INVESTIGATION AND CONTROL OF CONTAMINATED GROUNDWATER DISCHARGES TO SURFACE WATER IN THE SAN FRANCICO BAY REGION

The San Francisco Bay Estuary drains a watershed of approximately 59,000 square miles. Bay margins comprise over 200,000 acres of tidal marshes and mudflats, including intertidal and subtidal habitats. These habitats provide spawning, resting, and feeding areas for numerous aquatic and terrestrial organisms, such as salmon, sea lion, California Clapper Rail, and Salt Marsh Harvest Mouse. Although the volume of groundwater discharging into the Estuary and its tributaries comprises only a fraction of the total water input, these discharges affect all of the tidal, near-shore, and riparian habitats and are an important component of the ecosystem.

The land adjacent to the Estuary and its tributaries is highly urbanized, with a population of more than five million people. More than 5,000 contaminated sites have been identified around the Bay, including landfills, refineries, chemical manufacturers, gasoline stations, dry cleaners, and various other industries. Petroleum, chlorinated solvents, and perchlorate are the most common chemicals of concern. Most contaminated sites impact shallow groundwater (<20 m). The ages and lengths of chlorinated solvent plumes are used to estimate migration rates (e.g., 20 to 75+ m/yr) and the volume of groundwater discharging into the Estuary or its tributaries in a particular area (e.g., 10 to 100 m3/yr per meter of shoreline). Understanding these relationships aids in predicting timing, extent, and magnitude of impacts to the Estuary and its tributaries.

Impacts to the Estuary and its tributaries are mitigated by remediating contaminated sites within striking distance of important habitats. This includes screening groundwater data with respect to surface water protection goals. A groundwater "buffer zone", where concentrations of contaminants are not allowed to exceed specified goals, may be established at the downgradient edge of a plume. Remedial actions are site-specific but can include all typical technologies. These actions play an important role in restoring and enhancing the ecosystems of the San Francisco Bay region.