Chemical evolution of shallow groundwater as recorded by springs, Sagehen basin; Nevada County, California

Springs in Sagehen basin, California, were used to document the effect of chemical weathering on the chemical evolution and composition of groundwater in a high elevation catchment. Geochemical tracer ages were determined with chlorofluorocarbons (CFCs) and tritium/3He dating techniques. The spring water ages range from less than 5 years to almost 40 years. Mass balance calculations performed by NETPATH were combined with spring water ages to calculate chemical weathering rates observed throughout the basin, which range from 0.0116 to 0.0018 and from 0.0036 to 0.0006 mmol l−1 year−1, for plagioclase and hornblende, respectively. Major cation concentrations, pH, and spring water conductivity were found to correlate positively (R2=0.7) with spring water age. This suggests that shallow groundwater, as represented by the springs, is a chemically evolving system.