NATURAL AND ANTHROPOGENIC NITRATE CONTAMINATION OF GROUNDWATER IN A RURAL-COMMUNITY, CALIFORNIA

Contamination of small basins impacts the quality of groundwater resou rces and the health of residents dependent on local, limited water sup plies. Understanding contaminant sources is crucial not only to planni ng mitigation and cleanup but also to the rural community, who must pa rticipate in any scientific or regulatory effort. Groundwaters, rock, and soil samples representative of the Sierra Pelona Valley in an arid , ''Mediterranean'' climate region of southern California indicate sig nificant anthropogenic nitrate contamination. Groundwater nitrate from diverse sources can be differentiated on the basis of chemical and is otopic compositions. Samples analyzed for dissolved or leachable ion c oncentrations as well as delta(15)N provide both chemical and isotopic signatures that distinguish between potential contaminant sources. Da ta indicate a predominance of anthropogenic, organic human and/or anim al waste and decay of irrigation-enhanced vegetation. Natural nitrate sources are minor but include low concentration soil nitrate from deca y of sparse, natural vegetation and nitrogen-bearing rock units. Anthr opogenic sources almost certainly contribute more than half of the nit rate found in the basin as a whole and provide the dominant source in the approximate to 40% of water wells that routinely or occasionally e xceed the nitrate-Ri public drinking water standard concentration of 1 0 mg/L.