NEAR-SURFACE GEOPHYSICS TO MODEL GROUNDWATER FLOW AND RECHARGE OF INLAND FRESHWATER LENS IN ARID SOUTHWEST US

Groundwater flow monitoring is crucial for groundwater management, hydrostatic, and hydrodynamic processes, especially in semi-arid to arid lands due to sparse rainfall and high evaporation rates. Despite these harsh climatic regimes, local occurrences of fresh groundwater develop as precipitation from flash flood events rapidly infiltrates the vadose zone to the shallow and brackish aquifer. The upward force exerted by the denser host aquifer allows freshwater to float atop forming a lenticular-shaped lens also referred to as an inland freshwater lens or “IFL.” For this study, we investigate lateral and vertical groundwater movement in the shallow subsurface of a gypsum dune field using time-lapse electrical resistivity tomography throughout the North American Monsoon in the Southwest United States to understand IFL formation and evolution over time and space in a topographic depression and mound. We hypothesized vertical flow is dominant in highly conductive zones and topographic depression of the dune, whereas lateral flow is dominant along the windward slope of the parabolic dune system, and the antecedent soil moisture conditions impact on the soil’s hydraulic conductivity. Results show that low rates of water input to the soil allow dry soil regions to act as a barrier to lateral flow. Vertical flow, on the other hand, occurs in the same location despite the amount and magnitude of water input. We conclude that the timing, magnitude, and soil properties determine groundwater movement in the shallow subsurface in arid lands.