Groundwater quality is critical for safe drinking water and irrigation supplies but can be threatened by geogenic toxins that are difficult to predict. In the arid, high desert San Luis Valley (SLV), Colorado, a groundwater basin serves as the primary water supply with observed arsenic concentrations exceeding the maximum contaminant level (MCL) of 10 μg/L set by the U.S. EPA. However, the sources and processes responsible for As occurrence are unclear. Through a community-engaged sampling effort, we collected 244 groundwater samples and measured major/trace element concentrations. Long-term land subsidence and depth-resolved sediment texture were computed at the same locations. We tested three plausible geochemical processes responsible for As release: (1) overpumping-induced dewatering of As-bearing clays (proxied by land subsidence), (2) pH-promoted desorption as well as reductive dissolution of As(V)/Fe(III) (hydr)oxides, and (3) incursion of higher-As geothermal fluids (proxied by lithium, boron, tungsten, and molybdenum) into groundwater. We find that statistics, statistical/machine learning, and aqueous thermodynamics all agree that geothermal fluid mixing within the aquifer is the main source of dissolved As. Our findings suggest that overpumping draws higher-As thermal fluid from the bottom of the aquifer to pumping depth, leading to increased concentrations of As in drinking/irrigation water supplies at wells.