SUPPORTING MANAGED AQUIFER RECHARGE DECISION-MAKING USING HYDROGEOLOGICAL MODELLING SUPPORTED BY DENSE GEOPHYSICAL IMAGING

It is essential to evaluate managed aquifer recharge (MAR) viability before implementing a recharge strategy, but site evaluation has typically focused on surficial measurements and suitability maps derived from regional soil surveys. Recent work in California’s Central Valley illustrates the importance of evaluating sediment texture and water content in the region of the vadose zone below the shallow soil layers (>2 m depth), especially at sites with a deep water table (>15 m depth). We present recent work from the Central Valley that studies the viability of flood-MAR using geophysical scanning and logging tools. A towed-transient EM system (tTEM) was deployed to reconstruct a dense, 3D model of the subsurface electrical resistivity underlying the proposed site. Next, we followed up with direct push logging targeted based on the resistivity model to provide sediment texture and water content logs. Using these collocated logging data and the resistivity model, we extrapolated estimates of sediment texture, in the form of volumetric coarse fraction, and water content across the resistivity model domain. At some sites, these results were sufficient for making binary yes-no decisions regarding MAR implementation. However, these geophysical models were also used as the foundation for detailed hydrogeological modeling. We illustrate how to use a sediment texture model estimated from geophysical scanning and logging to parameterize a model domain and perform simulations using the integrated hydrologic code ParFlow. The results of these models can be used to further assess recharge viability, as well as inform water managers of realistic timescales for realizing the benefits of a new recharge project.