An assessment of groundwater contamination in Central Valley aquifer, California using geodetector method

ABSTRACT Groundwater nitrate contamination of the aquifer in the Central Valley, California is a major problem due to intense agricultural practice over the last decades. Excessive loading of fertilizer and manure in combination with hydrogeological parameters and geochemical conditions have enhanced the downward percolation of nitrate into the aquifer. A spatial variance-based geographical detector method was used at the watershed scale in Central Valley to identify the key determinants to elevated nitrate concentration, locate the risk areas and analyse the interaction between these determinants. Statistically significant difference was observed in percent of wells with above background concentration (5mg/L) between the areas with low and high fertilizer application. Higher number of wells was also contaminated in areas with higher manure, higher permeability and higher dissolved oxygen conditions. These factors interacted with the hydrogeological parameters in exacerbating the groundwater nitrate contamination in the study area. A distinctly higher nitrate concentration was observed in Tulare basin and San Joaquin basin compared to Sacramento Valley, which could be attributed higher fertilizer rate, coarse grained sediment and over pumping of groundwater altering the hydrogeological conditions.

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