Carbon sources and signals through time in an Alpine groundwater Basin, Sagehen California

In 2003, chlorofluorocarbon (CFC) apparent ages, major ion chemistry and C isotopes were determined in nine springs from Sagehen Basin, a high elevation watershed in the eastern Sierra Nevada. Springs with similar apparent ages, which ranged between 15 and 45 a, had very similar chemistry despite being found in different areas of the watershed. In agreement with earlier studies, concentrations of rock-derived cations (Ca2+ and Na+), conductivity, temperature and pH increase with apparent age, documenting the chemical evolution of this groundwater system. In contrast with the cation data, δ13C and ΣCO2 show no correlation with apparent age. δ13C displays a strong linear relationship with 1/ΣCO2 (R2 = 0.91). This is consistent with results from a previously developed soil respiration/diffusion model. Spring radiocarbon content ranged between 85 and 110 pmc and varied with apparent age, whereby the youngest groundwater has the highest radiocarbon values. The spring radiocarbon is set by the soil pCO2 and its trend can be best described assuming the soil CO2 is composed of a mixture of 50–66% fast- (15–25 a) and 33–50% slow- (4 ka) cycling components. These results are consistent with previous soil C studies. The C isotope data indicate that in Sagehen Basin the groundwater ΣCO2 is inherited from the soil zone with little, if any, contribution from the dissolution of disseminated calcite.