Understanding potential impacts of climate change and mining on mountain block recharge in a semi-arid headwater catchment via natural tracer and end-member mixing analyses

Claire Tritz1, Jennifer C. McIntosh1
1Department of Hydrology and Atmospheric Sciences, University of Arizona

Mountain block recharge (MBR), an important component of recharge in arid climates, contributes water to adjacent basin-fill aquifer systems. Basin-fill aquifers support large population centers and irrigated agriculture in the southwestern US. MBR is projected to decrease due to climate change and development within mountain blocks. The spatial distribution, quantity, and flow paths of MBR are often poorly constrained, including its hydrologic connections to surface flows and shallow alluvial or regional aquifers. Constraining these flow paths and seasonal surface-groundwater interactions will help to refine conceptual and numerical models. This study focuses on MBR in a semi-arid headwater catchment in southeast Arizona: Davidson Canyon. Previous studies in the area suggest that Davidson Canyon’s seasonal and intermittent baseflows in the mountain block are a mix of young (less than a decade old) local groundwater in the alluvial aquifer and older, more geochemically evolved, regional groundwater within the fractured mountain block. Geochemical analysis (stable isotopes, major ions, and tritium) coupled with principal component analysis (PCA) and end member mixing analysis (EMMA) are applied to a time series of streamflow, shallow alluvial aquifer, regional groundwater, and precipitation data. Preliminary results suggest minimal seasonal variations in the fractional contributions of precipitation and geochemically evolved groundwater, but do suggest spatial variations within the watershed. Understanding these variations is vital for determining how stresses on the system, such as changes in precipitation, land, and water use changes affect MBR to alluvial basin-fill aquifers via focused mountain front recharge.

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