GROUND-WATER QUALITY MONITORING AND ASSESSMENT (GAMA): IMPLEMENTATION IN THE NORTHERN SAN FRANCISCO BAY REGION

The State Water Resources Control Board, in collaboration with the U.S. Geological Survey (USGS) and Lawrence Livermore National Laboratory (LLNL), is implementing a statewide ground-water-quality monitoring and assessment program (http://ca.water.usgs.gov/gama/). The USGS developed a comprehensive approach for this effort (Belitz and others, 2003, http://water.usgs.gov/pubs/wri/wri034166/). As part of this assessment, the USGS sampled 97 public supply wells in the ~2700 km2 North San Francisco Bay "study unit", which consists of the Wilson Grove Formation, Volcanic Highlands, and Valley and Plains study areas. These samples were analyzed for a broad suite of constituents. The study was designed to provide a spatially-unbiased assessment of ground-water quality within the study unit, as well as a statistically-consistent basis for comparing water quality in different study units. The results of this study will also provide resolution of specific areas of concern, including the natural and human factors affecting water quality.

The suite of analytes include major ions, trace elements, nutrients, volatile organic compounds, pesticides, pharmaceutical products, waste water components, dissolved methane, nitrogen and noble gases (in collaboration with LLNL). Naturally occurring isotopes (tritium, carbon-14, and helium-4) also will be measured in these samples to help identify the source and age of the sampled ground water. Preliminary results show that relatively few compounds were found that exceed water-quality standards.

For spatially unbiased and consistent assessment of ground-water quality, each of the three study areas was subdivided into grid cells approximating 25 km2. A public supply well in each cell was then selected to provide a spatially distributed, randomized monitoring network for each study area. Not surprisingly, preliminary data indicate the fewest constituents of concern were found in the less populated Volcanic Highlands study area, while more compounds of concern were found in the Valley and Plains.

In addition, several focused studies were undertaken to answer questions specific to the region, including the source of high-salinity waters in the southern lowlands area, the contribution of hydrothermal water to the ground-water system, and the source and movement of ground water in the Alexander, Napa, Sonoma, and Santa Rosa Valleys.