Evolution of groundwater age in a mountain watershed over a period of thirteen years

We compile a unique 13-year record of groundwater age for 11 springs in Sagehen basin, a watershed in the Sierra Nevada Mountains of California, USA. Chlorofluorocarbon (CFC), sulfur hexafluoride (SF6), and tritium (3H) data collected in prior studies from 1997 to 2003 are re-interpreted and combined with new data collected in 2009 and 2010. The age record is analyzed to explore the potential value of groundwater age monitoring for (1) providing additional constraints on the age distribution in mixed-age samples, and (2) identifying temporal changes in groundwater recharge. Motivation for this study is provided by a lack of knowledge of how groundwater recharge and discharge (stream baseflow) in mountain watersheds might respond to climate change, and a resulting need to better understand mountain aquifer residence times, storage, and recharge. Piston-flow ages for the springs generally range from 10 to 50 yr. The plausibility of different age mixing models is tested by comparing observed temporal variations in age with those simulated using simple numerical models, and by examining plots comparing the concentrations of different age tracers. We find that most spring waters are best characterized by a bimodal mixing model consisting of a new (<1 yr old) fraction and a fraction that is older, but still modern (recharged after 1950). Identification of this mixing model would not have been possible without data from multiple age tracers and data from multiple years. Computed mean ages vary substantially (often by 3–7 yr) between sampling events for most springs, including those with ages of 20–50 yr. Mean age variations are likely controlled by variations in the magnitude of the new fraction, which is positively correlated with annual snowpack water content. Most springs show overall upward trends in mean age for the sampling period, consistent with decreasing recharge rates in response to diminishing snowpack. Groundwater age monitoring appears to be a potentially valuable tool for both constraining the age distribution in sampled waters and investigating recharge transients.

► We compile a 13-year record of groundwater age for springs in a mountain watershed.
► Annual variations in age are large and correlated with annual snowpack.
► Time-series data provide important constraints on age mixing models and mean ages.
► Generally upward trends in mean age suggest declining recharge.
► This work demonstrates that groundwater age monitoring can be a valuable tool.