Sources and Residence Times of Groundwater in Shasta County, CA Determined by Isotopic Tracers

Abstract:

Large-volume springs are a significant source of water to communities in Shasta County. Aquifers in this region are developed in young volcanic formations and the age and flow of groundwater is not well characterized, making predicting the impact of drought and climate change on spring flow difficult. To better understand the water resources and the hydrogeology of the region and to better constrain the age of water produced by springs, we have sampled water from wells, springs, and streams for a suite of geochemical and isotopic tracers. We are using isotopic tracers because of the limited number of sampling points over a large area, leaving traditional hydrogeologic methods such as water levels and pump tests inadequate for a regional study.

We analyzed samples for sulfur-35 (87.4 day half-life) and found detections in two springs, confirming the presence of a fraction of recently (1-2 years) recharged groundwater. Tritium (12.3 year half-life) activities show that some wells produce water recharged more than 5 decades ago, but most produce more recently recharged water. We will also report results for sodium-22 (2.6 year half-life), krypton-85 (10.8 year half-life), carbon-14 (5,730 year half-life), dissolved noble gases, stable isotopes of water, and helium isotopic composition. These isotopes are applied to determine the age (residence time) of groundwater over a broad age distribution, from less than one year to tens of thousands of years. These tracers should also provide information on aquifer volumes, help delineate groundwater flow, and help to identify recharge areas. A collection of groundwater ages from springs at high elevations to wells in the upper Sacramento Valley will help delineate groundwater flowpaths. Finally, groundwater residence times will help determine groundwater volume and recharge rates, and resolve questions related to drought vulnerability and effective adjustments in water resource management.