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Spatial and Temporal Variations in Nitrate Contamination of a Rural Aquifer, California

Dep. of Earth Sciences, Univ. of California, Riverside, CA 92521;

J. A. Johnson

Planning Div., U.S. Army Corps of Eng., 911 Wilshire Blvd., Los Angeles, CA 900017;

L. J. Lund

Dep. of Soil and Environmental Sciences, Univ. of California, Riverside, CA 92521;

Z. J. Kabala

Dep. of Civil and Environmental Eng., Duke Univ., Durham, NC 27708.

^* Corresponding author (williams{at}ucracl.ucr.edu).

ABSTRACT

The quality of ground water in the Sierra Pelona watershed, California is examined as an example of a small rural groundwater basin in a mountainous area of arid climate. Water quality in this region has been seriously impacted by nitrate (NO₃) contamination with 42% of wells sampled 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 concentrations reported from this region suggest degradation of water quality due to anthropogenic activity. Dissolved ion concentrations, particularly NO₃, chloride and calcium varied radically in 55% of well waters sampled prior to, following, and months after an unusually wet winter. Our extensive well sampling program, chemical results and delineation of spatial and temporal NO₃-N variation allow us to constrain possible contamination sources and transport mechanisms active in the Sierra Pelona basin. The spatial distribution and temporal variability of NO₃ indicate three patterns of contamination: (i) isolated wells impacted by numerous, localized NO₃ sources which erratically affect a single well without significantly contaminating neighboring ones; (ii) a tight cluster of wells, unusually low in other ions but consistently high in NO₃; (iii) moderate and generally consistent NO₃ concentrations, found over a large, diffuse region of the Sierra Pelona alluvial aquifer. An understanding of the differing NO₃ source(s) and contamination mode(s) that contribute to these observed contaminant patterns is critical to development and success of any strategy for contaminant mitigation and/or remediation.