EVALUATION OF NITRATE AND CHLORIDE CONCENTRATIONS IN CONJUNCTION WITH ISOTOPIC WATER AGES IN THE WESTSIDE GROUNDWATER BASIN, SAN FRANCISCO AND SAN MATEO COUNTIES, CALIFORNIA

The Westside Groundwater Basin, located on the San Francisco Peninsula, California, is a major source of drinking and irrigation water for local residents and institutions. Given that the Basin is open to the Pacific Ocean and the San Francisco Bay concerns about the Basin's water quality have risen over the last few decades specially in view of declining water levels in municipal wells since the 1950's. The San Andreas and Serra faults cause the Pliocene and Quaternary sediments of the Basin to be intensely folded and faulted; this may act as a natural barrier for salt water intrusion to the major pumping wells located east of the Serra Fault. In this study, we compared nitrate and chloride concentrations in conjunction with isotopic water ages for sixty wells screened at different depths. In eight wells, nitrate values are above maximum contamination levels (MCL). Pearson correlation coefficients indicate that chloride and nitrates were negatively (-0.26) correlated; nitrate concentrations decrease with depth, while chloride concentrations increase. Elevated chloride concentrations in the deeper hydrostratigraphic units are probably a result of connate water derived from superjacent aquitards. Isotopic age data indicates that nitrate concentrations are found above MCL in wells with a mean water age of about 100 years or younger. A review of historical land use showed that 100 years ago dairy, artichoke and hog farms were prevalent and they might have been the source of nitrate found in deep municipal and irrigation wells in Daly City and Colma. Wells in San Bruno that have a mean isotopic age of 700 years have not been impacted by nitrates.