SPATIAL AND TEMPORAL VARIATIONS IN NITRATE CONTAMINATION OF A RURAL AQUIFER, CALIFORNIA

The quality of groundwater in the Sierra Pelona watershed, California is examined as an example of a small rural groundwater basin in a moun tainous area of arid climate. Water quality in this region has been se riously impacted by nitrate (NO3) contamination with 42% of wells samp led exceeding the EPA public drinking water maximum contaminant level (MCL) of 10 mg/L as NO3-N at some point during 1992-1993. High NO3-N c oncentrations reported from this region suggest degradation of water q uality due to anthropogenic activity. Dissolved ion concentrations, pa rticularly NO3, chloride and calcium varied radically in 55% of well w aters sampled prior to, following, and months after an unusually wet w inter. Our extensive well sampling program, chemical results and delin eation of spatial and temporal NO3-N variation allow us to constrain p ossible contamination sources and transport mechanisms active in the S ierra Pelona basin. The spatial distribution and temporal variability of NO3 indicate three patterns of contamination: ii) isolated wells im pacted by numerous, localized NO3 sources which erratically affect a s ingle well without significantly contaminating neighboring ones; iii) a tight cluster of wells, unusually low in other ions but consistently high in NO3; (iii) moderate and generally consistent NO3 concentratio ns, found over a large, diffuse region of the Sierra Pelona alluvial a quifer. An understanding of the differing NO3 source(s) and contaminat ion models) that contribute to these observed contaminant patterns is critical to development and success of any strategy for contaminant mi tigation and/or remediation.