Pilot Approach for Assessing Salt and Nitrate Loading to Groundwater in California's Central Valley

A study is underway as a key initial step in the effort by the Central Valley Salinity Coalition (CVSC) to help develop a Basin Plan amendment to address the issue of salt and nutrient management in California’s Central Valley. The overall objectives of the study are to develop and document procedures and methodologies to quantify the significant salt and nitrate sources in the Central Valley.  The procedures are being piloted in selected areas to evaluate their appropriateness and region-wide applicability. The study employs the use of the Watershed Analysis Risk Management Framework (WARMF) watershed model in coordination with the finite difference groundwater flow model, MODFLOW, to evaluate salt and nitrate mass loading to groundwater in three pilot study areas. Previously developed MODFLOW groundwater flow models for the Modesto and the Tule River pilot study areas are being used to provide information about recharge rates and groundwater pumpage to the WARMF model; salt and nitrate mass loads developed from the WARMF model will be shared with the groundwater models. MODFLOW with the particle tracking program, MODPATH, is being used to provide a map of the distribution of groundwater originating from different sources using backward and forward particle tracking techniques. The USGS Central Valley Hydrologic Model (CVHM), which was released in 2009 and uses advanced components of MODFLOW like the Farm Process, is being used to provide information about groundwater pumpage and deep percolation to the WARMF domain in a third study area in Yolo County. Salt and nitrate mass loads in the Yolo area will be simulated with the WARMF model, and the results will be compared to historical groundwater quality observations.