THE EFFECTS OF AGRICULTURAL NITRATE SOURCES ON GROUNDWATER SUPPLIES IN CALIFORNIA

The California Water Resources Control Board, in collaboration with the US Geological Survey and Lawrence Livermore National Laboratory, has implemented the Groundwater Ambient Monitoring and Assessment (GAMA) Program to assess groundwater quality across the entire state of California (Belitz et al., 2004). A critical component of the program is to consider the major threats to groundwater resources that supply drinking water to Californians. Nitrate is the most pervasive and intractable contaminant in California groundwater and is the focus of special studies under the GAMA program.

Two of the major potential sources of nitrate to groundwater are agricultural (synthetic fertilizers and animal waste as fertilizer or from confined animal operations), and near developed areas these sources may intermix with nitrate from septic tank leachate. Agricultural animal waste and septic leachate have associated organic carbon inputs, a necessary constituent for heterotrophic denitrification. Our research focuses on nitrate source attribution, on close examination of the parameters that govern denitrification, and on quantification of denitrification in the saturated zone. A multi-disciplinary approach employing N and O isotopes of nitrate, excess dissolved nitrogen, tritium-helium groundwater age dating, dissolved organics analysis, and advanced microbiological techniques allows identification of the dominant nitrate source and examination of its fate in groundwater, and provides a predictive capability for the probability that deep groundwater used for drinking water will become contaminated with nitrate. Examples from dairy sites where saturated zone denitrification has been quantified, and from groundwater basins affected by decades of irrigated agriculture using synthetic fertilizer, illustrate the power of this integrated analytical approach.

Belitz, K., Dubrovsky, N.M., Burow, K., Jurgens, B., and Johnson, T. Framework for a Ground-Water Quality Monitoring and Assessment Program for California. U.S. Geological Survey Water Resource Investigation Report 03-4166; U.S. Geological Survey: Sacramento, 2003.

This work was performed under the auspices of the U.S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under contract No. W-7405-ENG-48.