LARGE-SCALE WATER, SALT, AND NITRATE BALANCE CALCULATIONS FOR GROUNDWATER AND SURFACE WATER FOR SALT AND NITRATE MANAGEMENT PLANNING IN CALIFORNIA’S CENTRAL VALLEY FLOOR

Rising salt and nitrate levels threaten California community drinking water supplies and agricultural productivity. The California State Water Board intends to have salt and nitrate management plans developed for all basins and subbasins in the state by May 2014. The Central Valley Salinity Coalition (CV-SALTS), a stakeholder group that includes regulators, agricultural entities, water agencies and others, was formed to help develop sustainable salt and nutrient management planning for California’s Central Valley. The Initial Conceptual Model (ICM) (i.e., Phase 1 of a Salt and Nitrate Management Plan (SNMP)) was recently completed to establish the foundation for the development of an SNMP for the Central Valley Floor. The main focus of the ICM effort for the ongoing CV-SALTS program is to develop an approach for estimating the movement of water, salt, and nitrate for groundwater and surface water in the Central Valley. This presentation will describe the approach developed for determining ambient concentrations of salt and nitrate in shallow and deep groundwater. The presentation will illustrate how this information, in conjunction with mass loading estimates developed from a watershed model and ambient surface water quality data, is linked to water budget components from the USGS’ Central Valley Hydrologic Model (CVHM) resulting in a transient salt and nitrate budget. The balance calculations for water, salt, and nitrate are performed over a 20-year simulation period to investigate the water quality trends in the Central Valley. This is a large-scale “30,000 foot level” analysis of the entire Central Valley Floor; therefore, the resolution of the budgets is at the scale of the 22 analysis zones established for the ICM. Priority areas for Phase 2 have been recommended and preliminary estimates of assimilative capacity for each analysis zone were developed for TDS and nitrate. Phase 1 efforts also included “proof of concept” analyses for two focus areas located in Stanislaus/Merced Counties and the Kings Subbasin. These additional studies involved application of MODFLOW (USGS Modesto Regional Model) and CVHM (Kings Subbasin) with MODPATH and MODPATH-OBS. The focus area analyses also compared ambient groundwater quality at the ICM analysis zone scale to the higher resolution model grid scale for the focus areas.