ESTIMATING THE PROPORTION OF CHEMICAL FLUX FROM AGRICULTURAL SOURCES IN THE GROUND WATER OF THE SAN JOAQUIN VALLEY BASED ON PRINCIPAL COMPONENT ANALYSIS OF MAJOR MINERAL CONCENTRATIONS

Groundwater samples from the eastern San Joaquin Valley represent mixtures of waters from different sources. The source of the natural ground water is mainly from the recharge by precipitation in the Sierra Nevada. Because the area is predominantly agricultural, natural ground water is mixed with irrigation waters drained from farmlands due to deep percolation. In addition, nutrient (nitrate) contaminated water from numerous confined animal facilities, septic systems, municipal waste treatment facilities of cities and towns also contribute significantly to the compositions of the groundwater samples. We collected 125 groundwater samples from agricultural and domestic water-supply wells in a mixed suburban and agricultural area, and analyzed them for major minerals. The data were subjected to statistical correlation and principal component analyses. The results indicate that no significant spatial distribution pattern of groundwater compositions can be identified. However, apparently they are mainly dependent on their original sources and the mixing proportions among the different possible components. Three principal components are identified and they account for 84% of the sample variances. Compositions of some samples cannot be explained by the 3-component mixing model. They are likely influenced by local contamination sources of specific compositions. Based on these analyses, the ratios of mixing and the proportion of chemical flux due to groundwater recharge derived from agricultural and other anthropogenic sources can be estimated.