Modeling managed aquifer recharge processes in a highly heterogeneous, semi-confined aquifer system

Widespread groundwater overdraft in alluvial aquifer systems like the Central Valley (CV) in California, USA, has increased interest in managed aquifer recharge (MAR). Like most clastic sedimentary basins, recharge to the productive semi-confined CV aquifer system remains a challenge due to the presence of nearly ubiquitous, multiple confining units (silt and clay) that limit recharge pathways. Previous studies suggest the presence of interconnected networks of coarse-texture sand and gravel deposits that bypass regional confining units over a small fraction of the CV near the American and Cosumnes rivers. Here, variably saturated infiltration and recharge processes were simulated across a domain that includes high-resolution representation of the heterogeneous alluvial geologic architecture in this area. Results show that recharge potential is highly dependent on subsurface geologic architecture, with a nearly 2 order-of-magnitude range of recharge across the domain. Where interconnected coarse-texture recharge pathways occur, results show that these features can (1) accommodate rapid, high-volume MAR and (2) propagate widespread and rapid pressure responses over multi-kilometer distances in the semi-confined aquifer system. For all MAR simulations, results show that the majority of MAR is accommodated by filling unsaturated-zone (UZ) pore volume. Results also show that coarse-texture UZ facies (where present) accommodate the majority of MAR volume during early time, but fine-texture facies ultimately accommodate the majority of the total MAR volume, even for coarse-dominated sites. These findings highlight the large variability of MAR potential across the landscape and demonstrate the importance of fine-texture facies for accommodating MAR in alluvial aquifer systems.