University of California

Presentations 2016

Young, Megan

Presentation Title
Stable isotopes as indicators of sources and processes influencing nitrate distributions in groundwater beneath dairy farms in California
United States Geological Survey
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Dairies represent the majority of animal feeding operations in California, and have been shown to be potential sources of nitrate and salinity, dissolved organic carbon, and pathogens to groundwater. In California’s Central Valley, nitrate contamination of drinking water wells is a significant concern, and there are multiple sources of nitrate including septic discharge, synthetic and manure fertilizers, and concentrated animal feeding operations. In order to better understand the potential contributions of dairy manure derived nitrate to both shallow and deep groundwater, we used a combined geochemical and stable isotope approach for water samples collected from a network of shallow groundwater monitoring wells on several California dairies located in two distinct geographic regions. In the northern region, the lower San Joaquin Valley, the water table is shallow (2- 5 m below surface) and therefore considered highly vulnerable to contamination, while in the southern region, the Tulare Lake Basin, the water table is much deeper (20 - 30 m).In each dairy, nitrate isotopes, water isotopes, nutrient concentrations, and other chemical and physical parameters were measured in monitoring wells located within different land use areas of the dairies. Monitoring wells were classified by the dairy-related land uses of corrals, fields receiving manure, waste lagoons, and mixed land use (undetermined). Across all sampled dairy monitoring wells, d15N-NO3 ranged from +2.9 to +49.4‰, and d18O-NO3 ranged from -3.3 to +19.2‰. Mean nitrate concentrations, nitrogen isotopic composition, and oxygen isotopic composition were significantly higher in the northern (Stanislaus County) dairy wells in comparison to the southern (Kings and Tulare Counties) dairy wells. Nitrate isotope measurements indicated that many of the northern monitoring wells had consistently high contributions of manure-derived nitrate to the shallow groundwater during the 16 month study. Monitoring wells located in relatively new dairies in the south region showed little evidence of manure-derived nitrate, while those located in much older dairies in the south region showed a very wide range of nitrate isotope values, indicating significant nitrate contributions from multiple sources including manure and industrial fertilizer and biological processing effects. Combined nitrate concentration and isotopic data from all the monitoring wells showed very little evidence of significant saturated-zone denitrification. Monitoring well networks within individual dairies showed wide ranges of nitrate concentrations, nitrate isotopic compositions, and geochemical compositions, confirming the heterogeneity of the nitrate loading across dairy facilities and indicating that measurements from any single monitoring well may not be representative of general groundwater quality downgradient of an individual dairy.

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