Selenium Contamination in Corbicula Transplanted into Agricultural Drains in the Imperial Valley, California
Studies of environmental contaminants in the Imperial Valley and Salton Sea area began in 1986 and selenium was identified as a major element of concern. Black-necked stilts, colonial waterbirds, and desert pupfish were all found to be at risk of reproductive problems as a result of selenium contamination. This study was designed to examine selenium on the scale of individual drains as opposed to an ecosystem basis. The Asiatic River clam (Corbicula sp.) was used to evaluate selenium bioaccumulation in several irrigation drains in the Imperial Valley. The objective was to identify drains with high bioavailability of selenium as potential targets for drain-specific selenium remediation. Clams were placed in upstream, midstream, and downstream locations in 18 drains and analyzed for selenium concentrations in January and February of 1995. Selenium concentrations in the different drains was expected to vary because the drains get different proportions of tile water. Tile water is the water from subsurface drains with the highest selenium load from soil accumulation.
Methods: Corbicula of approximately 2.5 centimeters in length or larger were collected from Cibola National Wildlife Refuge on the Colorado River upstream of Imperial Valley irrigation drainage on January 5-6, 1995. Twenty-five to fifty clams were placed in traps which were placed on the surface of the sediments in water less than 0.4 m deep. At this time dissolved oxygen, pH, temperature and conductivity were measured. The reference site was located along the East Highline canal. This water comes from the Colorado River via the All-American canal and reflects water quality prior to its agricultural use in the Imperial Valley. Clams were collected on either a weekly or biweekly schedule. Water quality measurements were repeated and the clams were submitted for selenium analysis and moisture content determinations.
Results: The lowest concentration of selenium in clams placed in a drain was 6.0 ppm and the highest concentration measured was 15.8 ppm. Reference sample concentrations ranged from 8.5 ppm to 15.5 ppm and clams collected from the Colorado River as controls ranged from 8.1 ppm to 11.9 ppm. No pattern emerged regarding upstream sites versus downstream sites or the length of time the clams were in the drains. No particular area within the Imperial Valley was found to have consistently elevated selenium in the drains. The concentrations measured in Corbicula are in the dietary range that has been found to cause reproductive effects in birds. Because the concentrations of selenium in clams from the main channel of the Colorado River were higher than expected, it is not possible to clearly identify the source of selenium measured in the clams which were placed in the drains. It is possible that the selenium seen in the clams was already present at the time of placement in the drains. This technique would have been more useful if a sufficiently large source of uncontaminated clams were available. Additional options for future selenium bioaccumulation studies include collecting species such as sailfin mollies or juvenile tilapia from areas with and without drainwater influences.
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