Geophysical characterization of structures and hydrogeophysical pathways that transmit groundwater along the mountain front of the Casper Aquifer near Laramie, WY

Abstract:

Basin aquifers are an important source of drinking water in the western United States that are increasingly becoming stressed. The Casper Aquifer, located in southeastern Wyoming, is an example of such an aquifer. It is recharged by the Laramie Range mountain front and is the majority source of municipal water for the residents in and around the city of Laramie, WY. Earlier studies indicate that groundwater in this aquifer flows primarily through secondary transmissivity via fractures and faults. However, distantly spaced monitoring wells and surficial observations of structures provides insufficient knowledge about the extent and depth of the primary permeability pathways within the aquifer. This research uses data from an airborne electromagnetic (AEM) survey to map the major recharge pathways of the Casper Aquifer and inform municipal water policy. The AEM survey was flown over a targeted portion of the mountain front and basin aquifer too better map these pathways spatially and at depth. Inversion of the AEM data corresponds with the existing surface mapping of some geologic structures, and large ranges in the inverted resistivity throughout the survey area suggest that the AEM data distinguishes major pathways that transmit water from the mountain front to the basin. Correlation of borehole geophysical logging with the AEM data helps define the hydrogeophysical layers, and a hydrogeophysical model of the survey area is created by combining geophysical, geologic, and hydrologic properties of the known lithologies. Additionally, surface resistivity and monitoring well water level data collected at peak and minimal recharge states is compared to understand how depth to the water table affects the measured AEM resistivity. Interpretations of the hydrogeophysical model will inform municipal water policy and the development of a groundwater flow model of the mountain front and basin aquifer.