SCOUTING THE WAY: GEOPHYSICAL METHODS AS INDISPENSABLE TOOLS FOR MONITORING AND TARGETING MANAGED AQUIFER RECHARGE IN CALIFORNIA’S CENTRAL VALLEY (Invited Presentation)

Managed aquifer recharge (MAR) is widely discussed as a critical tool to help achieve groundwater sustainability in California’s Central Valley. There are many unanswered questions regarding MAR that hinder full implementation, including how water can be most effectively redirected into the ground, how to select optimal sites for recharge, how much recharge water actually reaches the aquifer and the time scale of recharge, and the role recharge water in mobilizing and transporting contaminants and pathogens through the subsurface. Addressing such a broad range of questions will require a multidisciplinary approach, and one such aspect that has gained substantial attention recently is the use of geophysical methods for site assessment and monitoring. Geophysical methods are especially useful for providing high lateral resolution data to fill information gaps between traditional well logs, soil borings and monitoring wells. I will discuss a range of ongoing MAR projects around the Central Valley where we have used a suite of geophysical tools to guide the selection of MAR sites and methodologies. These projects include electrical resistivity tomography surveys to image the wetting fronts associated with drywell injections, airborne and towed-transient electromagnetics for imaging large areas to assess recharge potential, and the coupling of logging methods to better characterize the complex hydrogeology of the vadose zone. These examples will demonstrate both the great potential of geophysical methods for addressing a wide range of questions around MAR implementation, the crucial limitations facing all geophysical methods, and how pairing geophysical and traditional subsurface measurement techniques can give us the best chance of achieving optimal MAR outcomes.