The "Atascadero Area Subbasin", DWR Bulletin 118 Basin No. 3-004.11, was subdivided from the "Paso Robles Area Subbasin" (of the Salinas Valley Groundwater Basin) in 2016 based on information that showed the Rinconada Fault is a significant barrier to groundwater flow. The Paso Robles Formation makes up most of the water-bearing sediments for both subbasins and the lateral (outer) extents are primarily defined by the contact with the Monterey Shale (bedrock). The boundary between the Paso Robles Subbasin and the Atascadero Subbasin is defined by the Rinconada Fault.
In 2015, the State legislature approved a new groundwater management law known as the Sustainable Groundwater Management Act (SGMA). SGMA requires that high and medium priority basins comply with the new law. DWR designated the Atascadero Groundwater Basin as a high priority basin. The map below shows those areas subject to SGMA.