EXPLORING GEOLOGIC CONTROLS ON INFILTRATION AND GROUNDWATER RECHARGE ON AN INTERMITTENT RIVER: A COUPLED GEOPHYSICS AND MODELING APPROACH

Non-perennial rivers and streams are the main surface water resource in arid climates, and streambed infiltration in these systems is a vital component of groundwater recharge. Subsurface geology controls the extent and location of streambed infiltration and therefore impacts both streamflow and groundwater levels. This study explores geological controls on groundwater recharge through an intermittent river streambed using scenario evaluation with numerical models constrained by field observations. Our conceptual model included five fundamental variations in the system that could impact where and how much recharge is possible: 1) the presence of a fault; 2) variation in the alluvial aquifer hydraulic conductivity; 3) variation in the thickness of the streambed; 4) presence or absence of a confining unit; and 5) groundwater withdrawals via pumping. To achieve a realistic outcome, we parameterized the model using field observations from the Alamosa River in Colorado as an example. Scenarios that changed hydraulic conductivity values resulted in the most notable changes to infiltration, streamflow, and deep aquifer recharge; conversely, variations in streambed thickness had the least impact. The extent to which streambed infiltration occurs is dependent on streambed properties as well as the hydraulic properties of the underlying alluvial aquifer, and this in turn controls the impacts on streamflow. This research shows that subsurface heterogeneities are a fundamental control on non-perennial rivers’ hydrogeologic systems and are key to their resilience.