Occurrence and Aquifer Processes Affecting Nitrate in Groundwater in the Central Valley

A major groundwater quality concern in the agriculturally productive Central Valley of California in recent decades has been the growing occurrence of nitrate concentrations above drinking water thresholds. A synthesis of the current distribution of, and processes affecting, nitrate concentrations in groundwater was undertaken by using three data sources: (1) the California State Water Resources Control Boards’ Groundwater Ambient Monitoring and Assessment Priority Basin Project (GAMA-PBP), conducted in cooperation with the U.S. Geological Survey (USGS), (2) regulatory monitoring data in public-supply wells from the California Department of Public Health (CDPH), and (3) the USGS-National Water Quality Assessment (NAWQA) Program.  Spatially-weighted analysis of GAMA-PBP and CDPH data for the entire Central Valley indicate nitrate concentrations were higher than the U.S. Environmental Protection Agency Maximum Contaminant Level in about 3.5% of wells that represented depths of the aquifer used for public supply. Nitrate concentrations are highest in shallow groundwater and decrease with depth and age of the groundwater. Some of the groundwater at depths of the aquifer used for public supply predates the fertilizer use and increased pumping associated with modern land use in the Central Valley. Most Central Valley groundwater is oxic and, thus, denitrification is unlikely. Some Central Valley groundwater is reduced and analysis of dissolved-gas and nitrate-isotopic data suggest that partial denitrification may occur. Spatial and temporal trends of nitrate in Central Valley groundwater are primarily controlled by the patterns of landscape irrigation, groundwater pumping, and by the fluxes of nitrate from modern land-use superimposed on groundwater age and reduction-oxidation distributions.