Modeling groundwater pollution from nonpoint sources: How much does detail in time matter?

Efficient fertilizer use, appropriate crop selection, and agricultural managed aquifer recharge (Ag-MAR) are among the most promising approaches to address groundwater quality degradation from agricultural land use. For guidance on the deployment of these practices, assessment tools are needed to better understand the interaction between land use, land management practices (including Ag-MAR), and long-term nitrate dynamics in private and public water supply wells across groundwater basins. Here we investigate how the choice of time-resolution in modeling simulations of groundwater flow and nitrate transport affects our ability to predict future outcomes from nonpoint source (NPS) management.

A three-dimensional, fully transient flow transport model framework using MODFLOW and MT3D is developed for a 45-year study of nitrate improvements in an alluvial groundwater basin in Eastern San Joaquin Valley, CA.  Reference simulations (the design closest to "true" real world  scenario) are designed with high temporal and spatial grid resolutions for a large groundwater basin (1000s of square kilometers). We compare results of the reference simulations against those with up-scaled (coarsened) time-dependent recharge, pumping rate, and evapotranspiration boundary conditions.  Specifically, the reference simulations resolve these boundary conditions at a monthly scale (e.g., monthly pumping). Upscaled simulations simplified the temporal resolution to annual average and to a 45 year average. Furthermore, we also simulate a steady-state solution with long-term average stresses.  Our results indicate that the annual averaged recharge, well pumping, and evapotranspiration time-series are appropriate alternatives to using monthly time-series when simulating NPS (nitrate) transport for long-term scenario analysis. This finding suggests that numerical models of groundwater nitrate transport at regional scale can be developed using upscaled time-resolution boundary conditions which costs less effort computationally while creates accurate results.

temporal variability of "boundary" stresses in a groundwater system model

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