Exploring geologic controls on infiltration and groundwater recharge on an ephemeral river: a coupled geophysics and modeling approach

Abstract

Ephemeral streams are the main surface-water source in arid climates, and streambed leakage in these systems is an important component of groundwater recharge. Subsurface geology influences the extent and location of streambed leakage and therefore changes streamflow, impacts groundwater levels, and has the potential to influence confined aquifer recharge. This study looks at exploring geological controls on recharge from an ephemeral river through scenario evaluation with numerical models, constrained by geophysical observations. Drone magnetometer, electrical resistivity, hammer seismic, and hydrologic data were collected along the Alamosa River in southeastern Colorado, USA to constrain and parameterize a MODFLOW-SFR model based on the field system consisting of three layers: 1) an alluvial aquifer, 2) a confining unit, and 3) a confined aquifer. Four scenarios were explored beyond a base case to quantify controls on subsurface geology: 1) adding a fault; 2) changing the alluvial aquifer’s hydraulic conductivity; 3) changing the thickness of the streambed; and 4) removing the confining unit. A fifth scenario, adding a pumping well, was used to explore the role of this human influence on ephemeral river systems. The drone magnetometer and resistivity data indicated that the middle portion of the study area was geologically distinct from the surrounding area, and the resistivity and seismic results indicate the presence of heterogeneity in the subsurface. Modeling scenarios that changed hydraulic conductivity values resulted in the most notable changes to the river’s hydrologic system, altering streamflow, leakage, and deep aquifer recharge. Streambed thickness proved to be an unimportant parameter. Results here suggest that the extent to which streambed leakage changes is proportional to the ratio of alluvial aquifer hydraulic conductivity (K1) to streambed conductivity (Ks), and that this in turn controls the impacts on streamflow. This research suggests that subsurface heterogeneities are a fundamental control on ephemeral streams’ hydrogeologic system and are key to their resilience under climate change.