Sensitivity Analysis of Long-term Groundwater Nitrate Dynamics in the Central Valley, California

Abstract:

Nonpoint source (NPS) contamination is a major environmental threat to agricultural communities that rely heavily on groundwater not only for irrigation but also for public and domestic water use. Statistical and index-based methods have been used to identify diffuse polluters and delineate vulnerable zones. However, they are ill-suited to assess future aquifer response dynamics to modern nitrogen management practices in the agricultural landscape. We have adopted numerical simulation methods typically employed in point source characterization and remediation management and developed the Nonpoint Source Assessment Tool (NPSAT) to evaluate both, vadose zone and groundwater travel times at computational speeds that allow for optimization and user interaction. We assess the sensitivity of the Central Valley application of NPSAT to estimates of hydraulic conductivity, groundwater pumping, recharge and vadose zone travel time, and to various approaches for obtaining boundary conditions describing long-term steady flow conditions. The Central Valley is an intensively farmed irrigated region, covering over 5 million ha, with 10s of thousands of production wells and over 100,000 domestic wells. The Central Valley NPSAT computes nitrate breakthrough curves at these wells and at receiving stream reaches over a multi-century time period, beginning in the early 20th century. We rely on two existing groundwater flow models, MODFLOW-based CVHM and IWFM-based C2VSIM, to describe long-term average boundary conditions and hydrogeologic parameters for the Central Valley aquifer system. We consider the two models as expressions of conceptual uncertainty and develop multiple Central Valley NPSAT incidences for sensitivity analysis. For transport simulation boundary conditions, two existing spatially distributed models of the root zone - crop system exist for the Central Valley that provide a conceptual uncertainty range for nitrate loading. The sensitivity analysis focuses on 21st and 22nd century predicted nitrate dynamics across the simulated wells and stream reaches under business-as-usual and alternative nitrogen management practices. Results provide an important tool to further refine the simulation approach, while also informing stakeholders about predictive uncertainty.