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

• Published in: Journal of hydrology (Amsterdam) 2012-08, Vol.460-461, p.13-28
• Main Authors: Manning, Andrew H., Clark, Jordan F., Diaz, Stephanie H., Rademacher, Laura K., Earman, Sam, Niel Plummer, L.
• Format: Article
• Language: English
• Subjects: aquifers; basins; Climate change; Earth sciences; Earth, ocean, space; Exact sciences and technology; Geochemistry; groundwater; Groundwater age; Groundwater recharge; Hydrogeology; Hydrology. Hydrogeology; labeling techniques; mathematical models; Mineralogy; Mixing models; monitoring; Mountain groundwater; mountains; Silicates; snowpack; spring; springs (water); sulfur hexafluoride; temporal variation; tritium; water content; water distribution; Water geochemistry; Water resources; watersheds
• ISSN: 0022-1694; 1879-2707
• DOI: 10.1016/j.jhydrol.2012.06.030

► 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. 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 50yr. 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 (