CALIFORNIA GROUND-WATER QUALITY MONITORING: FRAMEWORK FOR A COMPREHENSIVE, STATEWIDE PROGRAM
Although there are more than 500 ground-water basins in California, a smaller number account for most of the ground-water use and most of the potential sources of contamination. Consequently, we identified 116 priority basins. These basins account for 76% of all public supply wells (95% of those in basins). The priority basins also account for most of the municipal pumping, agricultural pumping, leaking underground fuel tanks, and areas with registered pesticide use. The co-location of sources of supply and sources of contamination occurs not only at the basin scale, but also at the local scale (<30 km2), and not only within basins, but also outside them. Consequently, the monitoring program need not choose between areas that are sources of supply and areas that are sources of contamination.
Assessment of water quality will be based on existing and newly acquired data. About 3000 wells will be sampled, with samples age-dated and analyzed for a broad suite of constituents including major ions, nutrients, volatile organic compounds, pesticides, trace elements, stable isotopes, pharmaceutical compounds and other emerging contaminants. Not all constituents will be analyzed for in all samples. The new data will be used to evaluate the reliability of data acquired for compliance with drinking water regulations, will fill data gaps (location and constituents of concern), will allow for detections of compounds at lower concentrations than required by statute, and will provide a basis for a comprehensive and consistent assessment of ground-water quality at local, regional, and statewide scales.