The identification of current groundwater basins was initially based on the presence and a real extent of unconsolidated alluvial soils identified on 1:250,000 scale geologic maps provided by the California Department of Conservation, Division of Mines and Geology.

Well completion reports for wells present in basin areas identified from the geologic map were then reviewed to identify the depth to the top of the water table and the top of impermeable bedrock. If a less than 25 feet of permeable material was present or if there was no groundwater within the permeable material, the area was eliminated from the map. The well completion reports were also reviewed to determine if water supply wells located within the delineated basin area were extracting groundwater from the permeable materials underlying the area or from the bedrock beneath the permeable material. If the wells only extracted groundwater from the bedrock, the area was eliminated from the map. This resulted in the elimination of some areas identified as basins in previous Bulletin 118 reports. If there were no wells present in basin areas identified from the geologic map and no other information on the geology underlying these areas, the areas were retained in the current version of the map until additional information could be collected.

Groundwater basins were delineated and separated from each other by the following features or conditions:

1. Impermeable Bedrock - Impermeable bedrock consist of rocks of low water yielding capacity. These include rocks of marine origin and crystalline/or metamorphic rock.

2. Constrictions in Permeable Materials - A narrow gap in impermeable material, even though filled with permeable stream channel materials, generally forms a basin boundary
due to groundwater flow constriction in these areas.

3. Fault - A fault that crosses permeable materials generally forms a barrier to groundwater movement. This is usually indicated by noticeable difference in water levels and/or flow patterns on either side of the fault.

4. Low Permeability Zone - Areas of clay or other fine-grained material that have significant areal or vertical extent generally forms a barrier to groundwater movement.

5. Groundwater Divide - A groundwater divide generally forms a barrier to groundwater movement. Groundwater divides have noticeably divergent groundwater flow directions on either side of the divide with the water table sloping away from the divide.

6. Adjudicated Basin Boundaries - The basin boundaries established by court order were used for all adjudicated basins.