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<br />(Valdes-Casillas et aI., 1998a). The highest concentrations <br />in the delta are found in the Rio Hardy, in the Colorado <br />River downstream of the Rio Hardy confluence, in <br />evaporative basins, and in agricultural drains in Zone 4 <br />(Valdes-Casillas et aI., 1998a). Delta selenium levels have <br />been found to be 1.8 to 14.2 times the u.s. EP A limit of 5 <br />micrograms per liter (ug/l) for freshwater aquatic life. <br />Levels also exceed the Mexican limit of 8 ug/l. Agricul- <br />tural return flows in the Mexicali Valley appear to be <br />the biggest contributor to delta selenium levels. Because <br />selenium bioaccumulates in the food chain, high con- <br />centrations of selenium underscore the need for further <br />sampling and analysis to determine selenium levels in <br />a range of delta species (Garcia-Hernandez, 1998). <br /> <br />Research in la Cienega de Santa Clara (Zone 7) in 1996 <br />and 1997 found that selenium concentrations in drain <br />water entering the marsh are 2.5 times higher than those <br />in water at Imperial Dam in the United States. At the <br />southern end of the wetland, concentrations range from <br />5 to 19 micrograms per liter. Samples of sediments, <br /> <br />plants, and fish showed concentrations not considered <br />hazardous for wildlife or humans.38 However, further <br />deterioration of water quality could lead to higher <br />levels of selenium in fish and wildlife, and additional <br />monitoring of la Cienega is warranted. <br /> <br />Salinity concentrations in delta wetlands have been <br />monitored in various biological surveys, often in con- <br />junction with monitoring of other contaminants. Most <br />of this work has focused on la Cienega de Santa Clara <br />(Valdes-Casillas et aI., 1998a). Salinity sampling in the <br />Rio Hardy/Rio Colorado wetlands was conducted in <br />July, August, and November 1997. The results show that <br />salinity is highest in areas that receive agricultural drain <br />water (the Hardy and Pescaderos rivers) and in areas <br />influenced by tidal flows. As salinity has increased, so <br />has tamarisk, which has become dominant in delta <br />riparian areas. While flood flows have allowed some <br />native species to regain a toehold, tamarisk is extremely <br />competitive and, with its deep roots, a great consumer <br />of water. <br /> <br /> <br />38 Results showed selenium concentrations of 0.8-1.8ug/1 in sediments, 0.03-1.17 ug/I in plants and 2.5-6.4 ugll in fish (Garcia-Hernandez, 1998). <br /> <br />30 <br /> <br />