CONCENTRATIONS OF SELENIUM IN BIOTA, SEDIMENTS, AND WATER AT CIBOLA-NATIONAL-WILDLIFE-REFUGE

Selenium contamination was studied at Cibola National Wildlife Refuge (Cibola NWR) in the lower Colorado River Valley, California and Arizon a, USA. The objective was to determine whether local irrigation practi ces resulted in exposure of fish to toxic concentrations of selenium. Water, sediment, fish, and crayfish were collected from sites that rec eived irrigation return flows and sites that did not. At sites receivi ng irrigation return flows, selenium was below concentrations that are toxic to fish. However, at two sites receiving water directly from th e Colorado River, selenium was at the toxicity threshold for fish. Sel enium concentrations were also elevated in crayfish from the sites whe re concentrations in sunfish were at the toxicity threshold. Further i ncreases in selenium concentrations at sites that are already at the t oxicity threshold could impair reproduction of sensitive species. Site -specific limnological conditions may play a role in accumulation of s elenium to toxic concentrations, but major sources of selenium seem to be upstream in the Colorado River basin rather than from local agricu ltural practices. Selenium input to the Colorado River from irrigation projects, coal-fired power plants, and natural sources should be redu ced to minimize the potential for selenium-induced toxicity in fish in backwaters along the lower Colorado River.