Bioaccumulation Study of the Algal-Bacterial Selenium Removal System Diana Stuart Abstract Selenium levels in agricultural drainage water of the San Joaquin Valley, CA have been found to be toxic to aquatic ecosystems. Researchers have been looking for cost-effective ways to reduce selenium concentrations in drainage water. This study looks at the effectiveness of the Algal-Bacterial Selenium Reduction System to reduce the selenium available to aquatic communities. Microcosms containing four species of aquatic invertebrates were established in each part of the system and used to study bioaccumulation. Water and soil samples from the microcosms were also taken to analyze selenium speciation throughout the system. It was found that the system is successfully reducing total selenium levels in the drainage water. However, it is also releasing a significant amount of the remaining selenium in the forms of selenite and organo-selenium, which are more bioavailable to wildlife. Invertebrate bioaccumulation showed that body burdens increased with higher levels of organo-selenium in water. Due to increasing selenite and organo-Se through the system, the selenium in the effluent of the system may be more available to wildlife than the influent. The water and soil samples confirmed that selenium is being transformed into more bioavailable forms, especially by the High Rate Pond. Recommendations were made to modify the Algal Bacterial Selenium Removal System in order to reduce the formation of these more bioavailable forms. Removal of the High Rate Pond from the system may reduce the amounts of available selenium and will be investigated in further studies. When modified, the Algal Bacterial Selenium Removal System may prove to be an effective low-cost method of selenium removal.Introduction Selenium (Se) is a semi-metallic element, often called a metalloid (Wilber, 1983). Although it is an essential element for animal and human nutrition, in high concentrations it has been shown to create toxicity problems in livestock and wildlife (Bainbridge et al. 1988). These high concentrations of selenium can occur due to natural enrichment of Se in soil originating from Cretaceous marine sedimentary rock (Presser et al. 1994). Selenium exists in different oxidation states. Selenate (Se6+ ) and Selenite(Se4+ ) are both water soluble inorganic species commonly found in aerobic water sources and elemental Se is more readily found in anaerobic sediments (Rosetta and Knight, 1995). Se also forms into selenoproteins, such as selenomethionine, analogous to the essential amino acid methionine, in which sulfur is replaced by selenium (Alaimo et al. 1994). Toxic effects of inorganic and organic Se are caused by the alteration of protein three dimensional structure and the impairment of enzymatic function of an organism (Demayo et al. 1979). The San Joaquin Valley in California has been found to contain soils with naturally high concentrations of Se (Losi and Frankenberger, 1997). Agricultural use of the lands in the San Joaquin Valley have resulted in leaching of selenium into drainage water. Drainage water can contain selenium concentrations, mainly in the form of selenate, of 230 to 640 µg/liter (Cantafio et al. 1996). The USEPA safe criterion level for chronic exposure of aquatic life to Se is 5 µg/liter (Dobbs, 1997). In 1986, high concentrations of Se in the drainage water at Kesterson Reservoir were shown to be causing embryonic mortality and abnormalities in aquatic birds (Ohlendorf et al. 1986). These negative effects on aquatic birds from Se were due to biomagnification, the increase in concentration of a substance in living tissue as it moves through the foodweb. Through feeding and direct uptake Se can bioaccumulate, be absorbed and stored, in organisms such as small aquatic invertebrates. Birds then feed on these organisms and with each feeding more Se is being ingested than excreted and Se concentrations within the birds increase. Kesterson was declared a toxic site in 1987 and has since been filled (Presser et al. 1994). However, farmland irrigation water continues to be drained into the San Joaquin River or into on-farm evaporation ponds and levels of selenium are well above the suggested limit to protect aquatic wildlife (Presser, 1994). Many research groups have been looking for cost-effective methods of selenium removal. It has been found that certain bacteria have the ability to reduce Se into less toxic forms or intoforms that can be more easily removed (Losi and Frankenberger, 1997). An Environmental Engineering group at UC Berkeley led by Professor W.J. Oswald has developed a selenium removal system called the Algal-Bacterial Selenium Removal (ABSR) System. Agricultural drainage water enters the system where selenium levels are theoretically reduced by the bacterial and algal processes and the resulting discharge is less contaminated. This system uses a combination of ponds containing algae and bacteria in which selenate, the major form of selenium in the drainage water, is reduced to selenite and elemental selenium which are more easily removed from the system (Lundquist et al. 1994). Influent (IN) water undergoes four steps in the system. The steps of the process are summarized as follows: 1) In an anaerobic Reduction Pond (RP), bacteria reduce selenate to selenite and elemental selenium which precipitate out into a sludge on the bottom of the pond. 2) In a High Rate Pond (HP), water is re-oxygenated and cleaned for surface discharge by algae cultures and high rate movement. 3) In a Dissolved Air Flotation (DAF) device, ferric sulfate polymerizes algal cells into flocs which are carried to the surface by air bubbles and then skimmed off and removed. 4) In a slow sand filter (SSF), any remaining particles are filtered out of the water. A pilot study using this pond system has been set up in the Panoche Water District near Los Banos, approximately 285 miles Southeast from Berkeley. Preliminary results have shown that the method may be able to reduce the total Se in the drainage water up to 80%, causing discharged water to be at concentrations much closer to suggested levels for wildlife safety (Quinn et al. 2000). Although much of the total selenium may be removed using the ABSR system, it is essential that the remaining selenium is in a form that is not readily bioavailable to aquatic organisms. The toxicity of Se is directly related to its bioavailability (Losi and
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