Geospatially Analyzed Groundwater Residence Time as a Tool for Sustainable Groundwater Management

Abstract:

Managing groundwater during California’s drought and satisfying the requirements of the 2014 Sustainable Groundwater Management Act will require multiple approaches to quantifying rates of change in groundwater storage in the heavily exploited basins around the state. Mean groundwater residence times are useful for developing sustainability goals in that the mean residence time is a measure of the aquifer turnover, or renewal time. The California Groundwater Ambient Monitoring and Assessment program is unique among groundwater monitoring programs in that multiple analyses allow estimation of groundwater residence time. For example, over 4,000 tritium and noble gas analyses have been carried out in wells across California, allowing calculation of tritium-helium groundwater age, spatial analysis of groundwater residence times, and identification of the depth of the transition from modern to pre-modern groundwater. Areas of rapid turnover identified by young ages can be compared with areas that have been identified as being hydrogeologically vulnerable based on physical measures such as mapped permeability, confining conditions, or recharge/discharge rates.

Application of groundwater residence time as a tool for sustainable groundwater management has advantages and potential pitfalls. The uncertainty associated with calculated ages and the complexity of broad age distributions in long-screened wells are some of the associated challenges. However, geospatial analysis of isotopic age data prove useful for highlighting areas where isotopic ages are not in agreement with other measures of groundwater renewal time, and where ages may therefore be helpful in setting sustainability goals. Initial comparisons suggest that isotopic ages delineate the extent of influence of artificial recharge more precisely than numerical models and that some areas in the Sierra foothills and Coast Range, identified as active recharge areas, host pre-modern groundwater, suggesting the need for a cautious assessment of sustainability.