INDUCED RECHARGE AND CHANGES IN GROUNDWATER-SURFACE WATER INTERACTION IN RESPONSE TO LONG TERM PUMPING OF A MULTI-AQUIFER SYSTEM

The response to pumping in multi-aquifer systems involves complex processes which can significantly affect regional water budgets. Particularly where long term pumping has occurred, drawdown might take decades to propagate regionally. This can result in the mischaracterization of critical water budget components like groundwater-surface water exchange, inter-aquifer flow and recharge. An evaluation of historical water level data (1960s to 2010s) from wells in the Denver Basin aquifer system documents large hydraulic head declines (> 50 m in some areas) and a multi-decade transition to induced downward gradients and bedrock aquifer recharge near perennial streams. The implications for regional water budgets are evaluated using a 2D variably saturated finite-difference model which quantifies fluxes across stream, alluvium and bedrock interfaces in a vertical sequence. Modeling and field data indicate that long-term head decline can produce complex saturation conditions near stream channels, including a perched saturated zone in the alluvium underlain by an unsaturated region in the bedrock aquifer. Saturation levels and fluxes across interfaces are strongly dependent on the geologic heterogeneity, particularly the locations of channelized sandstones within the bedrock aquifer. The results of this study provide insight into the mechanics of long-term water budget change, including controls on the transition to induced recharge. This has important implications for assessing the aquifer response to ongoing and future stresses.