Hydraulic Disconnection Between Aquifers: Assessing the Hydrogeologic Controls on Inter-Aquifer Exchange and Induced Recharge in Pumped, Multi-Aquifer Systems

Unprecedented, long-term pumping is occurring in aquifers worldwide, necessitating a greater understanding of the impacts from significant water table drawdown. Drawdown-induced hydraulic disconnection can significantly alter rates of inter-aquifer exchange and recharge, yet it remains an understudied phenomenon in multi-aquifer systems. This study investigates the potential for drawdown-induced hydraulic disconnection and its impact on inter-aquifer fluxes between a perennially recharged alluvial aquifer and a heavily pumped bedrock aquifer. We employed three-dimensional, transient, variably saturated flow modeling, incorporating multiple realizations of varying sandstone channel fraction (20–75%), to simulate evolving saturation patterns and alluvium-to-bedrock (A-B) flow rates. The results demonstrate the initiation and propagation of inter-aquifer unsaturated zones within sandstone channels underlying thinner low-permeability mudstones, leading to a substantial reduction in A-B flow, with the normalized flow response function (ABRF) decreasing by up to 98%. Complex saturation patterns, dictated by sandstone–mudstone heterogeneity, emerged as controls for water table elevation, disconnection status, and flow pathways. Multiple linear regression (R2 up to 0.88) identified the bedrock aquifer sandstone fraction and the vertical span of saturated, connected channels as significant predictors of maximum A-B flow. Substantial variability in maximum A-B flow rates across scenarios with identical sandstone fractions (coefficient of variation 0.17 to 0.29) demonstrates the impact of geologic heterogeneity and saturation state on inter-aquifer exchange rates. The results of this study illustrate that hydraulic disconnection is not limited to near-surface environments and that geologic heterogeneity is a key factor controlling inter-aquifer fluxes in heavily pumped multi-aquifer systems.