California Hydrogeologically Vulnerable Areas Map

In response to Executive Order D-5-99, State Water Board staff created a map where published hydrogeologic information indicates soil or rock conditions that may be more vulnerable (or susceptible) to groundwater contamination: ”Hydrogeologically Vulnerable Areas.” The map was created due to groundwater concerns over releases of MTBE, primarily from leaking underground storage tank sites. However, areas that are vulnerable to MTBE may also be vulnerable to other contaminants released at the surface.

The map was created in 2000 using Department of Water Resources (DWR) and US Geological Survey publications. Data from these publications were used to identify areas where geologic conditions are more likely to allow recharge at rates substantially higher than in lower permeability or confined areas of the same groundwater basin.

The identified areas are associated by a metadata table (in the following pages) which identifies (1) DWR basin name and number, (2) published source of information (i.e., title, date, author, and appropriate page, figure, table, or plate number), and (3) specific criteria upon which the vulnerability category is based.

Groundwater resources underlying designated (i.e., published) recharge, rapid infiltration, or unconfined areas were considered categorically more vulnerable to potential contaminant releases than groundwater underlying areas of slower recharge, lower infiltration rates, or intervening low permeability deposits (i.e., confining layers).