Identifying Components of Groundwater Flow, Flux, and Storage in Tuolumne Meadows, Yosemite, California

Abstract:

High elevation meadows in the Sierra Nevada of California, USA represent mixing zones between surface water and groundwater. Quantifying the exchange between stream water and groundwater, and the residence time of water stored in meadow sediments will allow examination of the possible buffer effect that groundwater has on meadows and streams. This in turn has implications for the resilience of the ecosystem as well as the downstream communities that are dependent upon runoff for water supply. Stream flow was measured and water samples were collected along a 5 km reach of the Tuolumne River and adjacent wells during both spring runoff and baseflow. Water samples were analyzed for concentrations of dissolved noble gases and anions, sulfur-35, tritium and radon to study surface water-groundwater interactions and residence times. Although lower than average because of the ongoing drought in California, discharge in early July 2015 was about 35 times that measured during the previous fall. During baseflow, a small component of fracture flow (2%) is identified using dissolved helium. Radon, anions and stream discharge identify reaches of groundwater discharge. Anions show a steady increase in the groundwater component over the western portion of the meadow during baseflow, and over 50% of stream water is exchanged with meadow groundwater, without a net gain or loss of stream flow. Sulfur-35 and tritium results indicated that groundwater contributing to stream flow has recharged within the previous two years. With the current drought, estimated as the most severe in 1200 years, accurate estimations of water availability are becoming increasingly important to water resource managers.