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Administrative Draft: January 2018
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ATTACHMENT B – GROUNDWATER BASIN AND SUBBASIN
DESCRIPTIONS
This attachment provides detailed descriptions of the Salinas Valley Groundwater Basin and its subbasins that underlie the Proposed Project, which are summarized in Section 4 of the WSA. If a project’s water supply includes the use of groundwater, the WSA must include: a description of any groundwater basin or basins from which the proposed project will be supplied, a detailed description and analysis of historical and projected groundwater pumping, and an analysis of the sufficiency of the groundwater from the basin or basins from which the proposed project will be supplied to meet the projected water demand associated with the proposed project.
The Proposed Project is located in the Salinas Valley Groundwater Basin, near the intersection of three subbasins: the 180/400-Foot Aquifer Subbasin (Basin No. 3-004.01), the Eastside Subbasin (Basin No. 3-004.02), and the Forebay Subbasin (Basin No. 3-004.04), as defined by DWR Bulletin 118.1 The subbasins cover areas of of 140 square miles, 90 square miles, and 147 square miles respectively. In the area around the City of Gonzales, the subbasins are general bounded by the Gabilan Range to the east and the Sierra de Salinas to the west. The 180/400- Foot Aquifer Subbasin and Eastside Subbasin are both characterized as high-priority basins under the Sustainable Groundwater Management Act (SGMA), while the Forebay Subbasin is characterized as medium-priority. 2 All three subbasins have produced Groundwater Sustainability Plans, which were used by this WSA to evaluate the long-term reliability of groundwater supplies for the Proposed Project and the City of Gonzales. The Salinas ValleyWide Integrated Groundwater Sustainability Plan was also used to evaluate conditions across subbasin boundaries.
While current supply wells for the City of Gonzales are located only in the 180/400-Foot Aquifer and Eastside Subbasins, a portion of the City and the Proposed Project also overlies the Forebay subbasin. It is possible that water from any of the three subbasins would ultimately be used to serve the Proposed Project. Therefore, each of the three subbasins is described in turn below. See Figure B-1 for DWR’s representation of the Groundwater Basins in the Salinas Valley near Gonzales.
1 California Department of Water Resources, California’s Groundwater Update 2020 Highlights (2020). 2 CA Department of Water Resources. (2022). Basin Prioritization. water.ca.gov. Retrieved from https://water.ca.gov/programs/groundwater-management/basin-prioritization Vista Lucia – Water Supply Assessment
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Figure B-1 – Location of Salinas Valley Subbasins
B.1 180/400-FOOT AQUIFER SUBBASIN GEOLOGY3
The 180/400-Foot Aquifer Subbasin lies in northwestern Monterey County and includes the northern end of the Salinas River Valley. The Subbasin covers an area of 89,700 acres, or 140 square miles (DWR, 2004). It is bounded by the Eastside Aquifer and Langley Area Subbasins to the east, the Forebay Aquifer Subbasin to the south, the Monterey Subbasin to the west, and the Monterey Bay to the north. The 180/400-Foot Aquifer subbasin is located at the northern, downgradient end of the larger Salinas Valley Basin. Land surface elevations in the Subbasin range from approximately 500 feet above sea level along its border with the Sierra de Salinas to sea level at Monterey Bay.4 The geology of the 180/400-Foot Aquifer Subbasin is characterized by alluvium, terrace deposits, the Paso Robles Formation, and the Aromas Red Sands Formation.
The geology is a result of both fluvial sedimentary deposits from the Salinas River and its 3 Salinas Valley Basin Groundwater Sustainability Agency, Salinas Valley: 180/400-Foot Aquifer Subbasin Groundwater Sustainability Plan (2021)
4 180/400-Foot Aquifer Subbasin GSP (2021), Chapter 4.
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tributaries and marine deposits from the Pacific Ocean. The majority of the sediments in this subbasin are a mix of sands, gravels, and clays.5
The stratigraphic succession of geologic formations in the 180/400-Foot Aquifer Subbasin include, from oldest to youngest: Cretaceous basement rock; Tertiary Deposits including the Purisima Formation, Santa Margarita Sandstone, and Monterey Formation; Quaternary-Tertiary Deposits including the Paso Robles Formation; and Quaternary Deposits including terrace deposits, alluvial fans, Aromas Red Sands and similar, and alluvium form streams and small drainages.
The shallowest water-bearing sediments are thin, laterally discontinuous, and do not constitute a significant source of water for the Subbasin. These shallow sediments are therefore not considered a principal aquifer. These sediments are generally within 30 feet of the ground surface and are part of the Holocene Alluvium unit. Beneath the shallow sediments, the following series of aquitards and principal aquifers have long been recognized in a multitude of studies and reports. They are the distinguishing hydrostratigraphic features of this Subbasin:
Salinas Valley Aquitard
180-Foot Aquifer
180/400-Foot Aquifer
400-Foot Aquifer
400-Foot/Deep Aquitard
Deep Aquifers
Salinas Valley Aquitard
The Salinas Valley Aquitard is the shallowest, relatively continuous hydrogeologic feature in the Subbasin. The aquitard is composed of blue or yellow sandy clay layers with minor interbedded sand layers (DWR, 2003). The Salinas Valley Aquitard correlates to the Pleistocene Older Alluvium stratigraphic unit and was deposited in a shallow sea during a period of relatively high sea level. Most of the Salinas Valley Aquitard is generally encountered at depths of less than 30 feet. The Salinas Valley Aquitard overlies and confines the 180-Foot Aquifer. 180-Foot Aquifer
The 180-Foot Aquifer is the shallowest laterally extensive principal aquifer in the 180/400-Foot Aquifer Subbasin. This aquifer consists of interconnected sand and gravel beds that are from 50 to 150 feet thick. The sand and gravel layers are interlayered with clay lenses. This aquifer is correlated to the Older Alluvium or upper Aromas Sand formations. 180/400-Foot Aquitard
The base of the 180-Foot Aquifer is an aquitard consisting of interlayered clay and sand layers, including a marine blue clay layer similar to the Salinas Valley Aquitard, known as the 180/400- 5 180/400-Foot Aquifer Subbasin GSP (2021), Section 4.2 Vista Lucia – Water Supply Assessment
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Foot Aquitard. It is widespread in the Subbasin but varies in thickness and quality, and areas of hydrologic connection between the 400-Foot and 180-Foot Aquifers are known to exist. In areas where the 180/400-Foot Aquitard is thin or discontinuous, seawater in the 180-Foot Aquifer can migrate downward into the 400-Foot Aquifer in response to pumping. The 180/400-Foot Aquitard overlies and confines the 400-Foot Aquifer.
400-Foot Aquifer
The 400-Foot Aquifer is a hydrostratigraphic layer of sand and gravel with varying degrees of interbedded clay layers. It is usually encountered between 270 and 470 feet below ground surface. This hydrogeologic unit correlates to the Aromas Red Sands and the upper part of the Paso Robles Formation. Near the City of Salinas, the 400-Foot Aquifer is a single permeable bed approximately 200 feet thick; but in other areas the aquifer is split into multiple permeable zones by clay layers. The base of the 400-Foot Aquifer is the 400-Foot/Deep Aquitard. 400-Foot/Deep Aquitard
The 400-Foot/Deep Aquitard is primarily comprised of several blue marine clay layers. This aquitard can be several hundred feet thick (Kennedy-Jenks, 2004; Brown and Caldwell, 2015), consisting of mostly clay with sand and gravel lenses. The heterogeneous nature of the aquitard indicates there may be potential pathways for downward migration of water from the 400-Foot Aquifer to the Deep Aquifers. The 400-Foot/Deep Aquitard overlies and confines the Deep Aquifers.
Deep Aquifers
The Deep Aquifers, also referred to as the 900-Foot and 1500-Foot Aquifers, are up to 900 feet thick and have alternating sandy-gravel layers and clay layers which do not differentiate into distinct aquifer and aquitard units (DWR, 2003). The Deep Aquifers correlate to the lower Paso Robles, Purisima, and Santa Margarita formations where they exist. The Deep Aquifers overlie the low permeability Monterey Formation. While the Deep Aquifers are relatively poorly studied, some well owners have indicated that there are different portions of the Deep Aquifers with different water qualities. No public data exists to substantiate these statements.
B.2 EASTSIDE SUBBASIN GEOLOGY6
The Eastside Subbasin lies in northeastern Monterey County. The Subbasin covers an area of approximately 57,500 acres, or 90 square miles. The Eastside Subbasin lies along the east side of the Salinas Valley Groundwater Basin. It is bounded by the Gabilan Range to the east, the Forebay Subbasin to the south, the 180/400-Foot Aquifer Subbasin to the west, and the Langley Area Subbasin to the north. Land surface elevations in the Subbasin range from approximately 900 feet above sea level along its border with the Gabilan Range to approximately 20 feet above sea level where it meets the 180/400-Foot Aquifer Subbasin along State Highway 101 near the 6 Salinas Valley Basin Groundwater Sustainability Agency, Salinas Valley: Eastside Subbasin Groundwater Sustainability Plan (2021).
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City of Salinas. The geology of the Eastside Subbasin is dominated by alluvial fan deposits.
Surface-water drainages originating in the Gabilan Range deposited a series of interconnected alluvial fans that extend from the Gabilan Range in the northeast to the fluvial deposits that define the 180/400-Foot Aquifer Subbasin in the southwest. There are no known structural features that restrict groundwater flow within the Eastside Subbasin, such as geologic folds or faults. However, groundwater flow from the Eastside Subbasin to various other subbasins may be restricted due to lack of continuous sediments. Usually, groundwater flow follows the topography of the valley northwest toward Monterey Bay.
Major geologic units present in the Eastside Subbasin are described below, starting at the surface and moving down through the geologic layers from youngest to oldest:
Quaternary Deposits
Older Igneous and Metamorphic Rocks
Quaternary Deposits
The Quaternary Deposits are made of alluvium in streambeds and small drainages, hillslope deposits, alluvial fans, Aromas Red Sands and similar, and terrace deposits. Poorly to moderately sorted gravel, sand, and silt with discontinuous lenses of clay in some areas generally characterize these deposits. The Eastside Subbasin lacks the confining aquitards found in the adjacent 180/400-Foot Aquifer Subbasin.
Older Igneous and Metamorphic Rocks
The eastern border of the Subbasin is defined by the contact between the Quaternary sedimentary units described above and the Cretaceous igneous and pre-Cretaceous metamorphic rocks of the Gabilan Range. Hard rocks like these also form the basement below the aquifer.
B.3 FOREBAY SUBBASIN GEOLOGY7
The Forebay Subbasin lies in the middle of Monterey County, and the middle of the Salinas Valley Groundwater Basin. The Forebay Subbasin is bounded by the Gabilan Range to the east, the 180/400-Foot Aquifer and Eastside Subbasins to the north, the Sierra de Salinas to the west, and the Upper Valley Subbasin to the south. Land surface elevations in the Subbasin range from approximately 1,800 feet along the Sierra de Salinas alluvial fans to less than 200 feet at the boundary with the 180/400-foot Aquifer Subbasin. The geology of the Forebay Subbasin is characterized by 2 intersecting geologic facies: the fluvial and marine dominated deposits of the main Salinas Valley; and the Arroyo Seco alluvial fan originating in the Sierra de Salinas on the west side of the Subbasin. In general, the alluvial sediments encountered in the Arroyo Seco Cone are more coarse-grained than those found in the main valley’s fluvial and marine deposits. 7 Salinas Valley Basin Groundwater Sustainability Agency, Salinas Valley: Forebay Subbasin Groundwater Sustainability Plan (2021).
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Major geologic units present in the Forebay Subbasin are described below, starting at the surface and moving down through the geologic layers from youngest to oldest:
Quaternary Deposits
Quaternary-Tertiary Deposits
Tertiary Deposits
Cretaceous Rocks
Quaternary Deposits
The Quaternary Deposits consist of flood plains and stream channel deposits, alluvial fans, and landslide and terrace deposits. The flood plains and stream channel deposits consist of unconsolidated, relatively fine grained, mixed deposits of sand and silt. There are thin, discontinuous layers of clay present. The thicknesses of the youngest deposits are generally less than 20 ft. The alluvial fan sediments consist of weakly to moderately consolidated, moderately to poorly sorted sand, silt, and gravel deposits. Gravel content increases toward the head of the alluvial fans, particularly the Arroyo Seco Cone which is the most prominent alluvial fan in this subbasin. Finer sediments such as clay and silt increase towards the furthest extents of the Cone, interfingering with the silts and clays often found in floodplain and stream-channel deposits.
The landslide and terrace features occur as erosional remnants of former stream channels of the Arroyo Seco and consist of weakly consolidated to semi-consolidated, moderately to poorly sorted, fine- to coarse-grained silty sand with gravels and cobbles. Their thickness is highly variable. These quaternary deposits are sometimes grouped together in other reports as Alluvium or Valley Fill Deposits.
Quaternary-Tertiary Deposits
The Quaternary-Tertiary Deposits consist of the Paso Robles Formation. This Pliocene to lower Pleistocene (1.6 million to 5 million years ago) unit is composed of lenticular beds of sand, gravel, silt, and clay from terrestrial. The depositional environment is largely fluvial but also includes alluvial fan, lake, and floodplain deposition. The alternating beds of fine and coarse materials typically have bed thicknesses of 20 to 60 feet.
Tertiary Deposits
The Tertiary Deposits consist of the Pancho Rico Formation and the Monterey Formation. The Poncho Rico Formation is a Pliocene (1.6 million to 5 million years ago) unit consisting of sandy marine strata and interbedded finer grained rocks. This unit conformably underlies the Paso Robles formation and conformably overlies the Monterey Shale, or non-comformably overlies the basement rocks northeast of King City. The Monterey Formation is a Miocene (5 million to 24 million years ago) unit consisting of shale and mudstone, with lower deposits being slightly sandier and deposited in a shallow marine environment.
Cretaceous Rocks
The Gabilan Range, which borders the Subbasin to the northeast, is composed of Mesozoic intrusive rocks and is important as a geologic boundary in the Subbasin and greater Salinas Vista Lucia – Water Supply Assessment
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Valley Groundwater Basin. The Sierra de Salinas, which borders the Subbasin to the southwest, is composed of metamorphic and sedimentary rocks and is important as a geologic boundary in the Subbasin and greater Salinas Valley Groundwater Basin as well.