INVESTIGATION AND CONTROL OF CONTAMINATED GROUNDWATER DISCHARGES TO THE SAN FRANCISCO BAY ESTUARY

The San Francisco Bay Estuary serves as the outlet to the Pacific Ocean for waters draining an area of approximately 59,000 square miles. The Bay is encircled by over 200,000 acres of tidal marshes, mudflats and other intertidal and shallow subtidal habitats. These habitats provide spawning, resting and feeding areas for a multitude of aquatic and terrestrial organisms, such as the Chinook Salmon, California Sea Lion, California Clapper Rail and Salt Marsh Harvest Mouse. Although the volume of groundwater discharging into the estuary comprises only a fraction of the total water input, these discharges affect essentially all of the tidal and near shore habitats and are an important component of the ecosystem.

The Estuary is also encircled by intense, urban development with a population exceeding five million people. Over 5,000 contaminated sites have been identified around the Bay, including landfills, refineries, chemical manufacturers, gasoline stations, dry cleaners and various other industries. Petroleum and chlorinated solvents are the most common chemicals of concern. The majority of the contaminated sites include impacts to shallow groundwater (<20 m). The ages and lengths of chlorinated solvent plumes along with other hydrogeologic data can be used to estimate the migration rates (e.g., 20 to 75+ m/yr) and volumes of groundwater discharging into the Estuary in a particular area (e.g., tens to hundreds of m3/yr per linear meter of shoreline). Understanding these relationships aids in predicting the timing, extent and magnitude of potential impacts to the Estuary.

Mitigation of impacts to the Estuary is done through the identification, investigation and remediation of contaminated sites within striking distance of important habitats. This includes screening of groundwater data with respect to surface water protection goals. Groundwater "buffer zones", where concentrations of contaminants are not allowed to exceed specified goals, are established at the downgradient edge of a plume. Remedial actions are site-specific but can include slurry walls, reactive barriers, groundwater pump-and-treat systems, in-situ treatment of impacted aquifers and monitored attenuation. These actions all play an important role in continued efforts to restore and enhance the ecosystem of the San Francisco Bay Estuary.