Contamination of small basins impacts the quality of groundwater resources and the health of residents dependent on local, limited water supplies. Understanding contaminant sources is crucial not only to planning mitigation and cleanup but also to the rural community, who must participate 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 significantanthropogenic nitrate contamination. Groundwater nitrate from diverse sources can be differentiated on the basis of chemical and isotopic compositions. Samples analyzed for dissolved or leachable ion concentrations as well asδ15{sup left}N provide bothchemical and isotopic signatures that distinguish between potential contaminant sources. Data indicate a predominance of anthropogenic, organic human and/or animal waste and decay of irrigation-enhanced vegetation. Natural nitrate sources are minor butinclude low concentration soil nitrate from decay of sparse, natural vegetation and nitrogen-bearing rock units. Anthropogenic sources almost certainly contribute more than half of the nitrate found in the basin as a whole and provide the dominant sourcein the≈40 of water wells that routinely or occasionally exceed the nitrate-N public drinking water standard concentration of 10 mg/L.
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