The use of managed aquifer recharge (MAR) to supplement groundwater resources can mitigate the risks to an aquifer in overdraft. However, limited information on subsurface properties and processes that control groundwater flow may lead to low levels of recapture of infiltrated water, reducing the efficacy of MAR operations. We used long 1D electrical resistivity probes to monitor the subsurface response over one diversion season at five locations beneath an operating recharge pond in northern California. The experiment demonstrated the benefits of integrating geophysical and standard hydrologic measurements. The water table response interpreted from time-lapse electrical resistivity images was in good agreement with traditional pore-pressure transducer measurements at coincident locations. Moreover, the electrical resistivity measurements were able to identify vertical variations in water saturation that would not have appeared in pore-pressure data alone. Changes in saturation estimated from electrical resistivity models indicated large hydraulic gradients at early time and suggested the presence of highly permeable conduits and baffles between the surface and the screened interval of recovery wells. The interpreted structure of these conduits and baffles would contribute to the movement of a large amount of infiltrated water beyond the capture zone of recovery wells before pumping begins, accounting in part for the low recovery rates.

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