Denitrification and Nitrate Transport in Groundwater Underlying Large Dairy Operations in California?s Central Valley
Lawrence Livermore National Laboratory
California has the largest dairy industry in the nation. California also relies on groundwater for more than half of its drinking water supply, and nitrate contamination is a demonstrated impact of dairy operations. Denitrification, the microbially-mediated conversion of nitrate to molecular nitrogen, is the largest sink for nitrate in the subsurface. Quantifying denitrification is important to validating groundwater and farm models, to predicting the arrival of dairy nitrate at drinking water wells, and to developing and assessing best management practices. Denitrification also affects the use of new contaminant and water tracers, including nitrate dual isotopic composition for source attribution, and dissolved noble gases for tritium/helium-3 age dating and identification of lagoon seepage. In this presentation, we will discuss evidence for denitrication at dairy sites in both the vadose and saturated zone; the impact of denitrification on the use of dissolved gas tracers; and how best to use new non-traditional tracers for dairy groundwater monitoring. Over the past six years, Lawrence Livermore National Laboratory has conducted research on nitrate transport and denitrification at five dairies in the San Joaquin-Tulare basins of California. These studies have sampled water collected from production wells, monitor wells and manure lagoons; and core sediment and water samples from the installation of monitor wells. They have integrated the use of field and laboratory measurements of dissolved gas to identify excess nitrogen from saturated zone denitrification and to identify manure lagoon seepage; isotopic characterization of nitrate-N and O across a large dynamic range in nitrate concentration for both source attribution and identification of denitrification; quantitative real-time PCR to identify sites of active denitrification; direct push cone penetrometer surveys and geostatistics to characterize heterogeneity in subsurface hydraulic properties; and tritium/helium-3 age dating with multi-phase modeling of both tritium and helium in the saturated and vadose zones. On the most intensively studied dairy in Kings County, this approach has identified saturated-zone denitrification that is actively mitigating nitrate loading from the dairy operation, and may be enhanced by the operation?s water management practices. Denitrification has also been identified in shallow groundwaters underlying the northern dairies. The production of nitrogen through denitrification can drive gas stripping, and affect the interpretation of both nitrogen data used to determine the degree of denitrification, noble gas signatures used to estimate recharge temperature and identify manure lagoon seepage, and helium content used in the calculation of tritium/helium-3 groundwater age. The routine use of measurements of excess nitrogen, nitrate isotopic composition and groundwater age in dairy groundwater monitoring provides a powerful approach to source attribution and to assessment of the effectiveness of best management practices. This work was carried out with funding from Lawrence Livermore National Laboratory, the California State Water Board, and Sustainable Conservation in collaboration with the University of California, Davis and the USGS. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.