CALIFORNIA GROUNDWATER QUALITY MONITORING: USE OF AGE-DATING AND LOW-LEVEL VOCS TO ASSESS AQUIFER VULNERABILITY

The California Water Resources Control Board, in collaboration with the US Geological Survey and Lawrence Livermore National Laboratory, has implemented a program to assess the susceptibility of groundwater resources. Advanced techniques such as groundwater age-dating using the tritium-helium method, and analysis of common volatile organic compounds (VOCs) at ultra-low levels are applied with the goal of assessing the contamination vulnerability of deep aquifers, which are frequently used for public drinking water supply. This effort is an important prelude to the statewide groundwater quality monitoring and assessment (GAMA) program.

Over 1200 public drinking water wells have been tested to date, resulting in a very large, tightly spaced, collection of groundwater ages in some of the heavily exploited groundwater basins of California. When employed on a basin-scale, groundwater ages are an effective tool for identifying recharge areas, defining flowpaths, and determining the rate of transport of water and associated contaminants. De-convolution of mixed ages, using ancillary dissolved noble gas data, gives insight into the water age distribution drawn at a well, and into the effective dilution of contaminants at long-screened production wells. In combination with groundwater ages, low-level VOCs are used to assess the significance of vertical transport. For example, low-level analyses show that tetrachloroethylene (PCE), occurs much more frequently in basins from California’s Central Valley (49% of wells) than in coastal areas (9% of wells); the presence of continuous confining units in coastal basins likely prevents downward migration of PCE and other contaminants, while semi-confined aquifers in the Central Valley allow some vertical migration of solvents to deep groundwater. The advantages and limitations of applying these technologies, and of the public water supply well sample set, will be illustrated in a comparison of results from the coastal Santa Clara Valley Basin and the Sacramento Basin of the Central Valley.

This work was performed under the auspices of the U.S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under contract No. W-7405-ENG-48.