A Flow and Transport Model Developed as a Salt and Nitrate Management Analysis Tool for a Management Zone in California’s Eastern Kings Subbasin
Luhdorff & Scalmanini Consulting Engineers
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For the purposes of Salt and Nitrate Management Plans (SNMP), management zones (MZ) are created that cover a spatial area within which salt and nitrate can be managed by one or more entity. As an archetype for the Central Valley SNMP, an MZ was defined for Alta Irrigation District (AID), located in the eastern portion of the Kings Subbasin. Groundwater quality data are used to estimate the salt and nitrate occurrence for different zones of the saturated subsurface. Separately modeled loading of surface recharge (mass and volume) allows the assessment of the effects of various management regimes on groundwater quality (e.g., changes in irrigation efficiency and fertilization rates, artificial recharge projects, improving POTW effluent quality, etc.). A groundwater flow and transport model was developed using MODFLOW and MT3D to investigate the effects of different management practices on underlying groundwater over time.A local AID model was developed by extracting a portion of the Kings Subbasin from the USGS’s large-scale Central Valley Hydrologic Model (CVHM) (Faunt et al., 2009). Several transformations occurred to adapt the CVHM parent model into the smaller-scale local AID MZ Model. Previous efforts using CVHM for salt and nitrate movement assessments concluded that transient flow simulation durations were not sufficient to evaluate the impacts of management regimes on groundwater used for municipal drinking water purposes. Therefore, the AID MZ model was converted into a steady state model using averaged hydrologic conditions over a selected baseline hydrologic period for model inputs, including initial heads, general head boundaries on model edges, streamflow and stage data, diversion data, pumping data, and recharge data. The local steady-state AID MZ model was further refined by increasing the vertical and horizontal discretization. The baseline groundwater flow model was calibrated using water levels in monitored wells and contour maps of groundwater elevation. Management conditions were developed to represent ongoing shifts in land and water management. The Baseline condition involved historical surface application of N fertilizers and surface irrigation (i.e., before current regulatory program); Scenario 1 involved increased irrigation efficiency and artificial recharge projects; Scenario 2 involved lower fertilization rates in response to regulatory programs; and Scenario 3 combines the changes from Scenario 1 and 2. MT3D transport (transient) modeling included assignment of initial salt and nitrate concentrations to all cells, boundaries, and streams. Simulated recharge concentrations were provided on a cell-by-cell basis for the entire model area for each surface management condition. Transport modeling was run forward in time for 100 years to assess the effects of surface loading changes on different zones in the saturated subsurface.