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

Abstract:

Mountain block recharge (MBR) is known to contribute water to adjacent basin-fill aquifer systems, and is an especially important component of recharge in arid climates. Large population centers in the southwestern US and irrigated agriculture depend on groundwater from basin-fill aquifer systems for water supply, and climate change and proposed mining activities in mountain blocks are projected to decrease MBR. The spatial distribution, quantity, and flow paths of MBR are often poorly constrained, including its hydrologic connections to surface flows, shallow alluvial aquifers, and regional basin-fill or bedrock aquifers. To constrain these flow paths and seasonal surface-groundwater interactions, that will help to refine conceptual and numerical models, a headwater catchment in southeast Arizona, Davidson Canyon, is studied. Previous geochemical and isotopic studies in the area suggest that Davidson Canyon’s baseflows in the mountain block are a mix local groundwater in the alluvial aquifer recharged by recent (less than 10 years old) precipitation 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. While these arid, intermittent streams do not support sustained year-round baseflow, groundwater contributions occur during the two primary precipitation seasons (summer monsoons versus winter frontal storms). Preliminary results suggest that the fractional contributions of precipitation and geochemically evolved groundwater sources to streamflow vary between and within each season. Understanding these variations is vital for determining how stresses on the system, such as changes in precipitation due to climate change or land use changes and increased pumping due to mining, affect MBR to alluvial basin-fill aquifers via focused mountain front recharge.