NATURAL ACIDIC GROUNDWATER-SURFACE WATER OCCURRENCE, LADERA SANDSTONE, SAN MATEO COUNTY, CALIFORNIA

Naturally occurring acidic groundwater (pH 3.5 – 5) has been delineated within the Ladera Sandstone within the boundary of the Stanford Linear Accelerator Center (SLAC) in San Mateo County, California. In order to determine the origin and extent of the acidic water, a combined geochemical-stratigraphic investigation was performed that included an exploratory drilling program targeting areas underlain by both undisturbed soils and native fill. Groundwater, surface water, rock outcrop, and subsurface soil samples were collected to support a variety of data collection efforts and experiments, including biostratigraphic age dating, whole rock and trace element geochemistry, soil pH and leaching tests, hydrochemical characterization, and mineralogical testing. Mineral saturation calculations were performed using WATEQ4F to identify reactive mineral phases that could play a role in controlling groundwater pH. The data indicate that acidic groundwater is confined to a zone of Miocene-age claystone mapped locally as Ladera Sandstone and regionally equivalent to the Monterey Shale. The groundwater pH appears to be controlled by geochemical reactions within the saturated zone with acid-forming and acid-buffering minerals such as pyrite and jarosite. The origin of the pyrite may be the result of Miocene-aged regional hydrothermal activity. Neutral-buffering of soil and groundwater by ubiquitous carbonate-mineral occurrences within the Ladera Sandstone causes large variations in the observed groundwater pH, both laterally and vertically, and results in the formation of gypsum and extremely high concentrations of dissolved sulfate in groundwater. Discharge of acidic groundwater to the surface appears limited and is quickly neutralized before discharging to a seasonal drainage leading to San Francisquito Creek.