Groundwater and surface water flow to the Merced River, Yosemite Valley, California: 36Cl and Cl2 evidence

Our current understanding of water fluxes and flow paths within the mountain block is limited, and improved understanding is necessary to assess hydrology more accurately above the mountain front. Source waters and the processes controlling their mixing were characterized in the Merced River basin within Yosemite National Park, California, using Cl-36 and Cl-, supported by Rn-222, O-18, D, and streamflow data. Streams, snow, groundwater, and springs were sampled seasonally from July 2004 to October 2007. Three source water end-members were identified: (i) near surface runoff of recent meltwater containing bomb-pulse Cl-36 (Cl-36(BP)), (ii) shallow, evapotranspired groundwater, and (iii) groundwater containing Cl- derived through extended rock interaction. Both groundwater end-members mix in Yosemite Valley and then later discharge to the Merced River. Near surface runoff dominates all stream hydrographs during snowmelt, whereas the two groundwater end-members become significantly more important during base flow. Tributaries consist of mixtures of the shallow evapotranspired groundwater and near surface runoff, whereas the Merced River is composed of the mixture of all source water end-members. Snow is not an obvious end-member, and elevated Cl-36(BP) in the near surface runoff suggests that Cl-36(BP) was retained efficiently, and is being slowly released as meltwater interacts with the soil. The use of Cl-36 as a natural tracer is important in revealing the processes controlling streamflow generation in large montane catchments and the results will be helpful in configuring and calibrating hydrologic models.