Heritage Ranch Community Services District
Basin Boundary Modification - Scientific Justification

The Heritage Ranch Community Services District (HRCSD) believes that a minor modification of Department of Water Resources (DWR) Bulletin 118 should be considered based on the type of water production facilities and surface water entitlements of HRCSD, geology of the area, and history of Bulletin 118:

HRCSD Water Facilities and Entitlements

  • The only water available to HRCSD is surface water from the Nacimiento Reservoir. HRCSD has no groundwater wells since there is no groundwater within their service area.

  • HRCSD purchases Nacimiento water from San Luis Obispo County as part of the 17,500 acre feet of the County's Nacimiento entitlement.

  • The only HRCSD water well for the community of Heritage Ranch is in the Nacimiento River, mile downstream of the dam. This is a shallow horizontal gallery well, 6 foot below the river bed. This surface water well is totally dependent on water flow in the river.


  • In the vicinity of the HRCSD gallery well, the alluvium is 24 feet or less deep below the river flood plain (17 feet of saturated thickness), as observed in several borings, and 600 feet wide. This is less than the 25-foot depth required for aquifers within a basin per DWR boundary guidelines.

  • The alluvial deposits narrow where the USGS stream gage 1114940 is located on Camp Roberts' property (35 45' 41" latitude, 120 51' 16" longitude). The alluvium at the narrows is about 150 feet wide, within which the stream bed is about 100 feet wide. The narrows is located where Nacimiento River cuts through a wedge of steeply dipping Santa Margarita Formation sandstone that is located within a fault zone bracketed by the Rinconada fault and the San Antonio fault.

  • Non-water-bearing geologic units (including the Santa Margarita Formation and older formations) underlie the alluvium from the dam down to the Rinconada fault near the stream gage. Downstream of the Rinconada fault, the Plio-Pleistocene age water-bearing Paso Robles Formation underlies the alluvium to a depth of several hundred in syncline structures.

History of Bulletin 118 Paso Robles Formation

  • The Paso Robles Groundwater Basin is in the upper portion of the Salinas River watershed. The basin has been identified by the State of California as the Salinas Valley Paso Robles Area groundwater basin, basin 3-04.06.

  • The basin boundary was initially defined in the 1958 California Department of Water Resources San Luis Obispo County Investigation, Bulletin 18.

  • Along the Nacimiento River, this boundary was at the Rinconada fault, about 1.5 miles downstream of HRCSD's service area boundary and downstream of its water facilities.
  • The basin boundary at the Nacimiento Creek alluvial valley in the San Luis Obispo County's 2002 Paso Robles Groundwater Basin Study is similar to the Bulletin 18 map and the pre-2003 Bulletin 118 maps.
  • The Paso Robles Groundwater Basin boundary as defined in the 2002 Paso Robles Groundwater Basin study prepared for the County of San Luis Obispo is located downstream of the Jolon fault.
  • The boundary on the California DWR Bulletin 118 map followed this same precedent from its first publication in 1975 until 2003. In the 2003 Bulletin 118 map, the basin limit was extended to include the alluvium from the previous boundary to the dam.
  • The Sustainable Groundwater Management Act groundwater basin map, as prepared by the DWR, currently shows that the Salinas Valley Paso Robles Area, groundwater basin 3-04.06 extends up to the dam along the Nacimiento River alluvial valley from the basin area formerly defined in other reports.

Basin Boundary Modification

The most logical location for a basin modification would be to consider the boundary as defined by the DWR prior to 2003 and the 2002 Paso Robles Groundwater Basin Study.

The base of the alluvium upstream is at a depth of 24 feet below the river flood plain at the HRCSD gallery well, less than the 25-foot depth required for aquifers within a basin per DWR boundary guidelines. The USGS stream gage at the narrows near the fault zone that forms the edge of the water-bearing Paso Robles Formation provides a point of measurement for the inflow to the basin area upstream of where recharge can occur to aquifers beneath the alluvium. The contractual production of water downstream of the dam is related to the dam releases and is independent of other groundwater producers in the deeper basin sediment downstream.

Basin Boundary Modification Public Comment

This form allows for the public to provide comments related to the proposed Basin Boundary modification application for the Proposed Heritage Ranch CSD Groundwater Basin boundary modification. To submit comments, please fill out and submit the form below. These comments will be submitted to the Department of Water Resources (DWR) as part of the required application documentation.
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Heritage Ranch CSD Proposed Modification and Adjacent Agencies

Geological Map

Technical Memorandum - Tim Cleath, Certified Hydrogeologist

30-day Public Input Period Paso Robles Groundwater Basin

Heritage Ranch CSD Basin Boundary Modification Request - DWR Submittal