Environ. Sci. Technol. 1996, 30, 2613-2619
Selenium Fractionation and Speciation in a Wetland System YIQIANG ZHANG* AND JOHNNIE N. MOORE Department of Geology, University of Montana, Missoula, Montana 59812
Selenium fractionation and speciation in Benton Lake National Wildlife Refuge (Montana), a wetland system containing moderate levels of selenium, were studied to determine the biogeochemical processes of selenium in wetlands. Results showed that selenate was a major selenium species of dissolved selenium in drainage water. It decreased substantially through the pond system as the relative percentages of organic selenium and selenite increased. Elemental selenium and selenium associated with organic materials were the major fractions in sediments, accounting for a mean of 46% and 33% of total selenium, respectively. Concentrations of soluble selenium and adsorbed selenium were relatively low, respectively accounting for a mean of 5% and 13%. Within the soluble and adsorbed fraction of selenium in sediment, selenate and organic selenium were the major selenium species. Selenium associated with oxides was very low (less than 4%). In this wetland environment, microbial reduction of selenate to elemental selenium, selenium uptake by wetland organisms, and incorporation of these organisms into wetland sediment are the major processes removing selenium from the water column; selenium adsorption is relatively less important.
Introduction Selenium contamination of lakes and reservoirs mainly results from agricultural drainage from seleniferous soils (1-4), which are common in parts of the western United States. Bioaccumulation of selenium in organisms living in these wetlands creates serious hazards to fish and waterfowl (5-7). Inventories of National Wildlife Refuges have shown significant contamination of some wetland systems throughout the western United States (4). Concerns for the safety of these essential waterfowl production areas make understanding the complex biogeochemistry of selenium extremely important to wildlife managers. The fate of selenium in aquatic ecosystems is affected by a variety of physical, chemical, and biological factors. The chemistry of selenium is complicated because it exists in four oxidation states (-2, 0, +4, and +6) and in a variety of compounds (oxides/hydroxides, sulfides, organoselenium compounds, and selenides). Changes in selenium’s * Corresponding author e-mail address:
[email protected].
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1996 American Chemical Society
oxidation state and differences in chemical properties of various chemical forms strongly affect the movement and toxicity of selenium (2, 8-12). Many of the transformations of selenium from different oxidation states and between inorganic and organic forms are mediated by microbial processes (13-15). Wetland organisms can bioaccumulate selenium to a very high degree (5-7, 9), and the incorporation of live and dead organisms (especially plants and microbes) into wetland sediment forms a large reservoir of selenium in sediment organic matter. Clay minerals, oxides, and organic matter, all important constituents of wetland sediment, can directly adsorb different dissolved selenium species from overlying water and transfer selenium into the sediment (16-18). Because of this complexity, information about selenium fractionation in sediment is very important for determining the biogeochemical cycling of selenium in wetland systems. The purpose of the research reported here is to examine selenium fractionation in detail at a wetland typical of wildlife refuges with selenium contamination problems. We chose Benton Lake National Wildlife Refuge (BLNWR) in west-central Montana for this study because it is contaminated with selenium from agricultural runoff and ranks as one of the top waterfowl-producing refuges in North America (19). Selenium fractionation in surface sediment was determined for samples from seven sites within Benton Lake (Figure 1). The distribution of selenium fractions with depth was determined at one site containing high concentrations of selenium (Figure 1). Study Site. Benton Lake lies about 20 km north of Great Falls, MT, on the western edge of the Northern Great Plains. The refuge consists of 5020 ha (2750 ha in uplands and 2270 ha in ponds/marsh). The refuge’s shallow seasonal lake, Benton Lake, has been divided into six ponds (called pools 1-6) by dikes to facilitate management of water and habitat for waterfowl production (Figure 1). Currently, pools 1 and 2 are perennial wetlands and are managed to provide water about 1 m deep all year. Pools 3-6 are seasonal wetlands (mostly less than 0.5 m deep) that are filled in the spring to enhance nesting habitat for ducks, allowed to dry out during the summer, and then flooded again in the fall for use by migrating birds and to store water for the following spring. Water and selenium enter Benton Lake primarily through Lake Creek in an inlet channel of pool 1 and are derived mainly from natural runoff and trans-basin diversion of irrigation drainage (19). Also, localized “saline seeps” with high selenium concentrations flow into the lake at the SW edge of pool 4C.
Materials and Methods Samples were collected from Benton Lake and Lake Creek in 1992-1995. Eighteen water samples were collected from sites along Lake Creek and within the Benton Lake pond system (Figure 1). Water samples were filtered through a 0.45-µm membrane filter (using an acid-washed and deionized water rinsed 60-mL plastic syringe) into clean polyethylene bottles that had been rinsed two times in the field with filtered sample water. Surface sediments (0-5 cm) for determining selenium fractionation and speciation were collected from seven sites within the pond system
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TABLE 1
Sequential Extraction Procedure for Selenium in Benton Lake Sediment
FIGURE 1. Study site and sampling sites. P1, 2,... are designations for pool 1, pool 2, etc.
(Figure 1). These samples were collected using a plastic spoon. Four sediment cores were collected in a “highselenium” channel of pool 1 in individual 3.1 cm i.d. plastic tubes; maximum core depth was about 32 cm. Cored sediment was sectioned after removal from the core tube. “Bioaccumulation samples” from five sites (Figure 1) consisted of an aquatic plant (watermilfoilsMyrophyllum exculenta) collected by hand, Chironomid larvae (midge), and small plant roots collected from the sediment by sieving. All samples were transported to the laboratory on ice for preparation and analysis. Procedurally defined selenium fractions in sediment were determined mainly by modifying the sequential extraction technique summarized by Tokunaga et al. (20): soluble (0.25 mol/L KCl), adsorbed (0.1 mol/L K2HPO4), carbonate (1 mol/L sodium acetate followed by 0.1 mol/L K2HPO4), soil organic matter (NaOCl), easily reducible oxides (0.1 mol/L NH2OH followed by 0.1 mol/L KOH), amorphous oxides (0.25 mol/L NH2OH/HCl followed by 0.1 mol/L KOH), crystalline oxides (4 mol/L HCl), and amorphous aluminosilicates (0.5 mol/L NaOH). Our early method development showed that there was no detectable selenium in the carbonate fraction and there was little selenium in the easily reducible oxides, amorphous oxides, and crystalline oxides fractions in Benton Lake sediment. We therefore removed the carbonate fraction from the sequential extraction scheme and combined the three oxide fractions together and called it the “oxides” fraction (Table 1). We also found that selenium in the amorphous aluminosilicates fraction was much less than 1%, so we omitted it from the results. Because it is important to separate elemental selenium from selenium associated with organic matter, we added an extraction method to determine elemental selenium developed by Velinsky and Cutter (21). This step was added to the sequential extraction procedure after the adsorbed selenium extraction step (Table 1). There was good agreement between the total selenium concentration determined in separate sediment samples and the sum of the sequential extraction fractions. The
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fractions
extractant
ref
soluble adsorbed elemental organic material oxides
0.25 mol/L KCl 0.1 mol/L K2HPO4 1 mol/L Na2SO3 NaOCl 4 mol/L HCl
20 20 21 20 20
percentage recovery, expressed as the sum of selenium fractions divided by the total selenium, ranged from 95 to 104% with a mean of 99.4% for the extraction of the surface sediment. The percentage recovery in core sediment ranged from 89 to 110% with an average of 98%. To determine if drying had an effect on selenium fractionation, both field-wet and laboratory-dried (75 °C) surface sediments were used. Dry weight concentrations were determined by correcting for water content. These analyses showed that drying increases the concentrations in the soluble (44-225% increase) and adsorbed (0-260%) fractions and decreases concentrations in the organic matter fraction (0-30% decrease). This difference was substantial for individual samples, but the effect on the relative overall percentages was less important due to the low concentrations of soluble and adsorbed selenium. Data only from dried samples are presented for all selenium fractionation results, and experiments are underway to examine the details of this relationship. Samples of core sediments, watermilfoil, and small plant roots were dried at
