Numerical Modeling of Groundwater Depletion and Restoration Strategies in Coastal California

Numerical modeling applications in hydrogeology range from pore scale to global simulations, and help quantify fluid and solute transport dynamics in variably saturated, multi-permeability, and multiphase systems. Simulations of flow are essential for addressing issues of fundamental hydrologic science, and even more so for developing and testing management or remediation strategies. Numerical modeling can be used to explore and evaluate management options without expending the cost and time required by field testing. We present a brief overview of numerical modeling in hydrology, followed by a case study of a severely overdrafted coastal basin in California, USA. A finite difference fluid transport model was used to evaluate the efficacy of groundwater restoration strategies, specifically managed aquifer recharge (MAR) impact on groundwater level recovery and reduction of seawater intrusion into the local aquifers. First, we developed MAR project placement scenarios based on an analysis of surface and subsurface hydrologic properties and conditions, using a geographic information system (GIS). Second, we assessed the hydrologic impact of potential MAR placement and operating scenarios. For the region evaluated in this study, GIS results suggest that about 7% (15 km) of the basin may be highly suitable for MAR. Numerical modeling results suggest that simulated MAR projects placed near the coast help to reduce seawater intrusion more rapidly, but these projects also result in increased groundwater flows to the ocean (i.e. applied MAR water inefficiency). In contrast, projects placed farther inland result in more long-term reduction in seawater intrusion and less groundwater flowing to the ocean. This work shows how numerical modeling can assist with regional water supply planning, including evaluation of options for enhancing groundwater resources.