A MULTI-TRACER APPROACH FOR CHARACTERIZING MOUNTAIN-BLOCK RECHARGE: CASE STUDY OF THE TUCSON BASIN IN SOUTHEASTERN ARIZONA (Invited Presentation)

High elevation winter precipitation can comprise the majority of recharge to alluvial aquifers in the Basin and Range physiographic province via surface pathways (e.g. mountain-front recharge) and subsurface pathways (e.g. mountain-block recharge). This is evidenced by basin groundwater possessing a stable isotopic signature close to that of snowmelt and supported conceptually by evapotranspiration demand in the summer far exceeding monsoonal precipitation. Given the profound impacts of climate change on mountain precipitation, there is a clear need to be able to characterize the current sources and timescales of mountain recharge to these basins in order to project future water availability and well vulnerability. However, our ability to quantify mountain-block recharge remains limited, mainly due to the lack of deep wells in the mountain block and at the mountain front. What are often available in basin aquifers are deep, long-screened production wells, but the highly mixed flowpaths captured by these wells present a challenge to distinguishing and quantifying mountain-block recharge. Here, we present an approach to leverage multiple environmental tracers, including two novel age tracers (argon-39 and krypton-85), to characterize the age distributions of six production wells in the Tucson Basin in southeastern Arizona. Dissolved noble gas concentrations are interpreted for likely ranges of recharge elevation and multiple age tracers are combined to provide information on modern (tritium/helium and krypton-85), intermediate (argon-39), and old (radiocarbon) age fractions in the mixed age samples. We evaluate this approach for quantifying mountain-block recharge to basin aquifers and discuss our findings in terms of the complex stratigraphic and structural features commonly present in Basin and Range aquifer systems.