HYDROGEOLOGY AND GEOCHEMISTRY OF THE NORTHERN MIDCOAST GROUNDWATER BASIN, SAN MATEO COUNTY, CALIFORNIA

The Northern Midcoast groundwater basin located in San Mateo County, California, is a complex coastal aquifer system that’s future water demands are predicted to exceed its safe yield. Previous insightful groundwater studies by Ludhorff & Scalmanini (1987, 1991), Earth Science & Associates (1986, 1991), GeoConsultants (1987), Kleinfelder & Associates (1989), Hecht et al., (1989, 2001) revealed the physical boundaries of individual aquifer units based on well yields and storage estimates. However, few studies have focused on understanding aquifer response to precipitation events as well as assessing groundwater quality within the basin. Accordingly, the purpose of this study is to characterize aquifers of the Northern Midcoast groundwater basin by generating geologic cross sections and quantifying hydrogeologic and geochemical properties of specific water bearing zones. Geologic cross sections constructed from multiple well logs indicate that there are three formations (Cretaceous Montara granodiorite, Quaternary marine terrace deposits, Holocene alluvial and colluvial deposits), which serve as primary water bearing units. Not surprisingly, these primary water bearing units show a response to precipitation events; however, there is a net overall loss of water in the basin from the peak of a normal wet season (January 2001) to the peak of a normal dry season (August 2001). For example, aquifers composed of fractured and decomposing granodiorite lost approximately 82 acre-feet of storage, marine terrace deposit aquifers decreased approximately 156 acre-feet in storage, and aquifers comprised mainly of alluvium and colluvium decreased the most in storage, roughly 348 acre-feet. Geochemical signatures in groundwater were used to characterize aquifers of the basin, with major cations (Ca2+, Mg2+, Na+, and K+) and major anions (Cl-, HCO3-, and SO4-) classifying the waters of the basin as enriched in calcium-bicarbonate. Overall, these analyses were done in an effort to enhance the current conceptual model of groundwater behavior in heterogeneous coastal aquifer systems, as well as prevent significant health risks to the local community.