Ravindra Dwivedi1, Paul A. "Ty" Ferré1, Thomas Meixner1, Jennifer McIntosh1, Jon Chorover2, G.-Y. Niu1, Marisa M. Earll3, Chloe Fandel1, Alissa M. White1
1Department of Hydrology and Atmospheric Sciences, The University of Arizona, Tucson, Arizona
2Department of Soil, Water, and Environmental Science, The University of Arizona, Tucson, Arizona
3Department of Geosciences, Scripps Institute of Oceanography, San Diego, California
Groundwater in fractured bedrock aquifers is the essential component of mountain-block recharge, which is considered as the principal source of water for adjacent alluvial basins (e.g., Tucson Basin). Montane groundwater systems also sustain ecosystems and biological activity in surface through spring discharges and they drive chemical weathering processes within the deep Critical Zone. Given the long time scales of storage and flow in groundwater systems, groundwater plays a critical role in climate change resiliency. Despite its significance, our ability to characterize deep groundwater systems is limited, especially fractured bedrock aquifer systems in mountainous regions, due to remoteness of such areas and their steep topography.
We combined a multi-tracer approach with numerical modeling to determine the sources, flowpaths, and transit times of water to an ephemeral stream in a high elevation catchment (Marshall Gulch) in the Santa Catalina Mountains, Tucson, AZ, where there are no deep monitoring wells. Our results, such as seasonal water balance analysis and tritium model ages, indicate that stream baseflow is mostly composed of soil water and contributions from perched aquifers. The deeper fractured bedrock aquifers contribute to streamflow only during wet conditions such as snowmelt. Furthermore, the stable water isotope values, chloride concentrations and TIMS model results indicate that transpiration is the dominant mechanism of evapotranspiration water loss. In this way, our results have implications for not only predicting vulnerability of mountain systems to climate change, but they also suggest a need to reexamine our standard streamwater sampling procedure for better characterizing contributions from groundwater in fractured bedrock groundwater.