Defining Hydrogeophysical Layers With Multi‐Scale Geophysics for Increased Understanding of Mountain Basin Recharge.

Basin aquifers are important groundwater sources in the Western United States that are increasingly stressed due to growing populations, increased resource use, and the impacts of climate change. These aquifers are mainly recharged through melting snowpack in the surrounding mountains that infiltrates to the water table and flows directly into the basin (Mountain Front Recharge), or through deeper groundwater pathways that flow from the mountains directly into the basin aquifer (Mountain Block Recharge). However, the dominant system of recharge remains uncharacterized in many mountain basin aquifers. To address this challenge, near‐surface geophysical methods are being implemented to efficiently measure properties that govern groundwater storage and movement. This study infers groundwater storage and recharge to the Casper Aquifer around Laramie, WY, building off past studies that relied solely on sparse monitoring well data and observation of rainfall events. In this study, we use a clustering analysis on airborne electromagnetic data to define hydrogeophysical layers within the Casper Aquifer. These layers, which represent significant changes in bulk subsurface electrical resistivity, are integrated with existing hydrologic, lithologic, and smaller scale geophysical datasets to build a more representative hydrogeophysical model. Through this analysis, we define two sub‐aquifers within the larger Casper Aquifer system that are connected through structurally induced fractures and faults. This research highlights the importance of integrating geophysical data at multiple scales for defining hydrogeophysical layers that provide both a more complete understanding of basin aquifer recharge dynamics and constrain more detailed hydrologic models. Plain Language Summary: Growing populations, increased resource use, and climate change all lead to rising stress on groundwater resources in the Western United States. Groundwater in the western mountain region is generally stored in sedimentary basin aquifers that are understood to be recharged through precipitation that either: (1) infiltrates the water table and then flows directly into the basin, or (2) through deeper groundwater paths that flow from the crystalline cores of the mountains into the basin aquifers. However, relevant recharge systems are uncharacterized in many mountainous aquifers of the Western US. To address this challenge, near‐surface geophysical methods are being used to efficiently measure properties that govern groundwater storage and movement into these basin aquifers. This study uses hydrologic and multiple scale near surface geophysical data (airborne, surface and borehole) as inputs to constrain the storage and recharge dynamics on the Casper Aquifer near Laramie, WY. Integrating lithologic, hydrologic, and geophysical data at fine to large scale, highlights that the Casper Aquifer recharges primarily through snowmelt infiltration atop the nearby Laramie Range, where it flows downslope into the basin until it intersects fracture zone permeability pathways that facilitate movement of groundwater to shallower levels from a deeper primary subsurface aquifer layer. Key Points: Hydrogeophysical layers, distinct by their geophysical properties, are defined through the integration of multi‐scale geophysical dataNew mountain basin, front slope flow recharge regime defined for basin aquifers that outcrop from valley basin to summit of front slopeIntegrating multiscale geophysical data generates more complete understanding of groundwater storage and movement than traditional hydrologic studies [ABSTRACT FROM AUTHOR]