Application of multi-tracer methods to determine age distribution of nitrate contaminated wells in Turlock California

Nitrate contamination is the leading source of groundwater contamination in the United States. The problem is especially widespread in California, and particularly in areas where agriculture dominates land use. Turlock is located in Stanislaus County, which is one of California’s top ten agricultural commodity-producing counties and one of the top producers of agricultural commodities in the nation. Over the last one hundred years the primary crops cultivated in the vicinity of the City of Turlock are hay and almonds, which require heavy applications of nitrogen (N) fertilizers. Nitrate infiltration into the aquifer from over-applications of N fertilizers and manure from local dairies is beginning to present problems for the drinking water supply. Within the last 10 years, two of Turlock’s public drinking water wells were closed due to nitrate levels above the 45 mg/L Maximum Contaminant Level. Due to Turlock’s dependency on groundwater as the sole source of drinking water, coupled with its nitrate contamination issues, it is an ideal candidate for study. The objective of this study is to use geochemical and isotopic signatures to determine groundwater age and the source of nitrate in the groundwater of the Turlock Subbasin. Through analysis of sulfur-35, krypton-85, tritium, tritiogenic helium-3, terrigenic helium-4 and carbon-14, groundwater residence times of public drinking water wells in the City of Turlock were determined. Using this multi-tracer approach, groundwater age distributions revealed mixtures of modern (1950 to present day) and fossil (2-25 ka) groundwater. Recently recharged groundwater (0-20 years) was absent in all samples. The increasing nitrate concentrations which are expected to increase further, are explained by the groundwater residence time distributions. The risk of nitrate concentrations exceeding the health limit is significantly reduced by the fossil groundwater component, which is essentially nitrate-free.