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Published in Soil Sci. Soc. Am. J. 68:89-95 (2004).
© 2004 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA

DIVISION S-2—SOIL CHEMISTRY

Evaluation of Adsorbed Arsenic and Potential Contribution to Shallow Groundwater in Tulare Lake Bed Area, Tulare Basin, California

S. Gao*,a, R. Fujiib, A. T. Chalmersc and K. K. Tanjia

a Dep. of Land, Air, and Water Resources, Univ. of California, Davis, CA 95616
b U.S. Geological Survey, Sacramento, CA 95819
c U.S. Geological Survey, Montpelier, VT 05602

* Corresponding author (sugao{at}ucdavis.edu).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Elevated As concentrations in shallow groundwater in parts of the Tulare Basin, California, are a concern because of potential migration into deeper aquifers that could serve as a source of future drinking water. The objectives of this study were to evaluate adsorbed As and the potential contribution to groundwater using (i) isotopic dilution, (ii) successive extraction with an electrolyte solution resembling the pore-water chemical composition, and (iii) PO4 exchange for As. Sediment samples collected from 2 to 4 m below land surface in the Tulare Lake bed area contained a total As concentration of 24 mg As kg–1. Pore water extracted under hydraulic pressure contained a total As concentration of 590 µg As L–1, which predominantly contained As as arsenate [As(V), 97%], a minor amount of arsenite [As(III), 3%], and non-detectable organic As. The isotopic dilution method [73As(V)] estimated that the concentration of adsorbed As(V) on the sediment was 5.7 mg As kg–1 at pH 8.5 and 6.7 mg As kg–1 at pH 7.5, respectively. Fourteen successive 24-h extractions with the artificial pore water released up to 57 to 61% of the adsorbed As(V) that was determined by isotopic dilution, indicating that only a portion of the adsorbed As could be released to groundwater. The phosphate-exchangeable As (0.1 M PO4, pH 8.5 or 7.5) was 63% of the isotopically exchangeable As(V). Thus, extraction of As by 0.1 M PO4 at ambient pHs is recommended as a method to determine the potential amount of As(V) on sediments that could be released to the solution phase. The overall results indicated that adsorbed As could be a significant source of As to groundwater. However, other factors that affect As transport such as the leaching rate need to be considered.

Abbreviations: AAS, atomic absorption spectrometry • DD, distilled deionized • MCL, maximum contaminant level • RCF, relative centrifugal force


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
THE TULARE BASIN is located in the southern portion of the San Joaquin Valley of California (Fig. 1) and supports more than 1.2 million ha of highly productive, irrigated agricultural land. Parts of the basin are affected by saline shallow groundwater (approximately 155000 ha had shallow groundwater within 2 m of the land surface in 1987) and thus require artificial subsurface drainage to sustain agricultural productivity (Tanji and Hanson, 1987). The drainage water and groundwater contain elevated concentrations of As, especially in the southern Tulare Lake bed area (Westcot et al., 1989). Concentrations of As as large as 870 µg As L–1 was reported in shallow groundwater in the Tulare Basin (Swain and Duell, 1993), and as much as 110 µg As L–1 in drainage water entering an evaporation pond facility near this study site (Fig. 1) (Fujii, 1988). Research conducted by U.S. Geological Survey on groundwater at this study site showed concentrations of As(V) as large as 370 and 260 µg As L–1 as As(III) (Fujii and Swain, 1995; R. Fujii, unpublished data, 1996). These elevated concentrations of As exceed USEPA's criteria for both aquatic life and human health.



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  		Fig. 1. The study site in the Tulare Lake Bed area.

 
Arsenic compounds are carcinogens and the maximum contaminant level (MCL) for As in drinking water has been reduced from 50 to 10 µg As L–1 to protect consumers against the effects of long-term, chronic exposure of As in drinking water (USEPA, 2001). Human health problems associated with elevated concentrations of As in drinking water in the Tulare Basin were previously noted (Valentine et al., 1979). Thus, the elevated concentrations of As in shallow groundwater in Tulare Basin are of concern if downward migration of shallow groundwater containing relatively large concentrations of As reaches untapped deeper regional groundwater that could serve as a source of future drinking water. These concerns emphasize the need to understand the processes controlling As solubility and to determine the potential contribution of As from sediments to groundwater in the Tulare Basin.

The major processes controlling As solubility in groundwater include mineral (co)precipitation/dissolution, adsorption/desorption, chemical transformations, ion exchange, and biological activity. The conditions present such as pH, EH, solution composition, and the mineralogical composition of the aquifer determine the dominant processes affecting the environmental fate of As in the sediments. Arsenic exists in several chemical forms and species in nature. In the oxidized zone in Tulare Lake bed area, As occurs mainly in the oxidized form as arsenate [As (V)]. Sediments from surface to about 30 m below the land surface at this study site were extracted using a 0.1 M K2HPO4, pH 8 solution to estimate ligand-exchangeable As, a method previously used to estimate adsorbed selenite [Se(IV)] (Fujii et al., 1988; Chao and Sanzalone, 1989; Fujii and Burau, 1989). The results showed that 10 to 39% of total As in the sediments is phosphate extractable, suggesting that adsorption may be an important mechanism controlling As solubility and mobility in Tulare Lake bed sediments (Fujii and Swain, 1995).

Phosphate plays an important role in As solubility and mobility in the environment because of their chemical similarities. Addition of phosphate enhanced As release and movement from arsenate-contaminated soils (Woolson, 1973; Peryea, 1991; Melamed et al., 1995; Peryea and Kammereck, 1997). There is strong competition between phosphate and As(V) for adsorption sites in soils or on oxide surfaces (Hingston et al., 1971; Roy et al., 1986a,b; Jain and Loeppert, 2000). They both adsorb on soil components in an analogous manner, that is, form inner-sphere complexes. Phosphate was considered more strongly adsorbed than arsenate by sediments rich in Fe and Al oxides (Wauchope and McDowell, 1984). Arsenic(V) and PO4 single-anion adsorption envelopes on goethite and gibbsite were similar with substantial adsorption across a wide pH range (Manning and Goldberg, 1996). There was, however, evidence that some sites were uniquely available for adsorption of either As(V) or P. Because the arsenate molecule is larger than PO4, it interacts more strongly with some of the OH groups on goethite that may not be able to be exchanged by PO4 (Lumsdon et al., 1984). Further, increasing As mobility by PO4 exchange are also affected by PO4 concentration, pH, reaction time that determine the rates of As(V) desorption, and leaching rate (Barrow, 1992; Darland and Inskeep, 1997; Jain and Loeppert, 2000). Research results indicate that using PO4 exchange to evaluate the mobility of adsorbed As(V) may be a valid technique, but the relationship between PO4 exchangeable As(V) and As mobility in natural sediments is not well defined.

We hypothesized that sorption processes control the solubility of As in shallow groundwater in the southern Tulare Lake bed area. The objectives of this study were to evaluate the quantity of adsorbed As on sediment and the potential contribution to groundwater under oxidizing conditions using (i) isotopic dilution, (ii) successive extraction with an electrolyte solution resembling the pore-water chemical composition, and (iii) PO4 exchange for As.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Sampling and Analyses
Intact sediment core samples were collected from a depth of 2 to 4 m below land surface at Site 31D in the Tulare Lake bed area (Fig. 1). Groundwater was sampled using a closed flow-through system to prevent atmospheric gas contamination from a nearby observation well that had a screened interval from 3 to 6 m below land surface. Measurement indicated that the water at this depth was oxidized (Pt-electrode redox potential of +300 mV relative to H2/H+). Pore water was extracted under hydraulic pressure from intact sediment core samples and analyzed as described below. Total As concentration was 588 µg As L–1, 97% as As(V). Representative subsamples of the sediment cores were air-dried for characterization and the remainder of the sediment cores was kept moist and sterilized using {gamma}-radiation at the Crocker Nuclear Laboratory, University of California at Davis, to prevent microbial transformation of As in the samples.

Major cations and anions in the pore water were analyzed by atomic absorption spectrometry (AAS) and high performance liquid chromatography (HPLC). The trace elements were analyzed using inductively coupled plasma atomic emission spectroscopy (ICP-AES). The cation-exchange capacity of the sediments was determined by the Ba saturation-Ca replacement method at pH 7.0 conditions (Janitzky, 1986); organic matter content was determined by the dichromium reduction method (Nelson and Sommers, 1982); and CaCO3 content was determined by the gravimetric method (Richards, 1954). Particle-size fraction analysis was done using the pipette method (Gee and Bauder, 1986).

Clay mineralogy was determined by X-ray diffraction (Kunze and Dixon, 1986; Whittig and Allardice, 1986). Total surface area was measured by the EGME method (Carter et al., 1986). Iron, Mn, and Al oxides were determined by extraction with ammonium oxalate buffer (pH 3.0) after removal of CaCO3 using 1.0 M ammonium acetate pretreatment (pH 5.5) (Loeppert and Inskeep, 1996) as well as citrate-dithionite extraction procedure (Holmgren, 1967). The extracts were analyzed by ICP-AES. The U.S. Geological Survey determines the total As concentration in the sediments by using acid (HNO3–HCl-HClO4–H2SO4–HF) digestion and hydride generation atomic absorption spectrometry (HGAAS) method, a modification of the method described by Welsch et al. (1990). The total As concentration in the sediment was 24 mg As kg–1.

Pore waters were analyzed for As species, including organic forms, using a modified hydride generation with cold-trapping AAS technique (Masscheleyn et al., 1991; Crecelius et al., 1986). This method accurately detects inorganic As(V), As(III), methanearsonic acid, and dimethylarsinic acid. No organic As forms were detected. Therefore, only inorganic As(III) and total As(III + V) were determined using the methods described below in the Analytical methods for As(III) and As(III + V) section.

Chemical composition of the pore water was analyzed (Tables 1 and 2). The pore water was supersaturated with respect to calcite and dolomite as determined by geochemical speciation model MINTEQA2 (Allison et al., 1991). The pore water and sediment saturation paste pH were both 8.5. However, the pH of the groundwater sampled from a nearby observation well with a screened interval from 3 to 6 m below land surface was pH 7.5. Both pH conditions were chosen in this study. The sediment sample used in this study was clay in texture and dominant clay mineral was smectite (Table 3).


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  		Table 1. Pore water and artificial pore-water chemical composition.

 

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  		Table 2. Trace elements in pore waters.

 

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  		Table 3. Selected properties of sediment samples used in this study.

 
Estimation of Adsorbed Arsenic using Arsenic-73 Radioisotope Dilution Method
Arsenic-73 was purchased from Los Alamos National Laboratory (Los Alamos, NM) in the form of As(V) in 0.1 M HCl and the specific activity was 4.736 x 107 Bq mL–1 (1.28 mCi mL–1) at the time of purchase. The solution contained 0.057 µg 73As L–1 and 2.025 µg 75As L–1. The half-life of the 73As by electron capture to 73Ge is 80.3 d.

A solution of similar chemical composition to the pore water (Table 1) was made with reagent-grade chemicals and used for all experiments. The artificial pore water was made up by dissolving reagent-grade salts: 0.638 g NaHCO3, 0.877 g NaCl, 0.06 g MgSO4, 0.987 g Na2SO4, and 0.075 g CaSO4 into 1 L distilled deionized water (DD H2O). The final constituent concentrations are shown in Table 1. After dissolving all of these salts, the solution was kept in a container for about a week and shaken occasionally. Finally, the solution was filtered through 0.45-µm filter membrane to avoid any carbonate particulate interference for the experiments. The cap was sealed with Teflon tape to avoid absorbing CO2 from air causing changes in alkalinity.

Sediment suspension was initially prepared with a sediment/solution ratio of near 1:10 (w/v) by adding sterilized moist sediment (equivalent to 120 g oven-dry basis) to 1140 mL of solution in a 2-L plastic container. The suspension was mixed and shaken on a reciprocal shaker for 24 h before distributing 19 mL of the suspension to 50-mL Nalgene polypropylene tubes.

One milliliter of diluted 73As from the stock solution with an activity of 3.7 x 105 Bq (approximately 10 µCi) 73As(V) was added to the 50-mL tube that contained 19 mL of sediment suspension and 2 g equivalent oven-dry sediment. The final sediment/solution ratio was 1:10. This sediment/solution ratio was arbitrarily chosen and used for all the experiment throughout this study. Blanks were also conducted by adding 1 mL DD H2O. After shaking on a reciprocating shaker at about 300 strokes min–1 at room temperature (20–23°C), the supernatant was obtained by centrifuging for 20 min at 11720 x g relative centrifugal force (RCF) at 20°C using a temperature-controlled centrifuge and then filtered through 0.22-µm syringe filter membranes (mixed cellulose esters). Fifty microliters of the supernatant were added to Ready Caps (Beckman, Coulter, Fullerton, CA). Blanks and standards were prepared at the same time with the same procedure as the samples. After air-drying, the caps were counted on a BECKMAN LS 8000 Series Liquid Scintillation Systems (Beckman Coulter, Fullerton, CA) to determine the radioactivity. The first experiment determined the radioactivity of As in solution changes in triplicate samples that had reaction period of 3, 6, 12, 24, 36, 48, 72, and 96 h. After 24 h, the radioactivity in solution remained fairly constant (Fig. 2) . Then, a second experiment was conducted to determine the concentration of As adsorbed on the sediments at pH 8.5 and 7.5 with five replicates. After preparing the suspension with an approximate sediment/solution ratio of 1:10, the pH of the suspension was then adjusted to the desired pH (8.5 and 7.5) by adding diluted HCl with a pH auto-titrator (Mettler D21, Metterler Instrument Co., Hightstown, NJ). The suspension pH was maintained for minimum 24 h before distributing 19 mL of the suspension to 50-mL Nalgene polypropylene tubes. After adding 1 mL of DD H2O (control) or solution containing radioactive 73As, the final sediment/solution ratio of 1:10 was reached.



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  		Fig. 2. 73As(V) radioactivity decrease in solution at sediment suspension pH 7.5. Vertical bars are the standard deviation of the mean (n = 3).

 
Isotopic dilution method has been used to estimate adsorbed metals (e.g., Fujii and Corey, 1986). Estimate of adsorbed As from the radioisotope dilution method was based on the following assumption that at oxidation state V, radioisotope 73As is chemically identical to the abundant 75As in the nature and both isotopes exhibit the same adsorption properties. Thus, the distribution coefficients (Kd) of the two isotopes are equal:

[1]

The concentration of As added from the radioactive source to each tubes was extremely low (<0.002 µg As L–1) and so it did not significantly change the As(V) concentration in the solution phase, that is, As in solution with radioactive source approximately equals to As in solution in the control. The isotopically exchangeable As on the sediments is then derived based on the above assumption (Fujii and Corey, 1986):

[2]
where ml equals the liquid volume (mL); ms equals the mass of the solids (g); Asexch equals the concentration of As on the solid that is isotopically exchangeable (mg As kg–1); Asl equals the concentration of As in solution phase in the control (mg As L–1); Atotal equals the total radioactivity added (Bq); and Al equals the total radioactivity in solution at equilibrium (Bq).

Successive Extraction with Artificial Pore Water
To simulate the chemical composition of the natural solution phase in contact with the sediments for laboratory experiments, the sediment was successively extracted with the solution that had similar chemical composition to the pore water. The sediments were extracted in duplicate for 24 h at a solid/solution ratio of 1:10 (w/v), and pH of the suspension was adjusted to pH 8.5 or 7.5. The suspensions were then centrifuged at 11720 x g RCF for 20 min and filtered through 0.22-µm syringe filter. The supernatant was analyzed for As(III + V) and As(III) as described below. Arsenic (V) was calculated by difference. Artificial pore water was added to the remaining sediment (adjusted to a solid/solution ratio of 1:10 [w/v]) and mixed for another 24 h. The above procedures were repeated for a total of five successive extractions at pH 8.5 and 14 successive extractions at pH 7.5.

Phosphate Exchange for Adsorbed Arsenic
The purpose of this experiment was to determine if PO4 extraction could be used to estimate the amount of exchangeable As(V) potentially leachable to groundwater. Comparison of the results of PO4 extractions with those of the radioisotope dilution technique was used to assess the applicability of the method. The following procedure was used to determine the exchange time for PO4 and As(V): Sediment suspensions were first equilibrated with DD H2O at a sediment/solution ratio of 1:5 (w/v) and then 0.2 M PO4 (K2HPO4 + KH2PO4, pH 7.5) was added to the suspension to reach a final concentration of 0.1 M at a solid/solution ratio of 1:10 (w/v). After shaking for 1, 3, 6, 12, 24, 36, and 48 h, bulk of the solution was removed by centrifuging at 11720 x g RCF and filtered through 0.22-µm filter. The pH was monitored at the end of experiment, which remained within ±0.3 units. The concentrations of As(III) and As(III + V) in solution were determined as described below.

The exchange of PO4 for adsorbed As was further studied for three phosphate concentrations (0.1, 0.05, and 0.01 M at pH 8.5 and 0.1 and 0.01 M at pH 7.5) and two extraction times (24 and 48 h). After removing the first 24-h extraction solution with pore water for soluble As, 20 mL of PO4 solution with the concentration desired at pH 8.5 or 7.5 was added to about 2 g of sediment. After mixing for 24 or 48 h, the supernatant was obtained by centrifuging at 11720 x g RCF for 20 min and filtering through 0.22-µm filter. The concentrations of As(III) and As(III + V) in solution were analyzed.

Analytical Methods for As(III) and Total As(III + V)
The analytical method used for determination of As(III) and total As (III + V) was a modification of the method described by Glaubig and Goldberg (1988). Because a narrow pH is required for As(III) analysis, increased ionic strength and alkalinity often interferes with detection of As(III). Thus, analysis of As(III) in samples was done using sampling matrix matching in preparing the standards. Spiked samples were included in all analytical runs to assure adequate recovery (>95%).

Arsenic (III) determination: Sample containing 0.025 M HCl was introduced into the continuous hydride vapor generation accessory (VGA-76, Varian Analytical Instruments, Walnut Creek, CA). By introducing samples containing As(III) and mixed with the MES buffer (0.2 M MES [4-morpholineethanesulfonic acid]) and NaBH4 (0.6% NaBH4 + 0.5% NaOH), arsine was produced from As(III) only and introduced to the quartz cell of the AAS (Varian AA-1275, Varian Analytical Instruments, Walnut Creek, CA). The As was atomized and the absorbance was measured at 193.7 nm.

Total As(III + V) determination: Ten milliliters of samples containing <15 µg As L–1 As was placed in test tubes. Four milliliters of concentrated HCl was added to each sample, followed by addition of 4 mL of 12.5% (wt/v) urea. After 10 min, 2 mL of 12.5% KI was added and the solution was mixed using a VORTEX mixer (Scientific Industries Inc., Queens Village, NY). After about 1 h, the samples were analyzed on the AAS using VGA-76 with 10 M HCl and NaBH4 (0.6% [wt/v] NaBH4 + 0.5% [wt/v] NaOH).

Arsenic (V) was calculated as the difference between the As(III + V) and As(III) concentrations and As(V) was dominant in all the analysis in this study because of the oxidized nature of the sediment (97–99% of total As) and As(III) presented in trace amounts (<3%). Thus the As referred in the Results and Discussion was mainly As(V).


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Estimation of Adsorbed Arsenic (V) on Sediments using Arsenic-73 Radioisotope Dilution Technique
The loss of 73As from the solution to the sediment as a function of time is shown in Fig. 2. It was assumed that the loss of 73As(V) from solution resulted from only sorption. Within the first 3 h, the activity of 73As in the solution decreased from 18500 to 9250 Bq mL–1 (0.5 to 0.25 µCi mL–1) indicating a rapid rate of adsorption. During the next 21 h, the 73As activity decreased from 9250 to 7400 Bq mL–1 (0.25 to 0.20 µCi mL–1) suggesting a much slower rate of As adsorption. After 24 h, 73As radioactivity in the solution remained relatively constant (7030–7400 Bq mL–1 or 0.19–0.20 µCi mL–1), indicating that a steady state for isotopic exchange may have been achieved within about 24 h.

The amount of isotopically exchangeable As determined for the sediment using Eq. [2], was 5.7 mg As kg–1 at pH 8.5 and 6.7 mg As kg–1 at 7.5, respectively. Thus, adsorbed As [dominated by As(V)] was about 24% of the total As (24 mg As kg–1) at pH 8.5 and 28% of the total at pH 7.5.

Removal of Adsorbed Arsenic by Successive Extraction with Artificial Pore Water
Changes of As concentrations in successive extractions with the artificial pore water from the sediments are shown in Fig. 3 for both pH 8.5 and 7.5. The extracted As concentration decreased exponentially as the number of extractions increased but a power equation provided the best fit to the experimental data. Because a limited number of successive extractions (five 24-h extractions) were performed at pH 8.5, extrapolation was made using a power equation that best describe the data (r2 > 0.99). After 14 successive extractions, the As concentration in the 14th extract was 14 µg As L–1 at pH 7.5 and predicted to be 9 µg As L–1 at pH 8.5.



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  		Fig. 3. Arsenic [97 to 99% as As(V)] in successive extractions with artificial pore water at pH 7.5 and 8.5. Vertical bars are the range of duplicate values.

 
The cumulative As concentrations extracted from the sediments and the percentage of isotopically exchangeable As were computed based on As concentration in the extracts (Fig. 4 and 5) . After five successive extractions, the amount of adsorbed As removed at pH 8.5 was 2.1 mg As kg–1 (37% of isotopically exchangeable As) and at pH 7.5 was 2.37 mg As kg–1 (36% of isotopically exchangeable As). After 14 successive extractions at pH 7.5, 3.9 mg As kg–1 [57% of isotopically exchangeable As(V)] was removed. Assuming that the As release for continued successive extractions at pH 8.5 would have followed the same pattern observed for the extractions at pH 7.5, extrapolation to 14 successive extractions at pH 8.5 resulted in an expected value of 3.5 mg As kg–1, accounting for about 61% of the adsorbed As. In all cases, the As removed by 14 successive extractions resulted in a release of less quantities of As than that determined by radioisotope dilution technique, suggesting that only portion of the isotopically exchangeable or adsorbed As can be easily released to groundwater when As(V) was dominant species.



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  		Fig. 4. The cumulative adsorbed As removed from the sediment by successive extractions at pH 7.5 and 8.5. Vertical bars are the range of duplicate values.

 


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  		Fig. 5. Percentage of isotopically exchangeable As removed by successive extractions at pH 7.5 and 8.5. Vertical bars are the range of duplicate values.

 
Phosphate Exchange for Adsorbed Arsenic
Arsenic concentration in solution extracted by 0.1 M PO4 at pH 7.5 as a function of time is shown in Fig. 6 . The initial increase in As concentration in solution was relatively rapid up to 6 h. Between 6 and 24 h a much slower increase in As concentration occurred. After 24 h very little change was observed, indicating a near steady-state PO4 exchange for As(V) on the sediment.



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  		Fig. 6. Arsenic [97 to 99% as As(V)] concentration in solution extracted by 0.1 M PO4 at pH 7.5. Vertical bars are the range of duplicate values.

 
Further extractions using different concentration of PO4 for 24- and 48-h periods at pH 8.5 showed significant variations in terms of the percentage of adsorbed As (Table 4). Overall, As concentrations exchanged by PO4 were less than the isotopically exchangeable As.


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  		Table 4. Phosphate extraction of As from sediment at pH 8.5.

 
Statistical analyses of the extraction treatment for As were performed using PC/SAS version 6.04 (SAS Institute, 1991). An initial F-test showed significant differences among PO4 concentration extractions (p-value = 0.0005) and between the two extraction time periods (24 and 48 h) (p-value = 0.0001), with no interactions of the two factors (p-value = 0.2236). We used Fisher's LSD method, with an {alpha} of 0.01, for the multiple comparison test to determine differences in the means of As concentrations between the levels of PO4 treatments. These tests showed that the 0.1 M PO4 extraction was significantly different from the 0.05 and 0.01 M PO4 extractions, but the latter two were not significantly different from each other. Thus, 0.1 M PO4 extraction may estimate a larger amount of leachable adsorbed As (63%) that coincidently was close to the fourteenth extraction described above (57% at pH 7.5 and 61% at pH 8.5%, respectively).

Extractions of As on sediments with PO4 at pH 7.5 was done only for 24-h extractions and at concentrations of 0.1 and 0.01 M PO4, Results showed that 0.1 M PO4 extracted As (63% of adsorbed As) were significantly higher than 0.01 M PO4 extractions (54% of adsorbed As) (data not shown). Thus 0.1 M PO4 extracted As concentrations were larger than other PO4 concentrations at both pH 8.5 and 7.5 conditions and close to the fourteenth extraction results. This suggested that the phosphate extraction method (0.1 M PO4 at ambient pH, 24–48 h extraction) might be used to determine the amount of As potentially available to groundwater. Our results suggest that a 48-h extraction with 0.1 M PO4 at ambient pHs would provide the amount of As adsorbed on sediment that may be subject to leaching into groundwater.

The PO4 extractions have shown that PO4 cannot exchange all of the adsorbed As(V) even with the concentration (0.1 M) in solution that is 1000 times greater than the concentration of adsorbed As (0.0001 mol As kg–1) in the sediments. These results could be explained by the work of Lumsdon et al. (1984) who showed evidence that some adsorption sites were only available for arsenate because of the size difference between PO4 and arsenate. Comparison with the successive extraction results with artificial pore water suggested that PO4–extractable As may be used as a reasonable estimate of the amount of As on sediment available to leach into groundwater.

Estimates of As that may potentially contribute to soluble As indicate that some of the adsorbed As in Tulare Lake bed sediments could be a significant source to contaminate the underlying groundwater. The sediment used in this study contained a total concentration of 24 mg As kg–1. Adsorbed As concentrations were 5.7 to 6.7 mg As kg–1, or about 24 to 28% of the total As on the sediment. Using the successively extraction results as estimates, 57 to 61% of the adsorbed As was potentially leachable that correspond to As concentrations of 3.5 and 3.9 mg As kg–1 on sediments at pH 8.5 and 7.5, respectively. This amount of As could result in a significant amount of water yielding As concentration of 10 µg As L–1 or greater. Arsenic concentration in the solution phase after the 14 time successive extractions with artificial pore water was 14 µg As L–1 at pH 7.5 and predicted concentrations of about 10 µg As L–1 at pH 8.5. Up to this point, the sediment/water ratio was 1:140 (v/w), which means that each gram of sediment has the potential to contribute As to a volume of more than 100 mL of groundwater, which would exceed the MCL of 10 µg As L–1. This amount of As could be estimated by 0.1 M PO4 in this sediment although the actual amount of PO4 extractable As(V) available for leaching may vary because of differences in sediment properties such as clay content and bulk density. The sediment properties determine the reaction time between sediment and solution As, and the rate of leaching that are important to consider in evaluating As mobility in sediment (Barrow, 1992; O'Reilly et al., 2001). Nevertheless, the results from this study indicated that adsorbed As can be a significant source of As contamination to groundwater.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
A measure of the isotopically exchangeable As in sediments may provide information about the maximum amount of As that is available to groundwater under oxidizing conditions. Successive extraction with pore water that has similar chemical composition to pore water released 57 to 61% of the isotopically exchangeable As to solution when solution concentration approached about 10 µg As L–1, the drinking water MCL. The corresponding As concentrations on the sediments were 3.5 and 3.9 mg As kg–1 at pH 8.5 and 7.5, respectively. Extraction with 0.1 M PO4 for 48 h at ambient pHs (8.5 or 7.5) can determine this amount of sorbed As that has the potential to be released into groundwater. The results indicated that As via desorption could be a significant mechanism for contaminating groundwater in oxidized shallow groundwater zone in Tulare Lake bed. However, the contribution of As to groundwater also depends on the underlying sediment properties that determine the reaction time between the sediments and solution As, as well as the leaching rate.

Received for publication January 2, 2003.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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The SCI Journals Agronomy Journal Crop Science
Vadose Zone Journal Journal of Plant Registrations
Journal of Natural Resources
and Life Sciences Education		 Journal of
Environmental Quality