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 c hemical weathering on the chemical evolution and composition of groundwater in a high elevation catchment. Geochemical tracer ages were determined wit h chlorofluorocarbons (CFCs) and tritium/(3) He dating techniques. The spri ng 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 ra nge from 0.0116 to 0.0018 and from 0.0036 to 0,0006 mmol l(-1) year(-1), fo r plagioclase and hornblende, respectively. Major cation concentrations, pH , and spring water conductivity were found to correlate positively (R-2 = 0 .7) with spring water age. This suggests that shallow groundwater, as repre sented by the springs, is a chemically evolving system. (C) 2001 Elsevier S cience B.V. All rights reserved.