Environ. Sci. Techno/. 1995, 29, 148-153
Analytical Method for the
P o k m t s in CIq=Rich Mathials RICHELLE M . A L L E N - K I N G , * " HESTER G R O E N E V E L T , A N D DOUGLAS M. MACKAY Waterloo Centre for Groundwater Research, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada
A method using dialysis tubing for phase separation was developed for measuring nonionic, hydrophobic organic compound sorption in clay-rich geologic media such as aquitards. The complications caused by nonsettling particles when centrifugation is used for phase separation are eliminated. Slurried sample was sealed in the tubing and placed into a sample bottle, which was filled with synthetic groundwater amended with the sorbate of interest. Particles are retained within the tubing during equilibration, whereas the sorbate may diffuse through the tubing yielding equal aqueous-phase concentrations inside and out. Following equilibration, the aqueous phase outside the tubing was analyzed, and sorbed mass was determined by difference. Equilibration occurred within 48-72 h. Analyte loss through the Teflonlined septa was shown to be proportional to concentration and accounted for 1 6 % of the total analyte mass when the sorbed mass fraction accounted for 26-82%. Pentane extraction of selected samples showed comparable mass recoveries in samples and soi I-fr ee controls .
Introduction Clay-rich aquitards provide naturally occurring barriers to organic contaminant migration in groundwater. Sorption and retardation of contaminants as well as low permeabilities are relied on to limit contaminant transport to underlying aquifers. Similar expectations apply when compacted clay liners are installed as barriers at waste disposal sites. Although sorption in various sandy aquifer materials has been the focus of much recent research (191, sorption in clay-rich aquitards has been determined for materials from onlytwo sites: Sarnia, Ontario, Canada (1012) and Mexico City, Mexico (13). Sorption studies with clay-rich materials are susceptible to the nonsettling particle experimental artifact (14) when typical batch methods are used which rely on centrifugation for phase separation. Nonsettling particles (NSP),such as colloids or organic macromolecules, may remain in the solution phase after centrifugation. If the NSP sorb a significant mass of the analyte of interest, analysis of the supernatant will yield an overestimate of the dissolved analyte mass, and in methods which calculate the sorbed mass by difference,the sorbed mass will be underestimated. While ultracentrifugation can minimize or effectively eliminate the effect, not all researchers have access to an ultracentrifuge. Headspace analysis is another method which can be used to eliminate the effect (15, 16). This study reports the results of nonionic, hydrophobic organic compound (HOC)sorption experiments conducted using dialysis tubing to contain slurried clay-rich samples, thus allowing solution-phase sampling without complication from NSP. Other researchers have successfully used dialysis tubing to study organic pollutant interactions with dissolved humic materials (17). Because organic compounds can diffuse through synthetic polymers, such as PTFE in the septa commonly used to seal sample bottles (181,the amount and predictability of this sorbate loss was also investigated so that data interpretation could account for this process.
Materials and Methods Sorbents and Sorbate. Clay-rich samples were acquired from four aquitards with varying properties, as shown in Table 1 (hereafter referred to by the listed source location). The samples represent a wide range of clay mineralogies and organic carbon contents. The dominant clay minerals in each ofthe samples are as follows: muscovite and chlorite in Sarnia and Borden; beidellite, muscovite, and kaolinite in Birsay; and amorphous phyllosilicates in Mexico. Synthetic groundwater, prepared based on the major ion analysis of a groundwater sample from each sample site,was sterilizedbyautoclavingprior to use. The following ion concentrations were added to the synthetic groundwaters (per liter): Sarnia, 48 mg of Ca, 32 mg of Mg, 130 mg of Na, 170 mg of SO4, 21 mg of C1, and 330 mg of HC03 COSas HC03; Birsay, 50 mg of Ca, 16 mg of Mg, 20 mg of K, 480 mg of Na, 880 mg of S01,50 mg of C1, and 260 mg of HC03 + C 0 3 as HC03; Mexico, 26 mg of Ca, 64 mg of Mg,
+
+ Present address: Department of Geology, Washington State University, Pullman, WA, 99164-2812; e-mail address:
[email protected].
148 1 ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29. NO. 1,1995
0013-936)(/95/0929-0148$09.00/0 D 1994 American Chemical Society
TABLE 1
Grain Size and Fraction Organic Carbon Content (foe) of Aquitard Sediments grain-size analysis (YO) aquitard sample Mexico City Sarnia, ON Borden, ON Birsay, SK a
f, (Oh)'
sand
3.830(0.011) 0.80 0.500 (0.008) 12.58 0.184 (0.003) 3.38 0.413 (0.009) 33.90
silt
clay
25.49 49.80 47.29 40.01
73.70 37.62 49.32 26.08
Standard error of the mean shown in parentheses; n = 4.
168 mg of K, 414 mg of Na, 490 mg C1, and 560 mg of HC03
+ COSas HCO,; Borden, as described by ref 1. Nitrogen-
and phosphorous-containing compounds were not included to inhibit microbiological growth (7). Core material was mixed in a blender for 30 s (Sunbeam Osterizer Model 10) with synthetic groundwater to create a thick slurry (appoximately 0.10 g/mL for MX, 0.33-0.38 g/mL for the others). In order to avoid chemical alteration of the clay surfaces, ultrasonication was used to disperse the samples (19, 20). The slurry was sonicated for 2 min (Model XL, Heatsystems,Farmingdale,NY), and during dispensing, the slurry was homogenized using a motorized stirrer (RZRI, Caframo, Wiarton, ON). The solids content of the slurry, used to calculate the exact weight of dry clay per sample, was determined by drying (72 h at 105 "C) aliquots of slurry dispensed at the same time as the samples. Perchloroethene (PCE) was used as a model HOC. In addition to widespreadconcem about PCE as a groundwater contaminant, it is representative of moderatelyhydrophobic compounds, recalcitrant to biotic and abiotic transformation in aerobic systems (see ref 7 and references cited therein), and has been used as a model compound in many other sorption and transport studies (refs 4, 7, 21, and 22 to name a few). [14C]PCEwas obtained from Sigma (St. Louis, MO) and used as received. At the time of use, radiochemical purity was found to be significantly less than reported by the supplier, as described further below. [14C]PCE stock solutions were prepared in methanol such that the labeled concentrations were 500-1000 dpm/mL in the batch experiments. Unlabeled PCE (analyticalgrade, BDH, Toronto, ON) was added gravimetrically to adjust total PCE concentration in the stock solutions to result in the desired aqueous sample concentrations. Gas Chromatographic Confirmation of [14C]PCEDeterminations. Pentane and octanol extraction of water samples amended with stock solution showed that a 14Clabeled, unextractable, and hydrophilicimpurity remained in solution and accounted for about 17%ofthe total activity. A similar proportion (15%) of hydrophilic 14C-labeled impurity was determined by HPLC analysis (23) for the same [l4C1PCE chemicallot. Pentane extraction of selected samples following equilibration demonstrated the essentially nonsorbing nature of the 14C-labeledimpurity. Analysis of several Sarnia samples and controls were performed by both radiochemicaland gas chromatographic (GC) methods to investigate the impact of the 14C-labeled impurity on sorption analyses. GC analysis was performed on pentane-extracted water samples with a HP 5890 Series I1 GC with a DB-624 capillary column (J&WScientific, Folsom, CA) and electron capture detector. Analysis of variance showed that corrected radiolabeled apparent Kd's (calculated from the scintillation method and corrected
assuming that the 14C-labeled impurity is completely nonsorbing) were not significantly different from the GC results and that apparent &'s calculated without impurity correction were significantly different from both the GC and the corrected-radiolabeled results at the 95% confidence level. These results support the assumptions that the pentane-extractable 14Cis PCE and that correcting the scintillation results for the 14C-labeled impurity by the method proposed herein is satisfactory. BatchTests. Precut (20cm) lengths of 29-mm diameter Spectra/Por dialysis tubing (Canlab,Mississauga, ON) were washed before use. The molecular weight cutoff (MWCO) ofthetubingwas 12000-14000 (12-14K) withone exception where 3500 (3.5K) MWCO tubing was used. The tubing, knotted at one end, was filled with the desired volume of clay slurry (1-5 mL) using a pipettor (Oxford, Monoject Scientific, St. Louis, MO), knotted again, placed into a sample bottle (16.2 mL nominal volume), and filled with synthetic groundwater. Bottles were weighed at each step of preparation for determination of solid mass and aqueous volume. The desired volume of clay slurry to add was determined from preliminary experiments to ensure that at least 30% of the analyte mass would be present in the solid phase after equilibration, which has been shown to improve the accuracy of such sorption methods (7). The final solid mass per total volume water in the vials was 0.130 f 0.003, 0.0260 f 0.0005, 0.137 f 0.002, and 0.0214 f 0.0006 g/mL, for Borden, Mexico, Birsay, and Sarnia, respectively. Soil-free control vials were prepared with dialysis tubing and water only. To evaluate absorption by dialysis tubing, one set of control vials was preparedwith water only (tubingfree). Stock solutions were added to the samples using 5- or 10-pL syringes with a Chaney adaptor. The bottles were immediately sealed with 16-mm Tegrabond Teflon-lined septa (Chromatographic Specialties, Brockville, ON). The total injected mass was confirmed by direct additions into scintillation vials containing synthetic groundwater. The sample bottles were rotated at 3 rpm in the dark during the equilibration period, with daily vortexing at medium-high speed for 30 s to enhance mixing. After equilibration, 2.00 mL of the aqueous phase from outside the dialysis tubing was collected and counted twice in a scintillation counter (Packard 1900 CA Tricarb) for 10 or 20 min. An external standard (lS3Ba)was used to convert measured counts per minute to disintegrations per minute. The time to equilibrium was estimated for the Borden material using a series of sample bottles equilibrated for various times up to 11 days. To confirm that equilibrium had been reached in these studies, a separate experiment was conducted for a total of 45 days, using the glass ampule method of Ball and Roberts (7) to limit PCE loss from the sample container. To allow mass balance calculations, pentane was added to several sample bottles immediately after the initial aqueous sample was removed. The sample was shaken for at least 10 min at 360 rpm, then for at least 3 days at 3 rpm. Pentane and aqueous phases were analyzed by scintillation counting following extraction. Data Analysis. Sorbed mass was determined as the residual between the initial mass added, the mass in solution and gas phases after equilibration, and mass loss through the septum. Initial sorbate mass added (Mol was adjusted for loss through the septumusing the followingrelationship: VOL. 29, NO. 1 . 1 9 9 5 /ENVIRONMENTAL SCIENCE &TECHNOLOGY
149
M,' = M, - K,C
(1)
where 4 is the ratio of sorbate mass loss to equilibrium solution concentration determined from the soil-free controls (as discussed later), and C is the solution concentration. Following the lead of Young and Ball (23)but deriving equations directly pertinent to our experiment conditions, the 14C-labeledimpurity was assumed to be hydrophilic and nonsorbing, and the correction for its presence was calculated as follows
M , = M," (1 - X)
(21
0
Q
A 0
s
r;; 0.8
tn v)
and (3)
where x is the fraction of 14C-labeledimpurity, Vwis volume of water phase, and C " and M" reflect the apparent PCE concentration based on the combined activity of 14C-labeled impurity and [14C]PCE. Although the bottles contained no headspace, small air bubbles were trapped in the dialysis tubing when the clay slurry was added, and thus, gas-phase partitioning was accounted for using a literature value for the Henry's law constant (24) at ambient laboratory temperature (22-23 "C). Gas-phase partitioning is later shown to account for a low proportion of PCE mass in these experiments. Accounting for PCE mass in gas bubbles and diffused through the septum, the remaining PCE was attributed to sorption (modified from ref 7):
9'
[M,' - C(H&
+ VJ1
(4)
ms
where q is the sorbed mass, H, is the dimensionless Henry's constant, V, is the volume of gas phase, and m, is the dry mass of solids. The volume of the dialysis tubing, determined from its measured density (1.889 f 0.017 g/cm3, n = 4) and mass, was used in the calculation of exact volumes of gas and water. The dry weight of a clean 20-cm length of 12000-14000 MWCO tubing was 0.383 f 0.001 g (n= 10). An apparent Kd (Le., apparent linear sorption distribution coefficient) was defined by ref 7 as
Kd = 9IC
(5)
Results and Discussion PCE Loss in Controls. Solution 14Crecoveries in soil-free control vials after equilibration were 92.7 f 0.4% (n = 81, consistently less than loo%, suggesting PCE sorption to vial materials or loss. Consistency between activity recovered in pentane-extracted control vials (93.4 & 1.1%, n = 8) and solution recoveries before extraction suggests that the mass not recovered had been irreversiblylost from the vial, presumably by diffusion through the septum. Recovered PCE mass normalized to initial mass in control vials is shown for 20-mL controls over time in Figure 1. After 24 h, the rate of mass loss was essentially constant over the tested time interval. The relative mass lost from 20-mL vials (20-mm septa) was greater than from 16-mL vials (16-mm septa) at a particular time. For example, the relative mass loss after about 3 days was 10.3 1.4% (n= 6) and 7.8 f 1.6% (n = 6) from 20- and 16-mL vials, respectively. Therefore, to minimize mass loss, 20-mL vials
*
150
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 1, 1995
0.6
0
100
200
300
Time [hr] A
tubing-free
0
soil-free
FIGURE 1. [14C]PCEmass relative to initial mass in soil-free and tubing-free control vials over time for 20" vials.
.-0 CI
e 8 E: W =8
E 1000
2
100 lE4\ IO!
1
0.001 0.01
0.1
1
10
100
1000
PCE loss [ug] FIGURE 2. [''CIPCE mass loss from soil-free controls (16 mL) over a wide Concentration range. Samples taken after 3 days. Mass loss defined in text. 1(1 = 1.41 x L.
were used onlyfor equilibration time tests, and 16-mLvials were used for the rest of the study. Mass loss at a particular sampling time was proportional to concentration (Figure2). The slope of alinear regression of log transformed datawas 1.004,not significantlydifferent from 1.00. Therefore, the linear slope was determined by averaging the apparent slopes for each data point collected (7).
The fact that mass loss is proportional to concentration and increases with time suggests a diffusive mechanism for PCE loss from the vials. PCE diffusion through septum materials is expected based on previous research (18). Performance ofDialysis Tubing. Mean activity recovery in soil-free control vials with dialysis tubing was 99.1 f 1.5% (n = 6) of that for the tubing-free vials at 72 h, and 98.9 k 2.1% for 23 samples collected between 1 and 300 h (Figure 1). Comparable recoveries demonstrate that the dialysis tubing did not absorb significant PCE mass. Nonsettling particle effects were expected for the samples used in this study, Since the particles suspended in the
TABLE 2
Effect of Using Different Molecular Weight Cutoff (MWCO) Dialysis Tubing on Apparent Kd for Aywitanl Samples ~
1 m 1 4 o o oMWCO
3500 MWCO sample Birsay, SK Borden, ON Mexico City Sarnia, ON a
apparent & (SEW n
(0.18) (0.55) (0.98) (3.0)
8.70 15.3 41.6 151
l E 4 1 -
apparent&
(SEM) n
9.08 14.6 44.9 166
(0.42) 2 (0.20) 4 (1.8) 3 (7.8) 2
2 4 3 2
i'
SEM is standard error of the mean.
10
i 1:;;:;:
I
I
I
=;;:;:
i
I c [ug/ll 1
25
7
+
+
+
I
10
100
1000
FIGURE4. Isothermfor PCE in Borden aquitard sample. Symbols are measured data. Line is average &.
W
Y
TABLE 3 X
xc
Total 14C Recoveries for S o i l h e Controls and Aquitard Samplese
a 4+
5
1
10
100
1000
Time [hr] +
FIGURE 3. Apparent PCE Borden aquitard sample.
20ml
x
sample
n
fraction dpm fraction dpm normalized recoveredb recovery expectedbrE recoveryd
control Borden, ON Birsay, SK Mexico City Sarnia, ON
8 4 3 4 3
0.93 (0.010) 0.93 (0.014) 0.96(0.016) 0.97 (0.014) 0.92(0.016)
0.933(0.002) 0.952(0.003) 0.964 (0.003) 0.965(0.003) 0.976 (0.003)
1-00(0.010) 0.96 (0.015) 0.99 (0.017) 1 .OO (0.015) 0.94(0.017)
a Standard error of the mean shown in parentheses. Relative to initial. Based on the loss through septum as per eq 1; K = 1.4 x L. Fraction dpm recoveredfiraction dpm recovery expected.
16ml
& as function of equilibration time for
batch experiments are not expected to be transported under natural conditions in the aquitards, phase separation is necessary to achieve a sorption result pertinent to the field situation. Given that the pore size of the 3.5K tubing is 13- 15 8,it should exclude colloids and clay particles, which typically have diameters on the order of 30 A to 100 pm (25). A comparison between the 12-14K and 3.5K MWCO dialysis tubing showed no significant difference between the apparent &'s for all of the four samples tested (Table 2). Thus, it appears that the 30-35-A pore size of the 12000-14000K tubing is sufficiently small to achieve adequate phase separation. The larger MWCO (12-14K) dialysis tubing was selected for the remainder of the experiments because it is less expensive, more readily available, and easier to work with than the 3.5K MWCO tubing. Sorption in Aquitard Samples. Dispensing of samples as thick slurries minimized sample handling, and analyses of the aliquots yielded reproducible results, suggesting the slurries could be kept homogeneous during mixing. For example, the solids content of the Borden slurry was 0.330 f0.001g of dry clay to slurry (wt/wt) (n= 4). Homogeneity of the slurry was also evidenced by the reproducibility of the apparent & as discussed below. The apparent Kdwith time was determined for a Borden sample (Figure 3). In the experiments with septum-sealed bottles, equilibrationwas approximately75% complete after 24 h and was reached in 48-72 h. In the experiments
conducted in flame-sealed glass ampules, equilibrium was achieved in 24-48 h, with no change in solution concentrations for the remainder of the 45 days of the experiment (data not shown). This confirms that long-term experiments were not required to achieve equilibrium for these samples. It has been suggested that slow sorption equilibration in batch experiments with larger grain-sizematerials (sands, etc.) is due to diffusion limitations at the grain scale (3, 26). Rapid equilibration was expected in this study because of the small mean grain size. In the rest of this work, a 3-day (72 h) contact time was chosen for the experiments to ensure equilibration yet minimize PCE loss through the septum. A sorption isotherm for Borden aquitard material is shown in Figure 4. The results ofreplicate sorption analyses (Figure 4 and Table 2) demonstrate the reproducibility of the method. Despite the apparent precision of the method, careful interpretation of the data is required to ensure accurate results, as discussed below. Mass Balances and Check for Transformation. PCE sorption is determined by a mass balance approach, and therefore, it must be assumed and ensured that there are no unaccounted mass sinks in the experiments, such as transformation. The high mass recoveries (Table 3)in the extractions and the comparable results for the GC and scintillation methods demonstrate that all the PCE mass was accounted for at the end of an experiment and that significant transformation did not occur. PCE transformation products were not detected in the GC analyses. Thus, it did not appear necessary to utilize any of the various VOL. 29, NO. 1, 1995 I ENVIRONMENTAL SCIENCE &TECHNOLOGY
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111
chemical or physical methods to inhibit biological transformation, which in any case were not desirable as they may tend to affect the measured Kd (7). Sorption Calculations. Calculation of sorption included accounting for PCE loss through the septum and correcting for the nonsorbing 14C-labeledimpurity. Sorbate mass loss was established from the equilibrium solution concentration using the empirical relationship between concentration and mass loss determined from the controls (Figure2).The solution concentration was not as uniform over time in the sample vials as in the control vials. However, because the aquitard samples equilibrated relatively rapidly, the equilibrium solution concentration was expected to control diffusive mass loss. The activity recovered following pentane extraction normalized by the expected recovery (calculated accounting for diffusive mass loss) for the four aquitard samples was not significantly different from the controls (Table 31, confirming the accuracy of this method for calculating mass loss. PCE mass loss from samples was 1.5-6.1% when 28-82% of the PCE mass was sorbed. The gas-phase partitioning typically accounted for less than 1-3% of the total PCE mass because the trapped air bubbles comprised a relatively small proportion of the total bottle volume. In order to accurately estimate diffusive loss for interpreting sorption results, the relationship between mass loss and concentration should be determined empirically for the experiment system and compound of interest. The typical set of controls, spanning a range of concentrations, can be used to generate the empirical mass loss relationship, as shown in Figure 2. The potential for bias due to use of Teflon-lined septa has been noted (6,27)and, in previous studies (refs 1, 10, and 22, for example), sorption results have typically been corrected based directly on sorbate mass loss in an equivalently amended control. Such a method can result in significant bias in the results if the mass fraction sorbed is significant. Depending on the apparent & ofthe sample and the so1id:solution ratio, the equilibrium solution concentration in the sorption experiment can be either greater or less than the control. For example, in the current study, when the mass sorbed was 50-75% of the initial, the equilibrium solution concentration in samples was typically about half that of the comparable controls. The apparent Kd’s corrected for diffusive loss of PCE are 8-9% less than if the correction is made by assuming that sorbate mass loss in the experiments is the same as in controls. The difference between the results can be greater if (1) more mass is lost from the system by diffusion due to longer contact time or greater septum surface area-to-volumeratio and/or (2) a greater proportion of mass is sorbed. Although diffusion through the septum is a predictable and quantifiable mass loss mechanism, accuracy and reproducibility of the sorption analysis generally diminish as the mass loss from the vial approaches the masses in the solution and solid phases. Therefore, septum-sealedbottles are only appropriate for relatively short-term sorption experiments in which the diffusive loss of sorbate mass is a small percentage of the total sorbate mass.
Summary and Conclusions Fine-grained materials, such as the clay-rich aquitard samples tested in this study, are susceptible to nonsettling particle effects when sorption is measured by traditional methods relying on centrifugation for phase separation. 152
ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 1, 1995
Nonsettling particles, such as day colloids or organic macromolecules, are not expected to be transported within the matrix of these aquitard materials and can lead, therefore, to experimental artifacts in batch sorption analyses. Dialysis tubing was used in this study to eliminate complications from nonsettling particles in PCE sorption tests with four distinct clay-rich aquitard samples. The dialysis tubing did not absorb PCE. Reproducibility of the method was demonstrated. In this study, 12000-14000 MWCO tubing with pores of 30-35 A was sufficient to eliminate measurable effects of nonsettling particles on sorption distribution coefficients. Dispensing of samples as slurries resulted in reproducible sorption and other measurements with a minimum of sample handling. Loss of sorbate mass from control vials through Teflon-lined septa was shown to be linearly proportional to equilibrium solution concentration. Sorbate mass loss from sorption experiments in the same viallsepta systems could be estimated from the equilibrium concentration and the empirical relationship between mass loss and equilibrium concentration derived from controls.
Acknowledgments The authors wish to thank S. O’Hannesin and Dr. R. W. Gillham for providing access to the scintillation counter and GC; S. O’Hannesin for sharing her expertise and experience; and Dr. W. P. Ball for relevant information and discussions concerning 14C-labeledimpurities in the PCE. B. Parker, M. Broholm, and Drs. D. Rudolph, V. Remenda, and M. Mazari-Hirim generously contributed the aquitard samples used in the study. Funding for this research was provided by the University Consortium Solvents-inGroundwater Research Program, which is sponsored by The Boeing Co., Ciba-Geigy Corp., General Electric Co., Eastman Kodak Co., Laidlaw Environmental Services Ltd., Mitre Corp.,the Natural Sciences and Engineering Research Council of Canada, and the Ontario University Research Incentive Fund.
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(18) Reynolds, G.W.; Hoff,J. T.; Gillham, R. W. Environ. Sci. Technol. 1990,24, 135-142. (19) Hunter, C. R.; Busaca, A. J. Soil Sci. SOC.Am. J. 1989,53, 12991302. (20) Edwards, A. P., Bremner, J. M. J. Soil Sci. 1967, 18, 47-63. (21) Mackay, D. M.; Freyberg, D. L.; Roberts, P. V. WaterResour.Res. 1986, 22, 2017-2029. (22) Ptacek, C . J.; Gillham,R. W. J. Contum. Hydrol. 1992, 10, 119158.
(23) Young, D. F.; Ball, W. P. Environ. Prog. 1994, 13 (l), 9-20. (24) Gossett, J. M. Environ. Sci. Technol. 1987, 21, 202-208. (251 Stumm, W.; Morgan, J. J. Aquatic Chemistry: An Introduction Emphasizing Chemical Equilibria in Natural Waters, 2nd ed.; John Wiley & Sons: New York, 1981; p 647.
Ball, W. P.; Roberts, P. V. Environ. Sci. Technol. 1991,2512371249. Ball, W. P.; Roberts, P. V. In Organic Substances and Sediments in Water,Vol.2. Processesanddnulytical; Baker, R. A,, Ed.; Lewis Publishers: Chelsea, MI, 1991; pp 273-310.
Received for review April 26, 1994. Revised manuscript received August 31, 1994. Accepted September 15, 1994."
ES940258N @Abstractpublished in AdvunceACSAbstracts, November 1,1994.
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