

0.030

0.015

0.038

0.043 0.051 w0.002x 0.074

ND

0.024

0.075

ND

0.016

0.063

ND

0.007

0.056 0.058 w0.003x

91.0 w1.4x Ž2. 74.7 w0.3x Ž2. 67.7 w1.7x Ž2. 41.9 w1.6x Ž2. 23.4 w1.6x Ž2. 25.4 w5.2x Ž2. 27.2 w0.3x Ž2.

8.4

0.032

0.24 0.22 w0.027x 0.096

0.6 w0.1x Ž2. 11.4 w1.4x Ž2. 15.4 0.0 Ž2. 40.8 w0.8x Ž2. 56.2 w0.4x Ž2. 51.4 w1.1x Ž2. 46.8 w1.7x Ž2.

13.9

16.9

17.3

20.4

23.2

26.0

BET Surface area Žm2 rg.

Total Iron Ž% Fe.

Free Iron oxides Ž% Fe.

Total CEC Žmeqr100 g.

0.58 w0.004x

0.26 w0.01x Ž6. 0.47 w0.04x Ž6. 0.73 w0.02x Ž6. 2.38 w0.03x Ž6. 4.67 w0.08x Ž6. 4.78 w0.03x Ž6. 4.25 w0.04x Ž6.

0.080 w0.002x Ž2. 0.13 w0.003x Ž2. 0.26 w0.001x Ž2. 1.65 w0.08x Ž2. 2.92 w0.007x Ž2. 2.63 w0.06x Ž2. 2.41 w0.10x Ž2.

0.89 w0.14x Ž2. 0.68 w0.04x Ž2. 0.64 w0.18x Ž2. 4.2 w0.03x Ž2. 8.1 w0.58x Ž2. 9.6 w0.62x Ž2. 8.0 w0.10x Ž2.

1.07 w0.003x 3.48 w0.009x 23.60 w0.05x 44.81 w0.40x 42.51 w0.36x 36.15 w0.72x

Average wStd. Dev.x Žno. of replicates.; NDs not detected Žbelow method detection limit.; two analyses for pH, both are shown. bgss below ground surface, after removal of all surface litter. c Determined by difference from a single FArHA extraction. Second values below are Average wStd. Dev.x of 4 replicate analyses by direct measurement ŽUIC Coulometrics Models 5012, 5120.. d Determined by difference. b

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Soil horizon

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materials were then homogenized and stored in polyethylene bags. All subsequent splitting of samples were accomplished using either a 2-way or 16-way riffle splitter ŽModel SP-3 or SP-201, Gilson, Worthington OH. to ensure representative subsampling and to enhance reproducibility. Pulverized materials were obtained by grinding in a well-cleaned aluminum-oxide shatterbox Ž17.5 cm diameter, SPEX Ind.. for 1 min ŽBall et al., 1990; Ball and Roberts, 1991a.. This process results in a fairly uniform fine power, for which the median particle size is believed to be on the order of 10 to 20 m m ŽBall et al., 1990.. 2.2. Sorbent characterization Table 1 summarizes the characterization results for materials from the various soil horizons. Methods used for the physical and inorganic soil properties were as follows: Ø soil pH by probe at a 1:2 soil:water mass ratio ŽMcLean, 1982.; Ø textural analysis by sedimentation, using a hydrometer method after dispersion in sodium hexametaphosphate ŽGee and Bauder, 1986.; Ø specific surface area by single sample analysis in a commercial laboratory using 5-point BETrKrypton gas adsorption ŽDigisorb 2600, Micromeritics Instrument, Norcross, GA.; and Ø total iron following HF digestion ŽOlson and Ellis, 1982.; Ø extractable Žfree. iron oxides by citrate-bicarbonate-dithionite extraction ŽOlson and Ellis, 1982.; Ø cation exchange capacity ŽCEC. using unbuffered ammonium base saturation and KCl exchangeable aluminum and acidity ŽThomas, 1982.; and Ø soil mineralogy by X-ray diffraction and petrographic light microscopy ŽWhittig and Allardice, 1986.. Organic carbon analysis was conducted by means of high-temperature oxidation followed by coulometric CO 2 quantification ŽModel 5120 and 5012, UIC, Joliet, IL. on both pre-acidified and untreated samples, with inorganic carbon content independently verified ŽModel 5130, UIC.. Characterization of base soluble HA and FA followed the method described by Schnitzer and Schuppli ŽSchnitzer and Schuppli, 1989.. With this method, HA and FA are operationally defined on the basis of a single extraction of the soil in 0.1 M NaOH, with HA subsequently precipitated at pH 1. The aqueous Žbase-extracted. HA and FA components were analyzed for organic carbon content by high temperature combustion ŽModel TOC-5000, Shimadzu, Columbia, MD. and the humin component operationally defined as the unextracted SOM remaining on the soil, usually determined by difference from the TOC measurement with untreated soil. For three soil horizons ŽA, EB and B3., the mass fraction of humin was independently measured by direct analysis of the base-extracted soil residue using the high temperature oxidation method mentioned above. These tests demonstrated good mass balance on SOM ŽTablenote c of Table 1.. 2.3. Sorbates The organic chemicals used in this study were 1,2,4-trichlorobenzene ŽTCB. and tetrachloroethylene ŽPCE.. 14 C-radiolabelled chemicals were purchased commercially

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ŽSigma, St. Louis, MO. and subsequently diluted with unlabelled TCB and PCE in spike solutions made in HPLC grade methanol Ž99.9% purity, Aldrich Chemical, Milwaukee, WI.. All chemicals and spike solutions were stored at 48C prior to and between use. 14 C specific activities of the purchased materials were 2.67 mCirmmol and 9.42 mCirmmol for TCB and PCE, respectively. Radiochemical purity was reported to be ) 98% and was re-checked in our laboratories throughout the study, using both a gas chromatography-based verification of specific activity and HPLC techniques described elsewhere ŽNjoroge, 1994; Young and Ball, 1994.. The results confirmed better than 97% radiolabel purity for both chemicals. The unlabelled sorbates PCE ŽHPLC grade. and TCE Žspectrographic grade, both from Aldrich Chemical. were used as received. 2.4. Synthetic groundwater The water used in the sorption experiments is hereafter referred to as synthetic groundwater. This water was activated-carbon-treated double-deionized water ŽMilli-Q Plus, Millipore, Bedford, MA. that was supplemented with 0.01 M CaCl 2 and 0.02% NaN3 as a biocide. Although there is no evidence to suggest that aerobic biodegradation of TCB and PCE would be of concern in these studies, the sodium azide prevents biological activity from depleting oxygen concentrations or otherwise affecting conditions in the ampule over the long periods of equilibration used Žup to 124 days.. To our knowledge, no abiotic degradation pathways exist for the transformation of these two chemicals under the well-oxygenated conditions of these experiments. We predicate our subsequent interpretation of 14 C results on the assumption that such reactions do not occur at appreciable rates. 2.5. Soil pre-treatment Three of the soil horizons studied ŽA, EB, and B3. were subjected to selected chemical and physical methods for the removal of soil organic matter: a. base extraction identical to that used for humic substance classification; b. hydrogen peroxide oxidation ŽKunze and Dixon, 1986.; and c. thermal oxidation at 9508C. The thermal oxidation method was intended to achieve complete removal of all organic matter but has the unavoidable disadvantage of also altering mineral phases. For this method, the soil solids were placed in porcelain crucibles in batches of about 60 g and placed in a muffle furnace controlled at 950 " 108C, for 15 min. This treatment was found to remove virtually all of the organic matter Žfinal measured f oc - 0.005%. from all horizons except B3. In this latter case, 30-min treatment of a 30-g sample successfully achieved a non-detectable f oc . The final organic carbon contents of all three types of pre-treated solids were determined using the 9508C combustion method described under ‘Sorbent Characterization’. This method differs from the muffle furnace treatment in that smaller sample sizes are involved Ž- 2 g. and reaction occurs in a pure oxygen environment. 2.6. Sorption studies An important aspect of this work, K d measurement, was achieved using the application of precise experimental techniques, including a more critical examination of

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experimental methods and equilibration times than in most prior work with similar systems. The sorption methodology adopted was a batch-equilibrium technique described elsewhere ŽBall, 1989; Ball and Roberts, 1991a.. The batch sorption experiments were conducted in 10-ml flame-sealed glass ampules ŽWheaton Scientific Products, Millville, NJ. containing uniformly consistent headspace volumes on the order of 1 ml. All ampules were thoroughly cleaned prior to use Žacetone and Milli-Q Plus rinses followed by oven drying.. The 14 C labelled TCB and PCE were added as 3 m l of a methanol-based stock solution. Specific activity of the TCB and PCE spike solutions was adjusted as needed to provide the desired final solute concentrations and radiochemical activity. Sample mixing during equilibration was by rolling at 2 rpm, and solid:water separation prior to supernatant analyses was by centrifugation for 20 min at 800 g. The apparent sorption distribution coefficient for a given sample and time of equilibrium was deduced from a mass balance following scintillation-counter analysis of aqueous-phase activity. The precise radiochemical activity of the spike solutions was determined by means of replicate injections directly into scintillation fluid each time a set of blanks and samples was prepared. Solute losses to glassware and to the small headspace volume were accurately accounted through the analysis of blank samples Žsolute spike in soil-free ampule filled to same level as samples.. Two replicate blank samples were evaluated at each concentration studied and for each set of samples analyzed. On the basis of the blank results, minor corrections of calculated sorbed masses were made to account for solute losses to headspace, using methods described elsewhere ŽBall and Roberts, 1991a; Njoroge, 1994.. In no case did the observed losses in blank samples exceed 5% of the mass added and mass losses were confirmed to be proportional to the aqueous concentration Ži.e., no concentration effect on the blank correction factor, Ball and Roberts, 1991a.. For the 14 C sorption technique used, method detection limit ŽMDL. for K d is known to depend strongly on the reproducibility of influent spike mass, reproducibility of blank headloss measurements, and the fractional uptake from solution ŽBall, 1989; Ball and Roberts, 1991a.. On the basis of replicate measures with numerous blank samples, the mass balance with the 14 C-labelled technique is believed to be accurate to within roughly "1% Žone standard deviation.. We therefore take the minimum detection limit ŽMDL. of the system to be the amount of sorption needed to achieve a mass loss from solution Žowing to sorption. that is above that attributable to volatilization by more than three standard deviations above the blank level ŽLong and Winefordner, 1988. —i.e., the K d value at which sorptive uptake from solution is at least 3%, as dependent upon the given soil:water ŽS:W. ratio. Recognizing that fractional uptake from solution is equal to ŽS:W. K drwŽS:W. K d q 1x, we estimate the MDL for K d in this study to be roughly 0.01 mlrg for samples at the highest achievable S:W ratio Žapproximately 2.6 grml. and increasing to roughly 0.1 mlrg when S:W ratio is decreased to 0.3 grml. Of course, relative errors on the estimate increase greatly as the MDL is approached. For this reason, it is preferable to achieve much higher uptake from solution Žwell above 20% if possible. in order to achieve better precision of results ŽBall and Roberts, 1991a.. In the low S:W studies reported subsequently ŽS:W in the range of 0.25 to 0.3 mlrg., uptakes of 20% or more are achieved for all systems with K d ) 1 mlrg and uptake never exceeded 80%. Variability in results is therefore more a reflection of true replicate

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variability than of analytical precision. For studies with lower K d , comparatively poor analytical precision was achieved, and this is subsequently reflected in higher relative errors Žsample standard deviation divided by the mean. for these samples. For all isotherm work, ionic strength was held constant through use of the synthetic groundwater previously described. pH was allowed to be that of the unbuffered soil:water systems and was measured for each S:W used. For any given isotherm experiment, pH did not vary by more than 0.2 pH units among samples or with different times of equilibration. At the higher S:W ratio Ž1:1 to 2:1., pH values were as shown in Table 1. Lower S:W was used for the long-term isotherm work. For these samples, measured pH values were as follows Ž"0.1 pH units.: 5.0 for horizons A and E, 5.1 for EB, 4.4 for BE, and 4.0 for horizons B1, B2 and B3. Preliminary rate studies were conducted with three of the seven materials ŽA, EB, and B3.. These three materials are believed to represent the range of physical properties encountered ŽTable 1., with A and B3 representing ‘end point’ materials in terms of f oc and clay size content. Because rate effects would be expected to be most severe at high S:W ratio, and because K d detection limits are lower at such ratios, high S:W ratio Žin the range 1.0 to 2.6 grml. was used for these studies. The rate studies involved duplicate samples at each of six times Ž0.3, 1, 3, 10, 30, and ) 100 days., with final aqueous concentrations between 50 and 200 m grl. The rate studies were conducted with both pulverized and unpulverized samples, with the hope that the pulverized samples would attain a similar equilibrium distribution of chemical, but at sooner time. This finding had been previously observed with Borden sand, where the pulverization is believed to have reduced the path length for intraparticle diffusion ŽBall and Roberts, 1991b.. To evaluate the issue of sorption linearity, full range isotherm experiments were conducted with the same three soils investigated with regard to rate. The full range isotherms were obtained at the longest sorption time studied and were designed to achieve final Žequilibrium. aqueous concentrations of approximately 10, 100, 1000, 10,000, and 100,000 m grl for PCE and of approximately 10, 200, 2000, and 20,000 m grl for TCB. For both chemicals, the highest equilibrium concentrations achieved exceeded 50% of aqueous solubility Ž150 mgrl for PCE and 41 mgrl for TCB, Schwarzenbach et al., 1993.. Low S:W ratios were used, in the range of 0.17 to 0.33 mlrg, with the higher value used with the less strongly sorbing B-horizon materials in an attempt to maintain reasonably high chemical uptake and assure the cited MDL of 0.1 mlrg. Nonetheless, PCE uptake was below the detection limit of the method for these horizons. In addition to the full range isotherms, isotherms were conducted over a narrower range of concentrations Ž5 to 10, 25, 50, 75, 100, 200 m grl. with all seven soil horizons, and using two different equilibration times Žeither 10 and 30 days or 30 and 90 days, depending on the sorbatersorbent system.. These latter studies were also at low S:W ratio Ž0.1 to 0.3 grml.. Although lowering the S:W raised our K d detection limit as previously described, it eliminated a potential concern that particle aggregation might limit the rate of approach to sorption equilibrium. In fact, S:W effects were ultimately observed only with pulverized material, with all results on untreated samples showing similar results at both low and high S:W ratio.

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357

3. Results: effects of sorption methodology Although batch sorption techniques have been widely applied in the estimation of contaminant distributions in soil:water systems, recent work has shown that equilibrium can be slow to attain and that both long- and short-term batch study may be subject to experimental artifacts. Particular experimental concerns include the time for equilibrium to be achieved ŽBall and Roberts, 1991a; Miller and Pedit, 1992. and the effect of soil:water ŽS:W. ratio on results. In the latter regard, reduced measurements of K d at high S:W ratio have been attributed to incomplete separation of colloidal solids from the aqueous phase ŽMorel and Gschwend, 1987. as well as to other less well-defined causes ŽMcKinley and Jenne, 1991.. 3.1. Particle separation In this study, the synthetic groundwater used had sufficiently high ionic strength to allow good particle aggregation and therefore good solid:water separation during centrifugation. Visually clear supernatants were observed in all cases. Dissolved organic carbon ŽDOC. measurements on the supernatant indicated low DOC in the aqueous phase relative to that remaining on the soil. In particular, DOC was measured in the high S:W samples, where values were found to be on the order of 10 to 30 mgrl for A-horizon samples, 1 to 8 mgrl DOC for E, EB, and BE samples, and less than 1 mgrl DOC for all B-horizon samples. At the soil:water ratios employed for the above studies Žroughly 1 to 2 grml for these studies., SOM Žsolid-phase TOC. concentrations were on the order of 5000, 2000, 1000, 1000 and 600 mg-Crl for the A, E, EB, BE, and B samples, respectively. Potential errors associated with these DOC values would be greatest for TCB. Assuming a high K oc of 3000 mlrg for TCB, equilibrium partitioning calculations of the type proposed by Morel and Gschwend, 1987, would imply less than 9% error in the worst case—that is, for A horizon soil Žwhich has the highest ratio of DOC:SOM. and with the K oc of the dissolved material assumed to be 3000 mlrg for TCB Žthe highest value estimated for any soil.. This estimate is likely to be quite conservative, since the DOC of dissolved macromolecules or dispersed colloids is likely to comprise more polar components of the SOM. 3.2. Time to equilibrium In Table 2, the duplicate measures of K d at each point in time are reported. For purposes of comparison, the 10-, 30-, and 90-day ‘low concentration isotherm’ results Žobtained subsequently, and at lower S:W ratio. are also shown. First considering the duplicate results at high S:W and multiple time points Žwith A, EB, and B3 materials., we make the following observations. For five of the six systems evaluated Ži.e., all except A horizon with TCB., the K d value manifested at t ) 90 days is remarkably similar to that observed at 30 days. Only for the ArTCB system is there any indication of continuing uptake beyond 30 days. For four of the six systems Ži.e., all except the A horizon with either solute., the long-term equilibrium appears to be achieved by day 3. For the three soils where pulverized samples were studied ŽA, EB, B3., K d results with pulverized samples agree reasonably well with the long-term unpulverized results

358

Soil horizon

Apparent sorption distribution coefficient a , app K d Žmlrg., at time, t, in days

Soil:Water Ratio Žgrml.

t s 0.33

t s1

ts3

t s10

t s 30

t G90 b

Pulv’d Ž t )10. c

1,2,4-Trichlorobenzene: A High S:W 1.9 to 2.0 Low S:W 0.17 E Low S:W 0.25 EB High S:W 1.8 to 2.6 Low S:W 0.25 BE Low S:W 0.33 B1 Low S:W 0.33 B2 Low S:W 1:3 B3 High S:W 0.33 Low S:W 0.33

8.5, 8.3 y y 0.78, 0.96 y y y y 0.23, 0.04 y

7.5, 8.3 y y 0.79, 1.05 y y y y 0.21, 0.20 y

8.9, 11.2 y y 1.24, NAd y y y y 0.20, 0.18 y

8.6, 14.0 14.9 w1.6x Ž10. 3.53 w0.21x Ž10. 1.33, 1.31 1.46 w0.19x Ž9. y y y 0.18, NAd y

12.9, 13.9 15.3 w 1.6 x Ž 10 . 4.26 w 0.35 x Ž 10 . 1.33, 1.42 1.46 w 0.19 x Ž 9 . 1.59 w 0.27 x Ž 8 . 0.63 w0.08x Ž10. 0.29 w0.06x Ž10. 0.20, 0.22 0.14 w0.08x Ž10.

15.0, 16.3 y y 1.22, 1.41 y 1.41 w 0.17 x Ž 9 . 0.64 w 0.12 x Ž 10 . 0.39 w 0.07 x Ž 9 . 0.22, 0.22 0.18 w 0.05 x Ž 10 .

15.4 w4.6x Ž6. 31.8r31.9

Tetrachloroethene: A High S:W Low S:W E Low S:W EB High S:W Low S:W BE Low S:W

1.83, 1.84 y y 0.15, 0.15 y y

1.90, NAd y y 0.14, 0.23 y y

1.95, 2.19 y y 0.16, 0.16 y y

2.12, 2.01 2.03 w0.30x Ž10. 0.51 w0.10x Ž7. 0.17, 0.21 0.13 w0.04x Ž9. y

2.20, 2.30 2.56 w 0.29 x Ž 8 . 0.42 w 0.06 x Ž 10 . 0.17, NAd 0.14 w 0.07 x Ž 10 . 0.15 w 0.08 x Ž 10 .

2.39, 2.51 y y 0.16, NAd y y

2.64 w0.40x Ž6. y y 0.18 w0.05x Ž10.

a

2.0 to 2.1 0.25 0.25 2.0 0.33 0.33

1.42 w0.26x Ž6. 2.9r2.5 y y y y 0.23r0.26

Average value wStd. Dev.x Žno. of replicates.. When only two replicate samples were evaluated, both results are shown. Values in italics are used subsequently for comparative purposes Žsee Table 5.. b t s104 days for TCB high S:W samples; t s124 days for PCE high S:W samples; t s90 days for low S:W samples. c Avg. of all results at 10, 30, and )90 days. For low S:W pulverized samples, S:W was 0.10 to 0.12 grml and time was 30 days. d NA s not analyzed Žbreak in ampule seal..

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Table 2 Effects of batch conditions on apparent K d : equilibration time and soil:water ratio

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359

for both chemicals. However, we note that time course studies with pulverized samples ŽA and EB samples. with TCB Ždata not shown. did indicate a potential difference between measured K d at 104 days with those at shorter time intervals. This observation was most pronounced for the A horizon, where measured K d was 21.2 mlrg and 21.5 mlrg at 104 days versus 12.5 " 0.3 mlrg for the four samples at 10 and 30 days. Interestingly 10- and 30-day results were virtually identical, with slightly higher values at 10 days than 30. For PCE, 10-, 30-, and 90-day results with pulverized samples at high S:W were all in good agreement, implying that this less strongly sorbing HOC had attained equilibrium by 10 days. Next considering the low concentration isotherm data at low S:W ratio and two time points Žall soil horizons., we make the following observations. For all nine systems where comparison is possible Žseven horizons with TCB and three horizons with PCE., there is no significant difference Ž a s 0.05. between the early time Ž10- or 30-day. and longer time Ž30- or 90-day. results. In the five systems where S:W ratio comparison is possible ŽA, EB, and B3. the low and high S:W results show similar final K d . However, the A-horizonrTCB system appears to require ) 90 days to obtain equilibrium at high S:W. This system, at large particle size and high K d , is the one most susceptible to diffusive mass transfer limitations. For only four of the nine systems ŽErTCB, B2rTCB, B3rTCB, ArPCE. there is an apparent Žthough not statistically significant. increase of between 20% and 35% between the last two time points. Relative error of results Žstandard deviation divided by the mean. are on the order of 10% to 15% for all samples except the less sorbing systems ŽB3rTCB, EBrPCE, BErPCE., where relative error is as much as 50%. For the ArTCB and EBrTCB systems, results with pulverized material at low S:W ratio Žright-most column of Table 2. show large discrepancy relative to other results, either at higher S:W with pulverized samples or at any S:W with unpulverized samples. On the other hand, for the B3rTCB system, low S:W pulverized sample results were similar to those obtained at high S:W and only slightly higher than for unpulverized samples but not significantly different Žwithin one standard deviation of the mean., primarily reflecting the wide scatter in unpulverized experimental results with this marginally sorbing sample. The higher TCB-K d with low S:W pulverized A and EB material may indicate that TCB sorption equilibrium was not attained with either the unpulverized samples or the pulverized materials at high S:W. This may reflect, for example, a slow intraparticle diffusion in the comparatively larger grain sizes of the unpulverized A and EB material, or, in the higher S:W pulverized samples, slow mass transfer in colloidal aggregations of aluminosilicates, organic matter, and iron oxides ŽHolmen and Gschwend, 1997.. Other possibilities are that the high S:W in pulverized samples may have led to reduction in apparent K d through increased suspension of colloidal SOM Že.g., as suggested by Morel and Gschwend, 1987.. Although we saw no color or turbidity in sample supernatants, we did not measure DOC in the pulverized systems and cannot conclusively rule out this possible effect. We also cannot rule out a possibility that unpulverized samples were actually at equilibrium and that the high S:W pulverized samples

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were not. In this case, the low S:W pulverized results might imply that pulverization serves to create new sorption sites by exposing additional organic or inorganic surface area. Finally, it is possible Žalthough unlikely. that all four of the long-termrlow S:W pulverized TCB samples Žtwo in each size fraction. were affected by some undetected experimental artifact. In particular, ampule sealing is more problematic for pulverized samples Žowing to interference of the ampule seal by mineral fines. and it is conceivable that hairline cracks in seals went undetected. Although our inability to distinguish among these reasons has been bothersome to us, additional study was not possible within the project’s constraints. Overall, we note that results with unpulverized samples were always in reasonable agreement at the two final times studied, and at both S:W. On this basis, and because most of our long-term data were initially obtained at low S:W Žin order to avoid potential mass transfer problems in the clay soils., we subsequently take the low S:W unpulverized isotherm results at longer term Ž30 or 90 days of equilibration. to be indicative of the relative equilibrium sorption among these materials. These results are highlighted by italicized text in Table 2. In regard to this comparison, we have already noted that the comparatively few low S:W pulverized results in the ArTCB and EBrTCB systems suggest that true equilibrium sorption for TCB in these horizons could be somewhat greater than we have assumed. On the other hand, sorption results for the deeper material ŽB3. was fairly consistent among all systems studied Žpulverized and unpulverized at both S:W., serving to confirm the attainment of equilibrium conditions for this system. Thus, if biases in our relative estimates of sorption do exist, they will take the form of underestimation of TCB sorption in the shallow horizons and therefore underestimation of differences between the shallow and deep zones. Finally, we note that our equilibration conditions and times Žfull rotational mixing for 30 days. should be closer to equilibrium than those used in many previously reports of sorption with similar materials. In this context, our results serve to demonstrate how potential sorption nonequilibrium may go unnoticed, since it can be difficult to observe statistically significant difference without the proper combination of very long-term study, numerous replicates, precise analytical techniques, and study with pulverized material. For TCB sorption with the Duke Forest materials, more replicate rate work with both pulverized and unpulverized material would be useful avenues of future work. For our current purpose of comparing sorption among the soil horizons, we have found it nonetheless informative to consider the long-term Ž30- and 90-day. sorption isotherm results that have been obtained to date.

4. Results: comparative sorption studies For reasons noted above, our comparative study of sorption in the seven soil horizons focuses on the data shown in italics in Table 2. These results were all obtained under similar conditions: equilibration times longer than 30 days; a minimum of five different final aqueous concentrations between 7 and 200 m grl; and low soil:water ratio Ž0.1 to 0.3 grml.. Before elaborating on this comparison of low concentration results, we first address the issue of sorption linearity.

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361

Fig. 1. Sorption isotherm data and Freundlich isotherm fit for TCB with materials from Duke Forest soil horizons. Isotherm conditions were as given in Table 3.

4.1. Full-range isotherms in selected soil horizons The linearity of sorption isotherms was evaluated through full-range isotherms with three representative soil horizons ŽA, EB, and B3.. Both sorbates were studied, although sorption of PCE in the B3 system was too low to be accurately quantified. Figs. 1 and 2 show the results for TCB and PCE, respectively. In these figures, the results of the full-range study are shown as open symbols and are presented together with additional data subsequently collected for a low concentration Ž7 to 200 m grl. comparative study Žfilled symbols.. Freundlich isotherm fits from these two studies, as well as combined results from all data, are shown in Table 3. Note that some of the ‘average K d’ results presented in Table 3 Žthose in italics. have already been presented as part of Table 2, but are repeated here for purposes of comparison. As described elsewhere ŽBall and Roberts, 1991a., such an ‘average K d’ provides the most appropriate weighting of data if Ž1. linear partitioning holds, and Ž2. the isotherm method maintains approximately constant relative error Žas opposed to absolute error. among sorbed mass estimates for different concentrations. The second criterion is known to hold for the method used here, since 14 C counting was conducted to achieve similar precision of error Ž- 0.5% standard deviation. for all samples. As noted below, an assumption of linear partitioning is also reasonably appropriate with these materials—certainly for purposes of comparison, and possibly mechanistically. The results shown in Figs. 1 and 2 demonstrate at least an approximate isotherm linearity over the entire range of concentration investigated for both compounds— Freundlich exponents Ž1rn values. are above 0.9 for all soil horizons, and 95% confidence intervals extend beyond 0.97 or more for all systems except the A horizon

362

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Fig. 2. Sorption isotherm data and Freundlich isotherm fit for PCE with materials from Duke Forest soil horizons. Isotherm conditions were as given in Table 3.

with PCE. In general, the A-horizon data with both TCB and PCE do suggest a very modest degree of sorption nonlinearity, more than the other systems studied. Based on the ‘full-range’ data, which are more evenly distributed along the concentration axis, TCB and PCE Freundlich slopes Ž1rn values. were estimated as 0.96 and 0.92 for this horizon ŽTable 3.. Note that the additional low concentration data between 10 and 200 m grl Žfilled circles in Figs. 1 and 2. have not been included in calculations of ‘full range isotherms’. This has been done to heighten the independence of the compared data sets in Table 3, and is different from what was done with the ‘overall’ Freundlich fits. For the three soils where comparison is possible ŽA, EB, B3., there is very little difference in results for average K d calculated from either the full-range concentration set, the overall fit, or the results obtained only at low aqueous concentration Ž7 to 200 m grl.. The three data sets do not have significantly different mean K d Žmultiple two-sided t-test, a s 0.05. for any of the horizons with either sorbate. The similarity of the low concentration results to those obtained from full-range isotherms is a further indication of the isotherm linearity and suggests that similar mechanisms of association are occurring at all of the final equilibrium concentrations studied, which range up to over 50% of aqueous solubility. 4.2. Low concentration isotherms (all seÕen horizons) Given the results of Figs. 1 and 2, we felt justified in concluding that sorption isotherms were approximately linear for the Duke Forest soil horizons, and that the distribution coefficient could be reasonably well estimated by a less extensive range of aqueous concentrations, more easily achievable in the laboratory Ži.e., with fewer

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363

Table 3 Long-term sorption isotherm data for Duke soil horizons Equilibration time Ždays.

Freundlich isotherm parametersa K f wŽ m grg.r Ž m grg.1r n x

1r n

r2

Average distribution coefficient, K db Žmlrg.

1,2,4-Tetrachlorobenzene: A 7 to 200 Initial full range c All Data E 7 to 200 EB 7 to 200 Initial full range c All Data BE 7 to 200 B1 7 to 200 B2 7 to 200 B3 7 to 200 Initial full range c All Data

30 30 30 30 30 30 30 90 90 90 90 90 90

20.8 w15.6–27.8x 17.2 w13.7–21.5x 18.2 w15.7–21.2x 5.33 w4.40–6.47x 1.89 w1.07–3.33x 1.36 w0.94–1.95x 1.34 w1.07–1.67x 2.02 w1.48–2.74x 0.81 w0.40–1.63x 0.40 w0.17–0.98x 0.17 w0.05–0.50x 0.12 w0.09–0.16x 0.11 w0.07–0.16x

0.92 w0.85–0.99x 0.96 w0.92–0.99x 0.95 w0.92–0.97x 0.93 w0.88–0.97x 0.93 w0.79–1.07x 1.01 w0.96–1.06x 1.01 w0.98–1.05x 0.90 w0.82–0.98x 0.94 w0.77–1.11x 0.99 w0.78–1.20x 1.01 w0.74–1.27x 1.11 w1.07–1.15x 1.11 w1.05–1.19x

0.991 0.999 0.998 0.983 0.972 0.997 0.996 0.990 0.952 0.947 0.905 0.999 0.991

15.3 w 1.6 x Ž 10 . 13.5 w2.4x Ž8. 14.2 w2.3x Ž14. 4.3 w 0.35 x Ž 10 . 1.46 w 0.19 x Ž 9 . 1.48 w0.25x Ž9. 1.45 w0.22x Ž15. 1.41 w 0.17 x Ž 9 . 0.64 w 0.12 x Ž 10 . 0.39 w 0.07 x Ž 9 . 0.18 w 0.05 x Ž 10 . 0.24 w0.10x Ž7. 0.22 w0.08x Ž13.

Tetrachloroethene: A 7 to 200 Initial full range c All Data E 7 to 200 EB 7 to 200 Initial full range c All Data BE 7 to 200 B1 7 to 200

30 30 30 30 30 30 30 30 30

3.10 w1.96–4.87x 3.27 w2.85–3.76x 3.36 w2.99–3.77x 0.32 w0.21–0.51x 0.04 w0.007–0.23x 0.10 w0.04–0.27x 0.10 w0.05–0.20x 0.12 w0.01–1.16x NAd

0.94 w0.81–1.07x 0.92 w0.90–0.94x 0.92w0.90–0.93x 1.07 w0.94–1.19x 1.29 w0.82–1.96x 1.01 w0.87–1.16x 1.01w0.90–1.13x 1.09 w0.47–1.72x NAd

0.981 0.999 0.999 0.980 0.836 0.976 0.965 0.752 NAd

2.56 w 0.29 x Ž 8 . 2.01 w0.58x Ž8. 2.14 w0.58x Ž14. 0.42 w 0.06 x Ž 10 . 0.14 w 0.07 x Ž 10 . 0.12 w0.07x Ž9. 0.12 w0.07x Ž15. 0.19 w 0.08 x Ž 8 . - 0.05

Soil horizon

Aqueous concentration Ž m grL.

a

Fitted value w95% confidence limitsx, from linear regression of log-transformed data. Average value; w x sample standard deviation; Ž. number of replicate analyses. Italicized values are repeated from Table 2. c Based on initial ‘full range’ isotherm data, as described in text Ži.e., open symbols of Fig. 1 and Fig. 2.. d NA s Not analyzed. For all B-horizon samples with PCE, sorption was below detection limit. b

radio-labelled spiking solutions to purify and maintain.. Nonetheless, in order to assure a proper comparison among soil horizons, only the data from this same concentration interval Ž7 to 200 m grl. are used for comparative purposes, even where more complete isotherm data are available. As evident from Table 3, the low concentration isotherms for the four soil horizons not shown in Figs. 1 and 2 were also observed to be approximately linear with both chemicals. For TCB, the fitted Freundlich 1rn values fall between 0.9 and 1.06 in all seven horizons, and 95% confidence intervals on the low concentration data encompass 1rn values of 0.98 or higher for all seven soil horizons. For PCE, sorption was comparatively weak and isotherms exhibited considerably more scatter, particularly in the low carbon soil horizons ŽEB, BE.. Fitted exponent values ranged as high as 1.29 for PCE Žhorizon BE, low concentration data., but the PCE exponent value was not

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significantly different from 1.0 Ž95% confidence interval. for low concentration data with any of the four soil horizons evaluated. 4.3. Sorption results with treated solids For experiments with treated solids, a 30-day equilibration time was used, and final aqueous concentrations were similar to those used with rate studies, i.e., 30 to 200 m grl. Results are shown in Table 4. For horizons A and EB, the measured K d values after base pretreatment are not remarkably different from those measured for the untreated soil solids, despite the fact that the treatment had removed roughly 50% of the initial soil organic carbon. A less significant fraction of SOM was removed from B3 soil Žroughly 10%., and in this case the K d and apparent K oc of the base-extracted soil actually increased by over 4-fold, from 0.18 mlrg prior to treatment to 0.80 mlrg after. For PCE, results of base treatment with soil horizons A and EB are qualitatively similar to those observed with TCB. Lack of precision in the PCE results, owing to the low K d values measured, prevents a more quantitative comparison. Because trial sorption studies showed immeasurable association of PCE with the B3 soil, treated material from this horizon was not evaluated for PCE sorption. The results of both TCB and PCE sorption with peroxide treated solids are similar to those with base-extracted material. For all three soil horizons, the hydrogen peroxidetreatment led to greater reduction of both SOM and K d than did the base treatment,

Table 4 Organic carbon content and sorption distribution coefficients before and after pre-treatment Soil Pre-treatment a horizon

Percent organic carbonb Žg Cr100 g solids.

A

0.536 w0.040x y 0.218 w0.027x 41% 0.144 w0.008x 27% - 0.005 -1% 0.098 w0.006x y 0.051 w0.002x 52% 0.036 w0.002x 37% 0.009 w0.0007x 9% 0.063 w0.004x y 0.058 w0.003x 92% 0.055 w0.003x 87% 0.011 w0.0006x 17% - 0.005 -8%

EB

B3

a

Untreated 1 N NaOH Wash 30% H 2 O 2 Treatment 15 min Heat Trtmnt Untreated 1 N NaOH Wash 30% H 2 O 2 Treatment 15 min Heat Trtmnt Untreated 1 N NaOH Wash 30% H 2 O 2 Treatment 15 min Heat Trtmnt 30 min Heat Trtmnt

Percent 1,2,4-TCB of K dc Žmlrg. initial org. carbon 15.3 w1.6x Ž10. 13.9 w1.9x Ž4. 5.62 w0.61x Ž10. - 0.03 1.46 w0.19x Ž9. 1.39 w0.17x Ž3. 0.71 w0.14x Ž3. - 0.03 0.18 w0.05x Ž10. 0.80 w0.22x Ž5. 0.78 w0.07xŽ10. 0.56 w0.21x Ž4. - 0.03

Percent PCE of K dc Žmlrg. untreated TCB Kd y 91% 37% - 0.2% y 95% 49% - 2% y 440% 430% 310% -17%

Percent of untreated PCE Kd

2.56 w0.29x Ž8. y 1.23 w0.20x Ž4. 48% 0.67 w0.07x Ž8. 26% - 0.03 -1% 0.14 w0.07x Ž10. y 0.21 w0.09x Ž6. 150% 0.14 w0.05x Ž11. 100% - 0.03 - 21% - 0.03 y NA y NA y NA y NA y

Treatments as described in text. Average wStd. Dev.x of four to five replicate measurements of soil organic carbon. Method detection limit estimated as roughly 0.00005 or 0.005%. c Average wStd. Dev.x Žno. of replicates.; NA s not analyzed. Untreated soil data are from Table 2, repeated here for comparison. b

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although the effect of these two treatments on K d was quite similar in horizon B3 ŽH 2 O 2-treated K d s 0.78 mlrg versus 0.80 mlrg with base-treated.. The 9508C heat treatment was able to remove SOM to the method detection limit Ž- 0.005% carbon. in all three soil horizons treated; however, smaller sample size and longer treatment time were required for complete SOM removal from the B3 soil. For all three of these heat-treated samples, K d was also reduced to below detection limits. More specifically, the average K d-values of three replicate samples were below 0.03 mlrg Žs MDL at the S:W ratio of roughly 1.0 grml. for all three soil horizons after SOM removal. An interesting additional observation was obtained when a 60-g B3 sample was heated for only 15 min at 9508C. In this case, the heat treatment removed only 83% of SOM. Triplicate measurement with the resulting B3 material Žcontaining about 0.011% organic carbon. showed a significant uptake of TCB over a 30-day equilibration period Žmeasured K d of 0.56 " 0.21 mlrg.. Consequently, this material had one of the highest implied K oc values of any material studied, second only to the base-treated A horizon material. 5. Discussion For consistency in comparing K d among horizons, we use the K d s obtained at low S:W ratio and low equilibrium concentration, at the highest equilibration time evaluated. Table 5 Apparent K oc -values for TCB and PCE with Duke Forest soil horizons and treated soils 1,2,4-Trichlorobenzene

Tetrachloroethene

Kd Žmlrg.

K oc Žmlrg.

Kd Žmlrg.

K oc Žmlrg.

w K oc TCBxr w K oc PCEx

2850 2200 1500 1200 640 500 290

2.6 0.42 0.14 0.15 - 0.1 mlrg b - 0.1 mlrg b - 0.1 mlrg b

480 220 140 120 y y y

5.9 10. 11. 10. y y y

Base-treated solids (1 N NaOH Wash): A 0.218 14. 6400 EB 0.051 1.4 2700 B3 0.058 0.80 1400

1.2 0.21 not analyzed

560 410 y

11. 6.6 y

Hydrogen peroxide treated solids (30% H2 O2 ): A 0.144 5.6 3900 EB 0.036 0.71 2000 B3 0.055 0.78 1400

0.67 0.14 not analyzed

460 390 y

8.4 5.1 y

Soil horizon

O.C. Ž%.

a

Untreated soil horizons: A 0.536 15.3 E 0.191 4.3 EB 0.098 1.5 BE 0.127 1.5 B1 0.099 0.63 B2 0.079 0.39 B3 0.063 0.18

a

O.C.sOrganic Carbon; data for O.C. are from Table 1 and Table 4; K d data are from Table 2 Žvalues in italics. and Table 4. b Detection limit estimated at 0.1 mlrg for the low S:W used.

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These data, presented in italics in Tables 2 and 3, are included to two significant figures as part of Table 5. As evident from the table, K d decreases monotonically with depth, with the mean value for TCB in horizon A about two orders of magnitude greater than that observed in horizon B3. A similar trend in PCE K d data is observed, with highest K d in horizon A and lower values in the deeper horizons. In the sections which follow we discuss some of the potential causes for this behavior. 5.1. f o c Õariability with depth One obvious cause for decreasing K d with depth is that the lower soil horizons are lower in organic carbon content Ž f oc , Table 1.. If partitioning into organic carbon is the dominant mechanism of sorption and if K oc values are uniform among horizons, we might expect normalization by f oc to explain all of the depth variation. In Table 5 we provide such normalization by calculating ‘apparent’ K oc -values for the various soil horizons Ž app K oc s K drfoc . and for the treated solids. Because it includes any mineralrelated effects as well as organic partitioning, app K oc is not necessarily a mechanistic parameter. Rather, it is used here simply as a means of comparing observed values of K d to those that might be predicted from a simple partitioning model. As evident from Table 5, app K oc also decreases monotonically with depth in the soil profile—the apparent K oc of TCB for horizon A is an order of magnitude greater than that for the B3 horizon, and similar decreases with depth are observed for PCE. Treated soils show much higher app K oc than untreated material for all three horizons evaluated, but the trend of decreasing app K oc with soil depth is maintained after treatment. The situation is clearly illustrated for the case of TCB in Fig. 3, where TCB- app K oc is plotted

Fig. 3. Plot of app K oc -TCB versus soil horizon depth for untreated and treated materials from Duke Forest. Sorption data are from Table 5.

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against the horizon depth. Overall, these results make it quite evident that change in f oc is not the only factor affecting HOC sorption variability in these Duke Forest materials. In this regard, our results agree with those of Ainsworth et al., 1989, who found that a neutral but polar nitrogen-heterocyclic compound Žcarbozol. sorbed much less strongly to the organic carbon of deeper ŽC-horizon. soils. Despite the over ten-fold range in TCB- app K oc of the various materials studied, the ratio of values between TCB- app K oc and PCE- app K oc remains fairly consistent Žbetween 6 and 11. for all sorbents, and within expectations on the basis of K ow . More specifically, logŽ K ow . has been estimated to be 4.0 for TCB ŽSchwarzenbach et al., 1993. and between 2.88 and 3.4 for PCE ŽBall and Roberts, 1992; Schwarzenbach et al., 1993., yielding a TCB-K ow rPCE-K ow ratio of between 3.5 and 10.7, in very good agreement with the ratios shown in Table 5. The consistency of the ratios in Table 5 is somewhat remarkable, with the comparatively low value for horizon A possibly providing further evidence of nonequilibrium with TCB and that material. These results are consistent with a hypothesis that sorption is dominated by hydrophobic interactions, as further supported by the approximate linearity of the sorption isotherms ŽTable 3.. Although hydrophobic partitioning to mineral surfaces is possible, such adsorption does not appear to be occurring in the deeper soil horizons, despite their relatively high clay content and BET surface area ŽTable 1.. In fact, the K d values observed in the B horizon material are significantly less than what would be expected solely on the basis of adsorption to clean mineral surfaces—i.e., with application of Eq. Ž1. and assuming no SOM contribution to sorption. In this regard, Backhus ŽBackhus, 1990. studied mineral adsorption of TCB and other hydrophobic solutes with clean kaolin and reported a K SSA value on the order of 10y4 .6 Žmolrm2 .rŽmolrl., or 0.025 mlrm2 . Assuming a K SSA value of 0.025 mlrm2 for the B3 material and further assuming that the entire BET surface area is available for adsorption, we would predict a K d of 0.9 mlrg for this horizon. This contrasts with the observed K d of only 0.18 mlrg on untreated material and implies that some of the surface may be inaccessible, either because of SOM coverage, tight mineral aggregation Žas might be facilitated by the iron colloids., or both. Interestingly, the 0.9 mlrg K d estimation agrees reasonably well with horizon B3 TCB-K d values observed after base- or peroxide treatment Žapproximately 0.80 mlrg.. Because treatments did not remove all organic matter and because all of the inorganic surface may still not be exposed, this agreement is most likely coincidental. Although the treated sample results do not allow us to separate the individual contributions of mineral surface area and remaining SOM, the data do indicate that the in-situ Žuntreated. SOM and mineral surfaces in the deeper horizons are comparatively ineffective as HOC sorbents. We speculate that the improved sorption after treatment may reflect the removal, destruction andror chemical alteration of hydrophilic or polar organic moieties that mask potential sorption sites. Alternatively Žor additionally., the treatments may better expose mineral surface andror SOM by breaking up mineral aggregation. Overall, the results with both treated and untreated soils lead us to conclude that the reduction of app K oc in the deeper horizons must be attributable to some combination of the following two factors: depth-related changes in organic matter composition, as reflected by the greater FA:HA ratio in these deeper horizons, and variability in the

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accessibility of the sorbing solid phases to the aqueous HOCs, as might be affected by the increasing clay and iron content with depth ŽTable 1.. 5.2. SOM Õariation with depth The more hydrophilic organic molecules and bacterial degradation products are expected to move preferentially down the soil profile, together with complexed metal species and colloidal size mineral weathering products Ži.e., clay particles.. Because fulvic acid ŽFA. is by definition more soluble than humic acid ŽHA. under the low pH conditions of these soils, it is to be expected that FA:HA ratios will be elevated in the illuvial zones. Additionally, microbial breakdown of SOM should continue to occur at all depths, also leading to a higher propensity of the smaller molecular weight FA at greater depth. In fact, increases in FA:HA ratio with soil depth are commonly reported ŽMcKeague, 1968; West et al., 1994., and this ratio has been proposed as one criterion upon which to characterize and differentiate A and B subsurface horizons ŽMcKeague, 1968.. Complete absence of an identifiable HA fraction in the lower horizons, as observed in the Duke Forest, has not been as commonly reported. The increasing dominance of operationally-defined FA with soil depth ŽTable 1. suggests that the SOM of these deeper horizons may be of greater polarity than that in the overlying material and is qualitatively consistent with our observation of declining app K oc with depth. In considering the ability of HOCs to partition with the SOM, it is useful to compare our results to those from published linear regressions of logŽ K oc . to logŽ K ow .. Numerous regressions of this type have been developed in prior work, as summarized elsewhere ŽBall and Roberts, 1991a; Lyman, 1982; Schwarzenbach et al., 1993.. Using a correlation proposed by Curtis et al., 1986, that was developed largely on the basis of prior literature data with relatively nonpolar chlorinated compounds of the type used here, we estimate a K oc -value of 2800 mlrg for TCB Žlog K ow s 4.0. and between 260 and 790 mlrg for PCE Žlog K ow between 2.9 and 3.4.. Other published regressions yield somewhat different results, but estimated values generally fall within a factor of three of these estimates ŽNjoroge, 1994.. These estimates of TCB-K oc are in reasonably good agreement with observed values in the shallower A and E soil horizons, as well as with results on treated soils. However, deeper horizon values are considerably below these estimates. In Fig. 4, we present a summary of the calculated app K oc for TCB, together with values calculated from other published observations of TCB with subsurface soils and sediments ŽBanerjee et al., 1985; Chiou et al., 1983; Lee et al., 1989; Paya-Perez et al., 1991; Rebhun et al., 1992; Schwarzenbach and Westall, 1981; Weber et al., 1992.. The horizontal line on the figure is the K ow -based estimate of 2800 mlrg. The figure illustrates several points. First, the simple partitioning model is generally subject to errors of an order of magnitude or greater, even for a comparatively nonpolar HOC such as TCB. Second, the soil horizons at Duke Forest show an especially large amount of spread, roughly equivalent to that observed in the nonlinear systems studied by Weber et al., 1992. Third, 1,2,4-TCB sorption by the SOM of deeper Duke Forest materials is below that observed in any of these prior studies with other subsurface materials. As evident from Table 5, the estimated app K oc values for the three lowest horizons are roughly 5 to 10 times lower

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369

Fig. 4. Plot of app K oc -TCB versus organic carbon content of soil or sediment, using f oc and K d values obtained from the literature and from this study. `sdata from Banerjee et al., 1985; Chiou et al., 1983; Lee et al., 1989; Paya-Perez et al., 1991; Rebhun et al., 1992; Schwarzenbach and Westall, 1981; I s nonlinear data from Weber et al., 1992. Horizontal line is based on a regression equation of log K oc s 0.92 log K ow -0.23 ŽCurtis et al., 1986. and a log K ow of 4.02 for TCB ŽSchwarzenbach et al., 1993.. Squares represent data obtained from nonlinear isotherms, for which K oc was calculated at the upper and lower end points of the range of aqueous concentrations studied ŽWeber et al., 1992.. Arrows suggest uncertainty for porous silica and aluminum oxide samples, where f oc was reported - 0.0001 ŽSchwarzenbach and Westall, 1981..

than the prediction, despite the fact that these horizons have comparatively high clay content and specific surface areas. 5.3. Mineral-related effects on

app

K oc

Several investigators have observed that the uptake of HOCs by commercial humic and fulvic acid components adsorbed to oxides of iron and aluminum is less than the uptake by the same organic matter components in free solution ŽLaird et al., 1994; Schlautman and Morgan, 1993a.. In this regard, Schlautman and Morgan, 1993a observed a roughly 2-fold reduction in perylenerHA K oc after HA adsorption to aluminum oxide at pH 4. Reductions observed for perylenerFA K oc were more substantial, with negligible perylene sorption by the sorbed FA. These investigators hypothesized that the strong ligand exchange association of the macromolecules with mineral surfaces could result in the SOM being unable to make the conformational changes necessary to bind the HOC, and especially so for fulvic acid. Additionally, we speculate that the kaolinite, iron Žhydr.oxide solids, and SOM in these low pH soils may result in tightly aggregated microenvironments where access of aqueous-phase HOC to sorption sites may be prevented, or at least very severely rate-limited ŽHolmen and

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Gschwend, 1997.. In particular, at low pH, iron Žhydr.oxides and the aluminol surfaces of kaolinite will be positively charged and can tightly associate with both the anionic functional groups of the SOM and negatively charged mineral surface sites ŽRyan and Gschwend, 1992.. The latter will include mineral edge sites as well as silanol surfaces of quartz and kaolinite ŽRyan and Gschwend, 1992; Sposito, 1984.. The decline in app K oc with soil horizon depth may therefore relate to increasing iron and clay content at these depths, combined with associated effects on the conformation and accessibility of the SOM. Because iron oxides correlate positively with clay minerals in these soil horizons, it is difficult to assess the independent contributions of iron and clay. Also, because both components increase with soil depth, it is difficult to isolate the mineral effects from those associated with the observed compositional changes in the extractable SOM. Unfortunately, the studies with treated solids are also ambiguous in these regards. For each soil horizon, the app K oc of the treated material Žshown in Table 5. is considerably greater than that of the parent material. The increase in app K oc after treatment is presumably related to a combination of Ž1. changes in the composition of the remaining SOMrmineral complexes that render them better sorbents; Ž2. removal of hydrophilic organic matter from potentially adsorbent mineral surfaces; and Ž3. disaggregation of mineral aggregates, thereby increasing accessibility to potential SOM and mineral sorption sites. In general, our results are consistent with those of prior investigators, who have also observed higher app K oc values for various HOCs after base extraction ŽGarbarini and Lion, 1986; Rutherford et al., 1992; Weber et al., 1992., peroxide oxidation ŽShin et al., 1970; Walker and Crawford, 1968., and partial SOM removal by heat ŽWeber et al., 1992.. Unfortunately, however, neither the prior work nor our current study has been able to characterize the humin component of SOM in a manner that would allow us to differentiate among the alternative mechanisms noted above. The sorption observed with base- and peroxide pre-treated B3 solids is reasonably consistent with expectations for clean kaolin surfaces ŽBackhus, 1990.. However, the app K oc of treated materials is also consistent with a hypothesis of SOM-dominated sorption, since the measured app K oc of the treated soils is in close agreement with prior K oc values in the literature ŽFig. 3.. Finally, the negligible sorption after near complete oxidation of SOM at 9508C ŽTable 5. is an indication that the ‘clean’ heat-altered mineral surfaces do not adsorb TCB appreciably. However, the severe heat treatment undoubtedly altered the mineral structure and composition in important ways, such that a conclusion of negligible sorption by the original mineral surfaces cannot be inferred. 5.4. Regression of TCB data against soil composition We have further investigated the observed variability of sorption at this site through the use of multivariate correlation. In this task we used only the TCB data since PCE was not sorbed by the B soils, leaving only 4 K d s to regress for PCE. Even in the TCB data there are only 7 K d values, which severely limits the number of regression variables one can use while retaining statistical reliability. Nonetheless, the results are

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Fig. 5. Parity plots comparing measured TCB-K d values to those predicted by several models. Ža. Regression Eq. Ž1. with no mineral contribution: K oc s1210 mlrg. Žb. Regression Eq. Ž2.: K oc-shallow s 3600 mlrg; K oc-deep s 420 mlrg.

suggestive. We present the major result here while referring the reader elsewhere for a thorough discussion of alternative regression forms ŽNjoroge, 1994.. Fig. 5a shows a simple correlation of K d against f oc in each soil, assuming a simplified form of Eq. Ž1. with no mineral contribution to sorption. Clearly there is a systematic error in this correlation. Because of the nature of the deviations, correlation with inclusion of K SSA as a second parameter Žfollowing Eq. Ž1. resulted in a higher K oc Ž2360 mlrg., offset by a negative coefficient for the mineral surface area Žy0.037 mlrm2 .. This negative correlation with surface area is one means of simulating the reduced sorption in the deeper, high SSA horizons.

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To explore the idea that the SOM character varies with soil depth, several alternative regressions were considered, with the premise of all being that the organic matter in shallow and deep horizons does not have uniform K oc . The most successful alternative regression was achieved by assuming that SOM of the two shallowest soils comprised one type of organic matter Žhere designated as ‘oc-shallow’., as suggested by the fact that the extractable fraction was dominated by HA in these horizons. In a similar manner, this regression assumes that the SOM of the three deepest soils behaved as if it were composed entirely of a less strongly sorbing organic matter Žhere designated as ‘oc-deep’., as suggested by the fact that 100% of extractable SOM from these horizons was classified as FA. For the remaining two soils ŽEB and BE., the regression equations assume that ‘oc-deep’ and ‘oc-shallow’ occur in the same ratio as FA and HA in the base-extractable SOM fraction. Two important limitations of this approach are that the FA and HA characterization techniques used are operational definitions Žby no means indicative of molecular structure. and that the unextracted Ž‘humin’. phase could not be independently characterized to justify the assumption that it was of like character as the extracted material. However, because the humin in each soil horizon is believed to have derived from organic acids leaching downward through the soil, it may be reasonable to suppose that the FA and HA characterization method captures the ‘degree of hydrophilicity’ of the SOM. We note, however, that the inferred properties of our oc-deep fraction will also reflect any effects owing to complexation of the deeper organic acids with metals, metal Žhydr.oxides, and clay mineral phases. Mathematically, we regressed K d s K oc shallow Ž f ocyshallow . q K oc deep Ž f ocydeep .

Ž 2.

where f oc-shallow and foc-deep are mass fractions of shallow-like-oc and deep-like-oc respectively. Using our assumptions relative to the FArHA fractionation, we write: X f ocyshallow s f HA q f HA

Ž 3a .

X f ocydeep s f FA q f FA

Ž 3b .

and

where X f HA s

ž

f HA f FA q f HA

/

f humin

Ž 4a .

/

f humin

Ž 4b .

and X f FA s

ž

f FA f FA q f HA

The quantities f HA and f FA are the mass fractions that have been identified as FA and HA, respectively, with their sum representing the entire base-extractable carbon X X content. The quantities f HA and f FA are respectively the oc-shallow-like ŽHA-like. and Ž . oc-deep-like FA-like fractions of the base unextractable SOM Žhumin. in the soil. For X . is taken to be equivalent to the f oc Žand the A and E horizons, the quantity Ž f HA q f HA

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X . is taken to be zero., under an assumption that this is 100% the quantity Ž f FA q f FA oc-shallow material. Converse definitions Ž100% deep K oc . are provided for the B horizons directly through Eqs. Ž3a., Ž3b., Ž4a. and Ž4b., since f FA s 1 and f HA s 0 for those materials. Under these assumptions, the untreated K d data in Table 5 were analyzed using a multi-variate linear regression analysis ŽStatgraphics Plus Statistical software.. The software program evaluated the model unknowns Ž K oc-shallow and K oc-deep . and provided analysis of variance ŽANOVA. statistics for the full regression. Table 3 shows that, to a first approximation, the relative error in measured K d is uniform Ž10 to 30%., so the absolute error in the measured K d value varies considerably from soil to soil. For this reason, weighting factors of 1rK d measured were applied to the residual errors in all regression analyses. The effect of this was to minimize the sum of squares of the relative errors Žrather than the absolute errors. between measured and regressed results. This prevented over-weighting the results of high K d soils with their higher absolute errors. The resulting correlation of Eq. Ž2. provided a great improvement in the fit of the data over Eq. Ž1., as evident from Fig. 5b. The regressed parameters give roughly an order-of magnitude higher K oc for the shallow SOM Ž3600 mlrg. than for the deep SOM Ž420 mlrg.. These estimates of relative K oc values for shallow SOM and deep SOM are in reasonably good agreement with prior investigations using extracted HA and FA which have also shown FA to be roughly one order of magnitude less effective than HA in the uptake of HOCs ŽGarbarini and Lion, 1986; Gauthier et al., 1987; Schlautman and Morgan, 1993a; Tell and Uchrin, 1991.. The addition of soil surface area as a regression parameter to account for possible mineral sorption had little effect. More specifically, when the term Ž K SSA = SSA. was added to the right hand side of Eq. Ž2., K SSA was regressed at a negligibly low value of 0.005 mlrm2 , or roughly five times lower than found by Backhus ŽBackhus, 1990. with pure mineral phases. K oc estimates of the organic fractions were therefore only slightly affected by the 3-parameter regression Ž K oc-shallow s 3700 mlrg; K oc-deep s 160 mlrg..

6. Conclusions This work obtained a set of sorption isotherms under well controlled conditions, thus allowing a good comparison of results among seven well-characterized soil horizons. Although hydrophobic partitioning is believed to dominate sorption in these systems, variability in the soil organic carbon content could not account for the observed variability of K d . A 10-fold decrease of apparent app K oc with soil depth was observed, and we attribute this primarily to depth-related changes in the composition, conformation, and accessibility of the SOM. Extractable organic matter was increasingly dominated by FA at greater depth, and we believe that the higher FA:HA ratio in the deep soil reflects an increasing hydrophilicity of the SOM with depth. Iron oxide and clay particle size abundance also increase with depth, but the additional mineral surface area does not contribute to sorption in a simple additive way. In contrast, the minerals may serve to further reduce K oc with depth by altering SOM configuration andror accessibility. However, the relative importance of these possible mechanisms could not be tested with the data obtained.

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These results thus provide important field-based evidence on the variability of soil organic matter with depth. The data clearly demonstrate that the mineral surface area and SOM fractions present in the lower horizons are not effective for HOC sorption, especially in comparison with clean kaolin surfaces or extracted humic acids. Further studies will be needed to better elucidate the role of soil clay and mineral oxides in the sorption of HOCs in subsurface environments, and to further validate the SOM partitioning differences proposed in this work.

Acknowledgements We thank Dr. Daniel Richter ŽDuke University. for his assistance with obtaining and classifying the soil horizons. Valuable assistance with sorbent characterization was provided by Paul Heine, Daniel Markewitz ŽDuke University. and Guoshou Xia ŽJohns Hopkins University.. Dirk Young ŽJohns Hopkins University. assisted with checks of radiochemical purity. G. Xia assisted with the data analysis and figure production. Funding for this work was made available through a Fulbright Grant to B. Njoroge, administered through the Institute of International Education, and through research grants to W. Ball from the Lord Foundation of North Carolina, the National Science Foundation ŽPresidential Young Investigator Award, BCS-9157902 and BCS-929624., and EG & G Idaho, former management and operations contractor at the Idaho National Engineering Laboratory. Finally, we thank several anonymous reviewers for their helpful comments on this work.

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