Hydrogeologically Vulnerable Areas (HVAs) were determined by CA State Water Resources Control Board staff in 2000 in response to Executive Order D-5-99. The map displays where published hydrogeologic information indicates soil or rock conditions that may be more vulnerable (or susceptible) to groundwater contamination.

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.

Data and information used included CA 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).

Areas Outside Basins

Staff did not map areas in the fractured rocks that occur in mountain and foothill areas (e.g. Sierra Nevada), or in permeable volcanics which may provide primary recharge for those areas. By default, the groundwater resources in these areas are considered hydrogeologically vulnerable. Some small alluvial groundwater basins in regions of the Mojave and Colorado Deserts were not mapped due to their sparse population.

These maps and metadata tables were distributed to DWR and the Department of Public Health for review and revision. This map should be used as a large scale tool and does not override more recent or detailed site-specific knowledge.