Movement of Salt and Nitrate in Shallow Groundwater in California's Central Valley - Large Scale Water, Salt, and Nitrate Balance Calculations
Abstract
A large-scale analysis of salt and nitrate was performed for the shallow groundwater aquifer of the entire California Central Valley floor (about 20,000 square miles). This analysis combined many different platforms of data in order to complete water and mass balance calculations. Groundwater and surface water quality test data were used in combination with mass loading from a watershed model (the Environmental Protection Agency's Watershed Analysis Risk Management Framework, or WARMF), as well as an integrated hydrologic model that simulates the use and movement of water coupled between the landscape, surface water, and groundwater (the U.S. Geological Survey's Central Valley Hydrologic Model, or CVHM). For this analysis, the Central Valley floor was divided into 22 zones, and the movement of shallow groundwater, surface water, salt, and nitrate was simulated in, out, and between the zones on a quarterly basis for a 20-year simulation period. In this analysis, shallow groundwater is defined by an estimate of the vertical distance water will travel from the water table within 20 years. Fluxes of mass from deep ambient groundwater and ambient surface water quality were estimated from measured concentration data. Quantities of mass were acquired for recharge (from WARMF output) or calculated using concentrations and other water budget components. Flow and volume components were extracted by post-processing CVHM output data. This resulted in a transient water, salt, and nitrate budget for each of the 22 zones. Simulated shallow groundwater concentrations were calculated to investigate water quality trends for the Central Valley. Four zones were identified as areas with the highest concentrations of salt (TDS) in the southwestern portion of the Central Valley; and six zones were identified as areas with the highest nitrate concentrations, mostly in the southeastern portion of the Valley. Additional analyses intended to shift from the large-scale balance calculations to a higher resolution analysis of the movement of water, salt, and nitrate was performed as a 'proof of concept' for two focus areas located in Stanislaus/Merced Counties and the Kings Subbasin, using MODPATH and MODPATH-OBS. Particle tracking was employed for both focus areas to observe the movement of water, salt, and nitrate from recharge zones to monitored wells, or on a cell-by-cell/layer-by-layer basis.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.H13E1387D
- Keywords:
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- 1800 HYDROLOGY;
- 1829 HYDROLOGY Groundwater hydrology;
- 1831 HYDROLOGY Groundwater quality;
- 1847 HYDROLOGY Modeling