Environ. Sci. Technol. 2003, 37, 2415-2420
Selenium Speciation, Solubility, and Mobility in Land-Disposed Dredged Sediments P E T E R T . Z A W I S L A N S K I , * ,† SALLY M. BENSON,‡ ROBERT TERBERG,‡ AND SHARON E. BORGLIN‡ Lawrence Berkeley National Laboratory, 1 Cyclotron Road, 90-1116, Berkeley, California 94720, and LFR Levine-Fricke Inc., 1900 Powell Street, 12th Floor, Emeryville, California 94608
Pilot-scale tests for the land disposal of Se-enriched sediments from the San Luis Drain were performed in the San Joaquin Valley, California. Three test plots were instrumented and monitored on a dirt-road embankment near the sediment source area, providing an opportunity to measure Se oxidation and solubilization rates over a period of 2-3 yr. Soil, soil water, and groundwater data indicated that the amendment did not cause movement of dissolved Se below a depth of 15 cm. The low permeability of underlying sediments and the overall low Se solubility limit Se movement toward the groundwater table. Selenium remained in reduced forms and largely immobile at this site, although in-situ Se oxidation was measurable. Soluble Se concentrations increased from less than 0.5% to approximately 2.5% in the first 207 d following sediment application. Minor Se solubilization occurred after 439 and 704 d. Changes in Se fractionation measured using sequential extractions and Se speciation based on X-ray spectroscopy (XANES) results were in qualitative agreement. XANES results indicated initially rapid oxidation of organoSe and/or elemental Se to selenite during the first 207 d, followed by minor oxidation after 439 d. Further solubilization of the Se inventory is anticipated, but at a low rate of 1-2% per year, comparable to rates measured in other studies.
Introduction Pilot tests of land disposal of Se-enriched sediments were conducted in the San Joaquin Valley, California. Agricultural drainwater from farmland in the Grasslands Water District (Figure 1) is channeled via the San Luis Drain (SLD), small sloughs, and the San Joaquin River toward the San Francisco Bay Delta. The drainwater, which has an average selenium concentration of 60 µg L-1, deposits Se-rich sediments that are further Se-enriched through biogeochemical processes (1). Sediments have been accumulating in the SLD since its completion in 1974. Dust, wind-blown plant debris, algae, cattails, and suspended sediments have accumulated largely upstream and downstream from checks or weirs in the drain. An estimated 100 000 m3 of sediments currently resides in the SLD, with Se concentrations as high as 186 mg kg-1 (2). * Corresponding author e-mail:
[email protected]; phone: (510)596-9685; fax: (510)652-2246. † LFR Levine-Fricke Inc. ‡ Lawrence Berkeley National Laboratory. 10.1021/es020977z CCC: $25.00 Published on Web 04/26/2003
2003 American Chemical Society
The need to periodically dredge and remove the sediments from the SLD has prompted research into practical means of disposal. Land disposal is an attractive option due to its low cost, the proximity of large areas of available land, and relatively low Se solubility in chemically reduced sediments (3, 4). This paper presents the results from a pilot test of the application of SLD sediments to a nearby road embankment. Field experiments were carried out to measure the geochemical stability of Se in the applied sediments, rates of Se oxidation and solubilization, and transfer from the surface to underlying soils. In this paper, we compare measured Se oxidation and solubilization rates with results from previous studies. Selenium Geochemistry in Sediments. Selenium solubility in soils and sediments is controlled by Se speciation, which in most environments is dominated by chemically reduced and adsorbed forms (5-7). Under saturated or submerged conditions, Se occurs primarily in reduced forms (8, 9). The more oxidized Se species are generally most soluble. Selenate (Se(VI)) is the most soluble Se species, followed by selenite (Se(IV)), which sorbs strongly onto oxide surfaces (10); clay minerals (11); and soil organic matter (12). Elemental Se (Se(0)) has negligible solubility under environmental conditions (13). The oxidation kinetics of Se(0) are very slow (6), leading to its long-term stability, even in suboxic and oxic environments. Organic Se species are commonly found in soils although their chemistry and transformations are less well understood, in part due to the potentially large number of such molecular forms (14) and the difficulty in chemical separation. Furthermore, Se forms sometimes referred to as “organic” may be associated with soil organic matter (SOM) in only a physical sense (i.e., adsorbed or occluded). The predominant organo-Se forms found in SOM include selenomethionine, selenocystine, and selenocysteine (15). The determination of Se speciation in solids is challenging. Sequential or selective extraction techniques (8, 16-18) provide information on the associations of Se with soil or sediment fractions but cannot objectively discriminate among Se species. Nonetheless, sequential extractions provide the only currently available means for studying Se speciation in soils and sediments containing less than 10 mg kg-1 Se, albeit indirectly. Nondestructive X-ray spectroscopic methods have been used to determine Se speciation in samples with higher concentrations (19). X-ray absorption near-edge structure (XANES) spectroscopy can directly determine the valence of elements, including Se, and distinguish among organo-Se, Se(0), Se(IV), and Se(VI). Distinguishing the different organo-Se forms is difficult because of the variety of environmentally relevant species and the similarity of their spectra.
Materials and Methods Study Area. The SLD and surrounding agricultural areas are located in the San Joaquin Valley, in Fresno and Merced Counties, at an elevation of approximately 40 m above sea level. Average annual rainfall is from 150 to 360 mm, while average annual evapotranspiration exceeds 2000 mm. The average winter, summer, and annual air temperatures are 12.8, 35.1, and 22.2 °C, respectively. The area is dominated by irrigated farm fields and is crossed by several major and many smaller water distribution and drainage water canals. The primary crops are cotton, fruit, and nuts and to a lesser degree vegetables and feed grains. The SLD is a concrete-lined canal that runs SSE to NNW, roughly parallel to the San Joaquin River (Figure 1). The total length of the SLD is 136 km, but only the northern 64 km is VOL. 37, NO. 11, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
2415
FIGURE 1. Location of the San Luis Drain and test plots in the San Joaquin Valley, California.
TABLE 1. Test Plot Soil and San Luis Drain Sediment Characteristics preapplication soil characteristicsa date sampled
plot
EP1 12/22/98 EP2 6/29/99 EP3 6/29/99 a
Sec
total Se soluble (mg kg-1) (mg kg-1) 0.75 0.78 0.81
0.10 0.09 0.10
San Luis Drain dredged sediment characteristicsb ECc
date applied
6.8 6.9 6.8
1/14/99 9/3/99 9/3/99
OC pHc (g kg-1) (dS m-1) 8.0 8.2 8.1
nad na na
Values averaged over five soil profiles to 200 cm.
b
9
13.3 6.9 8.4
2.56 37.1 19.5
0.021 0.099 0.042
Average of 10 samples. c Measured in 1:5 soil:water extract.
currently used for agricultural drainage water conveyance. Drainage water is introduced to the SLD via the Grassland Bypass Channel (GBC) (Figure 1) and flows northward from there to the Kesterson Reservoir, where it is diverted to Mud Slough, which in turn delivers the water to the San Joaquin River, which drains into the Sacramento River Delta. Sediment Sources and Test Plot Soils. Sediments for the test were collected from three separate areas within a 0.5km section of the SLD. The areas were selected to obtain sediments with different selenium concentrations, nominally, 2, 20, and 40 mg kg-1 total Se. The sediments were applied to test plots EP1, EP2, and EP3 located on a nearby embankment (Figure 1). The embankment was chosen because of its proximity to the SLD and well-compacted, engineered soils (silty clay and clayey silt), which limit 2416
OC vol applied total Se soluble Sec ECc (m3) (mg kg-1) (mg kg-1) pHc (g kg-1) (dS m-1)
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 37, NO. 11, 2003
7.4 7.7 7.7 d
14.3 22.3 16.6
2.7 3.8 3.9
na, not analyzed.
infiltration. The depth to groundwater fluctuates seasonally from 150 to 200 cm below grade. Being part of an embankment road, the EP plots were regularly devegetated prior to sediment application. Sediment Dredging and Application. Dredging and application dates and sediment volumes applied for each experimental plot are shown in Table 1. Sediment was dredged from the SLD using a trackhoe. With the exception of plot EP1, batches of dredged sediment were turned 100 times in a cement mixer to ensure homogeneity throughout each application. Minor amounts of SLD water were added to the sediment to facilitate mixing and spreading. A 15-cm thickness of homogenized sediment was then applied to rectangular test plots using the cement truck chute and a cement rake. After drying and settling, the applied
TABLE 2. Sequential Extraction Procedure for Se Fractionation in Soils and Sediments solid:solution mass ratio
procedure
soluble adsorbed organic matter associated
target Se fraction
0.25 M KCl 0.1 M Na2HPO4 0.02 M NaOH
1:5 1:10 1:10
elemental soil organic matter
1 M Na2SO3 (pH 7) 4% NaOCl (pH 9.5)
a 1:4
residual (oxide-bound and recalcitrant)
(HNO3/H2O2/HCl)
b
shaken for 1 h shaken for 24 h heated at 85 °C for 2 h, shaken for 5 min every 30 min a heated at 100 °C for 30 min, centrifuged, decanted, repeated b
a
Ref 9.
b
solution/reagents
Ref 2.
sediment was 10 cm thick. Each plot was roughly 3 m by 15 m. Sediments applied to EP1 were initially dried in a pile for 35 d, then placed on the test plot using a grader, and disked into the underlying soil to a depth of 15 cm below original grade. Sampling and Monitoring. Soil profiles in each test plot were sampled prior to sediment application, using a directpush rig. Core samples were collected down to and slightly below the groundwater table. Cores were subdivided into 15-45-cm intervals. Five cores were collected from each test plot, and additional “control” cores were collected adjacent to the test plots. The applied sediment was sampled separately for each test batch. Ten sediment grab samples were collected within hours of dredging, except for test plot EP1, where 43 grab samples were collected. Sets of five postapplication soil cores were collected in EP1 after 85, 168, 439, 671, and 936 d and in EP2 and EP3 after 207, 439, and 704 d. Several nests of soil water samplers, tensiometers, neutron probe access casings, and groundwater wells were installed in each of the plots and were used to monitor movement of water and solutes. Monitoring was conducted approximately monthly for 1 yr and quarterly thereafter. Extraction, Digestion, and Analytical Methods. All soil samples were analyzed for near-total and soluble Se. A strongacid digestion procedure (HNO3-H2O2-HCl) was used to extract near-total Se in the soil and sediment (20). Soluble Se was removed from the soil or sediment sample using a 1-h 1:5 soil:water extraction. Selenium was analyzed using hydride generation atomic absorption spectroscopy (HGAAS) (21) following sample filtration (0.45 µm). Sequential extractions were used to identify and quantify dominant Se fractions in applied SLD sediments at the time of application and 207 d later. The sequential extraction procedure (Table 2) was based on existing techniques used for Se fractionation (8, 16-18, 21, 22). The KCl, Na2HPO4, and NaOH extracts were performed on field-moist soils; the Na2SO3 extract and subsequent extracts were performed after NaOH extraction, drying, and grinding. Residual Se is defined as the difference between total Se and the sum of sequentially extracted Se. All supernatant solutions were passed through a 0.45-µm nitrocellulose filter immediately after extraction. The NaOH extraction is intended to quantify Se associated with organic soil fractions (23). This fraction includes both inorganic and organic species, which are bound to soil organic matter (SOM). The NaOCl extraction completely breaks down SOM and thereby releases the remainder of Se associated with SOM (7). Organic carbon (OC) content was measured using the Walkley-Black dichromate procedure (24). X-ray Spectroscopy. XANES was used to identify and quantify the dominant Se species in the SLD sediments at the time of application and after 207 and 439 d. Selenium K-edge X-ray spectroscopy data for SLD sediment were collected at the Stanford Synchrotron Radiation Laboratory (SSRL) on Beam Line 4-1. X-rays were passed through a Si(220) double crystal monochromator detuned to 50% to remove higher-order harmonics. Samples were stored frozen
until immediately before the run. The samples were packed in a holder with dimensions of 28 mm × 2 mm × 2 mm and placed at a 45° angle to the beam. Fluorescence X-ray spectra of the samples and of Se standards were measured using a Lytle detector with a xenon-filled chamber. A Se(IV) reference standard was run with each sample and was used to correct for beam energy shifts. Multiple scans of the data were averaged using the R-Space X-ray Absorption Package (RSXAP) (25), the background was subtracted, and the edge step was normalized to one. The data were then fit with up to three model Se compounds. Model compounds were chosen to represent the most likely Se species present in SLD sediments (3).
Results and Discussion Soil and Sediment Selenium. The chemical characteristics of the embankment soils and the sediments applied to them are shown in Table 1. Sediments applied to EP1, EP2, and EP3 contained 2.56, 37.1, and 19.5 mg kg-1, respectively. Following application, only minor changes in total Se concentrations were observed in soil profiles in each test plot after 207 and 439 d, as represented by typical data from EP2 (Figure 2A). On day 439, increases in total Se were observed in all plots at depths between 50 and 75 cm below the ground surface. Corresponding increases in soluble Se (Figure 2B) and salts (EC increased to 8-10 dS m-1) were observed at the same depths. The test plots were not cleared of vegetation during the field test due to the presence of monitoring equipment. The invasion by plants resulted in progressive drying of the soil profile (data not shown) and evapotranspirative accumulation of salts and Se in the root zone, generally peaking at around 50-70 cm below the ground surface. Samples taken from a control site (Figure 2A) show a similar trend, indicating that Se increases in the root zone were not caused by leaching from the applied sediment. Total Se increased in all plots at a depth of 15 cm below original ground surface, from a background value of around 0.8 mg kg-1 to between 2 and 4 mg kg-1 on day 439. This depth represents the maximum depth of Se leaching from the applied sediment. With the exception of this shallow interval, total Se in the soil profile in EP1, EP2, and EP3 remained at or below 2 mg kg-1. Groundwater and Soil Water Selenium. Although Se concentrations in test plot groundwater varied over time, the same seasonal changes were observed in groundwater from control wells (Figure 3), indicating an absence of deep leaching. It should be noted that background groundwater Se concentrations in this area are generally high, as is typical for this part of the San Joaquin Valley (26). The observed groundwater Se variations are likely due to seasonal fluctuations in groundwater table elevation, which ranges up to 75 cm in any given year. Soil water Se concentrations increased relative to background values at a depth of 15 cm but not at greater depths. Selenium levels observed at the 15-cm depth corresponded to the concentrations of Se in applied sediments. Shallow soil water Se was highest in EP2 (175-250 VOL. 37, NO. 11, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
2417
FIGURE 4. Water-soluble Se in SLD sediments applied to plots EP2 and EP3. Results represent the mean of five data points, and bars indicate 1 SD.
FIGURE 2. Total (A) and water-soluble (B) Se concentrations normalized to soil mass in plot EP2. Results represent the mean of five data points and bars indicate 1 SD. The asterisk (*) indicates that preapplication soil profile data (6/29/99) are combined with SLD sediment data collected on day of application (day 0).
FIGURE 5. Selenium fractionation in SLD sediment applied to plot EP3. Results represent the mean of five data points, and bars indicate 1 SD.
FIGURE 3. Groundwater Se in test plot wells and control wells. µg L-1), somewhat lower in EP3 (100-250 µg L-1), significantly lower in EP1 (30-60 µg L-1), and similarly low in the control plot (25-60 µg L-1). Selenium Solubility. The aqueous solubility of Se in SLD sediment applied to the test plots increased significantly (ANOVA; P < 0.05) from less than 0.5% of total at the time of application to between 2 and 2.5% of total approximately 207 d later (Figure 4). After that initial increase, soluble Se in sediment applied to EP2 did not change significantly and remained around 2.5% for the remainder of the experiment. In the sediment applied to EP3, soluble Se increased slightly but not significantly after day 207. These results suggest that either the net rate of Se solubilization decreased markedly after the first few months or that soluble Se was being leached out of the applied sediment. However, soil profile data (Figure 2) do not indicate significant soluble Se leaching. Although seasonal Se oxidation and reduction along with other processes may be occurring, the average moisture content of the applied sediment decreased with time. Therefore, the applied sediment likely became on average more oxidized over the course of the test. The net Se solubilization rate, which would represent the combined effect of all processes, 2418
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 37, NO. 11, 2003
can be modeled using a first-order reaction rate constant (k). The average net Se solubilization rate in the applied sediment was approximately 1-2% per year (k ) 0.015 yr-1). In the underlying test plot soils, soluble Se ranged from 5 to 15% of total, similar to levels prior to SLD sediment application and in line with previously discussed findings of a lack of deep leaching of Se. Selenium Fractionation. Sequential extraction results illustrate the dominance of reduced and insoluble Se in SLD sediments (Figure 5). Approximately 90% of the Se immediately after application was either organic, organically associated, elemental, or refractory. Two-hundred and seven days after application soluble and adsorbed Se increased, while the more reduced fractions decreased. Increases in soluble and adsorbed Se and decreases in elemental Se were significant at the 95% confidence interval. Measurements of OC content show net oxidation of the sediment samples. Organic carbon decreased after 207 d from 22.3 ((1.5) to 18.0 ((1.3) g kg-1 in EP2 and from 16.4 ((0.6) to 14.5 ((0.5) g kg-1 in EP3. On the basis of differences between initial and day 207 concentrations, the Se oxidation rate was on the order of 1% per month. However, as readily oxidizable Se was depleted (6), oxidation rates decreased, a trend noted in the longer-term measurements of soluble Se (Figure 4). Selenium Speciation. X-ray spectroscopic results confirm the dominance of reduced Se species in SLD sediments. Spectra for samples collected during the initial application (day 0), on day 207, and on day 439 from EP3 are compared to each other and to Se standards (Figure 6). A substantial peak shift is observed between day 0 and day 207. The differences between the peak positions in samples collected
sorbed (5, 11). Such low solubility will limit the long-term transport of Se to the groundwater table, making land disposal on low-permeability soils a viable option for the management of SLD sediments.
Acknowledgments The authors thank Scott Mountford, Thana Alusi, and Donna Smith for sample analysis. For assistance in the field, we thank Ray Solbau, John Clyde, Sid Patel, and the Panoche and Grasslands Water Districts. We thank Jeremy Hughes, Andrew Zawislanski, Mike Alberg, and Stuart Counsell for sample preparation. We appreciate the assistance of Mike Gardiner, Dave McEuen, and Joe McGahan. We thank Kota Farms of South Dos Palos for use of the road embankment for sediment application. This manuscript benefited from reviews by Tetsu Tokunaga, Mitch Herbel, and three anonymous reviewers. This research was funded by the U.S. Bureau of Reclamation (Michael Delamore, Program Manager). FIGURE 6. Selenium K-edge absorption spectra of Se standards and SLD sediment applied to plot EP3. on days 207 and 439 are much smaller. The day 0 spectrum is best fit with 100% selenomethionine, although a 100% fit with selenocysteine is also satisfactory. The day 207 spectrum is best fit with either 49% selenomethionine and 51% selenite or 39% selenocysteine and 61% selenite. The day 439 spectrum is best fit with 36.8% selenomethionine, 57.7% selenite, and 5.5% selenate. These results indicate fairly rapid Se oxidation during the first few months after application, followed by much slower oxidation during the subsequent period. The appearance of selenate in the day 439 sample must be cautiously interpreted because of the very low relative concentration implied by the data. Likely, the method is only marginally able to quantify selenate at 5.5% of 19.5 mg/kg. It should be noted that XANES data for an SLD sediment sample from EP2 (day 0) were fitted well with either 100% Se(0) or 53% Se(0) and 47% organo-Se. This illustrates the difficulty of fitting the results to the reduced Se compounds. It also suggests the likely and expected presence of Se(0) in SLD sediments. XANES results are in qualitative agreement with Se solubility and fractionation data. All approaches show more rapid oxidation/solubilization of Se during the first few months after application and much slower oxidation/ solubilization thereafter. On the basis of XANES results, a first-order rate for organic Se oxidation is estimated at 0.82 yr-1 but is strongly affected by the early rapid oxidation. Longterm rates are likely to be approximately 1 order of magnitude lower. Furthermore, there are probably several reduced Se species present, and their oxidation rates will vary widely (6). Comparison with Previous Studies. Overall, these findings agree with field (23, 27) and laboratory (6) studies in which long-term Se oxidation rates are very slow following an initial more rapid release of bound Se. The apparent predominance of organic Se forms may explain the initial release since oxidation rates for organic Se (28), algaeaccumulated Se (1), and even biogenic Se(0) (29) can be an order of magnitude faster than the oxidation of abiotic Se(0). The estimated net Se oxidation rate based on XANES results (0.82 yr-1) is within a range of values measured in laboratory experiments (6, 29). Initially very rapid Se oxidation in formerly ponded and dried out sediments was also observed by Tokunaga et al. (3). The estimated net Se solubilization rate (0.015 yr-1) is within a range of values measured in previous field experiments (0.01-0.09 yr-1) (5, 27, 30). Slower oxidation in the field is anticipated because of seasonal reduction/oxidation cycles. Overall, Se remains largely water-insoluble in soils because Se(IV) is strongly
Literature Cited (1) Borglin, S. E.; Benson, S. M.; Zawislanski, P. T. In Proceedings of the 6th International Conference on the Biogeochemistry of Trace Elements (ICOBTE), Guelph, Ontario, Canada, 2001. (2) Zawislanski, P. T.; Benson, S. M.; TerBerg, R.; Borglin, S. E. In Proceedings of the 6th International Conference on the Biogeochemistry of Trace Elements (ICOBTE), Guelph, Ontario, Canada, 2001. (3) Tokunaga, T. K.; Pickering, I. J.; Brown, G. E., Jr. Soil Sci. Soc. Am. J. 1996, 60, 781-790. (4) Martens, D. A.; Suarez, D. L. In Environmental Chemistry of Selenium; Frankenberger, W. T., Jr., Engberg, R A., Eds.; Marcel Dekker: New York, 1998; p 61. (5) Tokunaga, T. K.; Zawislanski, P. T.; Johannis, P. W.; Benson, S. M.; Lipton, D. S. In Selenium in the Environment; Frankenberger, W. T., Jr., Benson, S. M., Eds.; Marcel Dekker: New York, 1994; p 119. (6) Zawislanski, P. T.; Zavarin, M. Soil Sci. Soc. Am. J. 1996, 60, 791-800. (7) Zhang, Y. Q.; Moore, J. N. Environ. Sci. Technol. 1996, 30, 26132619. (8) Weres, O.; Jaouni, A.-R.; Tsao, L. Appl. Geochem. 1989, 4, 543563. (9) Velinsky, D. J.; Cutter, G. A. Geochim. Cosmochim. Acta 1991, 55, 179-191. (10) Hamdy, A. A.; Gissel-Nielsen, G. Z. Pflanzenernaehr. Bodenkd. 1977, 140, 63-70. (11) Bar-Yosef, B.; Meek, D. Soil Sci. 1987, 144, 11-19. (12) Ylaranta, T. Ann. Agric. Fenn. 1983, 22, 29-39. (13) Masscheleyn, P. H.; Delaune, R. D.; Patrick, W. H. Environ. Sci. Technol. 1990, 24, 91-96. (14) Abrams, M. M.; Burau, R. G.; Zasoski, R. J. Soil Sci. Soc. Am. J. 1990, 54, 979-982. (15) Doran, J. W.; Alexander, M. Appl. Environ. Microbiol. 1977, 33, 31-37. (16) Lipton, D. S. Ph.D. Dissertation, University of California, Berkeley, 1991. (17) Tokunaga, T. K.; Lipton, D. S.; Benson, S. M.; Yee, A. Y.; Oldfather, J. M.; Duckart, E. C.; Johannis, P. W.; Halvorsen, K. E. Water Air Soil Pollut. 1991, 57, 31-41. (18) Martens, D. A.; Suarez, D. L. Environ. Sci. Technol. 1997, 31, 133-139. (19) Pickering, I. J.; Brown, G. E., Jr.; Tokunaga, T. K. Environ. Sci. Technol. 1995, 29, 2456-2459. (20) Zawislanski, P. T.; Mountford, H. S.; Gabet, E. J.; McGrath, A. E.; Wong, H. C. J. Environ. Qual. 2001, 30, 1080-1091. (21) Weres, O.; Cutter, G. A.; Yee, A.; Neal, R.; Moehser, H.; Tsao, L. Standard Methods for the Examination of Water and Wastewater, 17th ed.; Section 3500-Se; American Public Health Association: Washington, DC, 1989. (22) Velinsky, D. J.; Cutter, G. A. Anal. Chem. Acta 1990, 235, 419425. (23) Martens, D. A.; Suarez, D. L. J. Environ. Qual. 1997, 26, 424432. VOL. 37, NO. 11, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
2419
(24) Nelson, D. W.; Sommers, L. E. In Methods of Soil Analysis, Part 2-Chemical and Microbiological Properties; Page, A. L., Miller, R. H., Keeney, D. R., Eds.; American Society of Agronomy: Madison, WI, 1982; Vol. 9, p 539. (25) Bridges, F.; Booth, C. H.; Li, G. G. Physica B 1995, 208-209, 121-124. (26) Deverel, S. J.; Fio, J. L.; Dubrovsky, N. M. In Environmental Chemistry of Selenium; Frankenberger, W. T., Jr., Engberg, R A., Eds.; Marcel Dekker: New York, 1998; p 157. (27) Benson, S. M.; Daggett, J.; Zawislanski, P. T. In Proceedings of the 5th International Conference on Biogeochemsitry of Trace Elements (ICOBTE), Vienna, Austria, 1999.
2420
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 37, NO. 11, 2003
(28) Martens, D. A.; Suarez, D. L. Soil Sci. Soc. Am. J. 1997, 61, 16851694. (29) Dowdle, P. R.; Oremland, R. S. Environ. Sci. Technol. 1998, 32, 3749-3755. (30) Wahl, C.; Benson, S.; Santolo, G. Water Air Soil Pollut. 1994, 74, 345-361.
Received for review October 14, 2002. Revised manuscript received March 13, 2003. Accepted March 26, 2003. ES020977Z
