Data Series 258
U.S. GEOLOGICAL SURVEY
Data Series 258
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The Monterey Bay and Salinas Valley (MS) study unit covers approximately 1,000 mi2 in Monterey, Santa Cruz, and San Luis Obispo Counties across the central coast region of California (fig.2). It lies within the Southern Coast Ranges hydrogeologic province (Belitz and others, 2003), and includes eight ground-water basins and eight subbasins as defined by the California Department of Water Resources (California Department of Water Resources, 2003) (fig. 2). The wells sampled as part of this study generally are located around the Monterey Bay and along the Salinas River Valley.
The Salinas Valley is the largest of the intermontane valleys of the Southern Coast Ranges, and extends southeastward 120 mi from Monterey Bay to Paso Robles. The Salinas Valley formed, in part, as a result of normal faulting along the King City (Rinconda-Reliz) Fault that runs north along the western margin of the valley from King City in the south to Monterey Bay in the north (California Department of Water Resources, 2003). Normal movement along the fault, valley-side down, allowed the deposition of a westward thickening alluvial wedge (Showalter and others, 1983). The Salinas Valley has been filled up to 10,000 ft on the east, and up to 15,000 ft on the west, with Tertiary and Quaternary marine and terrestrial sediments that include up to 2,000 ft of saturated alluvium (Showalter and others, 1983). Water-bearing units, which lie above mostly non-water-bearing and consolidated granitic basement, include the Miocene-age Monterey Formation and Pliocene- to Pleistocene-age Paso Robles Formation, and Pleistocene to Holocene alluvium.
The climate in the Monterey Bay and Salinas Valley area is characterized by warm, dry summers and cool, moist winters. At the National Climate Data Center (NCDC) station in Monterey, on the basis of a 50-year record, the average annual temperature is 57ºF, and the average annual precipitation is 20 in., occurring as rain during the winter and early spring. However, the distribution of precipitation across the area is dependent on the topography and the prevailing winds, with an increase in precipitation concomitant to an increase in altitude. Precipitation also decreases with latitude from north to south in the study unit. Fifty-year climate records from NCDC stations from Santa Cruz to Paso Robles show that the mean annual precipitation decreases from 31 in. in Santa Cruz in the north, to 13 in. in Paso Robles in the south (fig. 2).
The MS study unit ground-water basins are drained by several rivers and their principal tributaries, including the Salinas Valley drained by the Salinas River; the Pajaro Valley drained by the Pajaro River; the Santa Cruz area drained by the San Lorenzo River; and Carmel Valley drained by the Carmel River (fig. 2).
Sources of ground-water recharge include percolation of precipitation, river and stream infiltration, and agricultural irrigation and return flow. Amongst the four study areas, the Santa Cruz study area has the least recharge from irrigation, whereas the Paso Robles study area has the least input from precipitation. The contribution of these inputs is also dependent on the hydrogeologic setting of each area, which is described below from north to south.
The Santa Cruz (MSSC) study area includes the following ground-water basins, the Felton Area; Scotts Valley; Santa Cruz Purisima Formation Highlands; West Santa Cruz Terrace; and Soquel Valley (fig. 2), all as defined by the DWR Bulletin 118 (2003). For the purposes of this study, these basins were grouped into the MSSC study area on the basis of having underlying Purisima Formation geology; however, two wells near the town of Felton were sampled to represent the Felton Ground-Water Basin, which is metamorphic terrain (fig. 2). The MSSC study area is bounded to the north, east, and west by the Santa Cruz Mountains, with altitudes as high as 2,900 ft, and to the south by Monterey Bay and the Pajaro Valley Ground-Water Basin.
Mean annual precipitation at Santa Cruz is 31 in. and mean annual temperature is 57ºF, based on a 50-year record from NCDC. The MSSC study area is drained by the San Lorenzo River and numerous creeks and their tributaries (fig. 3). Sources of ground-water recharge include percolation of rainfall, and river and stream infiltration.
In the north of the MSSC study area, the Santa Cruz Purisima Formation Highlands Ground-Water Basin is defined by the geologic boundary of the Purisima Formation. The upper Pliocene Purisima Formation is the primary water-bearing unit and consists of poorly consolidated, silty to clean, very fine to medium-grained sandstone beds interbedded with siltstone. The formation ranges in thickness from 600 ft in the north to 1,000 ft in the south near Soquel (Muir, 1980).
The West Santa Cruz Terrace and Soquel Valley Ground-Water Basins lie to the south of the Santa Cruz Purisima Formation Highlands Ground-Water Basin. In the Soquel Valley Ground-Water Basin, the water-bearing sediments consist of the Pliocene Purisima Formation, overlain by the Pleistocene Aromas Sands Formation and by Quaternary terrace deposits. The Purisima Formation and Quaternary terrace deposits have been incised locally by streams, and these channels have been filled with Quaternary alluvium (Muir, 1980). The Purisima Formation is a sequence of gray-to-blue, moderately consolidated, silty to clean, fine to medium sandstone containing siltstone and claystone interbeds (Greene, 1970). To the southeast, the Purisima Formation is overlain by hydraulically unconfined Aromas Sands. The Aromas Sands Formation is brown to red, poorly consolidated, fine to coarse-grained sandstone containing lenses of silt and clay (California Department of Water Resources, 2003). The West Santa Cruz Terrace Ground-Water Basin contains water-bearing sediments derived from the Purisima Formation, Quaternary terrace deposits, and alluvium along the San Lorenzo River and other streams (fig. 2). The Purisima Formation, the main water-bearing formation, is a thick sedimentary sequence with a fossiliferous marine rock base that grades to continental deposits in its upper portion. The thin terrace deposits and alluvium are poorly cemented, moderately permeable gravel, sands, silts and silty clays, and yield only minor quantities of ground water to wells (Greene, 1970).
The Scotts Valley and Felton Area Ground-Water Basins are small alluviated valleys located in the Santa Cruz Mountains (fig. 2). The 2-mi2 Felton Area Ground-Water Basin and the 1.2-mi2 Scotts Valley Ground-Water Basin contain the following formations, from oldest to youngest: granitic basement, Tertiary Lompico Sandstone, Monterey Shale, Santa Margarita Sandstone, and Quaternary alluvium. The principal water-bearing formation is the unconfined Santa Margarita Sandstone, which is up to 350 ft thick. The underlying Lompico Sandstone also yields water to a lesser extent, and is up to 600 ft thick.
The Monterey Bay (MSMB) study area, as defined for the MS study unit, extends from east of Santa Cruz south along the Monterey Bay to the Forebay of the Salinas Valley. The MSMB study area covers approximately 450 mi2 and includes most of the Quaternary sediment filled basins in this area (figs. 2 and 4), which include the Pajaro Valley, Carmel Valley, and the following subbasins of the Salinas Valley: 180/400-Foot Aquifer, Eastside Aquifer, Seaside Area, Langley Area, and Corral de Tierra Area, as defined by the DWR Bulletin 118 (2003). For the purposes of this study, these basins and subbasins were grouped together in the MSMB study area because these basins contain similar Quaternary deposits.
Mean annual precipitation at Monterey is 20 in, and mean annual temperature is 57ºF, based on a 50-year record from NCDC. The MSMB study area is drained by the Salinas and Carmel Rivers and their tributaries (fig. 2). Sources of ground-water recharge include percolation of precipitation, agricultural return flow, and river and stream runoff infiltration in the unconfined areas, but surficial recharge does not occur in the confined areas. In the confined areas, recharge is from underflow originating in upper valley areas, and ground water flows north and west towards the discharge zones in the walls of the submarine canyon in Monterey Bay (Greene, 1970; Durbin and others, 1978).
The MSMB study area is bounded to the west by Monterey Bay and to the southwest by the Sierra de Salinas Mountains, which have altitudes as high as 4,470 ft. It is bounded to the east by the Santa Cruz Mountains in the north, and the Gabilan Range further south, which have altitudes as high as 3,450 ft. The study area is bounded to the north by the surface expression of the geologic contact between Quaternary alluvium of the Pajaro Valley and marine sedimentary deposits of the Pliocene Purisima Formation (California Department of Water Resources, 2003).
The northern Pajaro Valley Basin of the MSMB study area contains water-bearing geologic units that include, from oldest to youngest, the Purisima Formation, the Aromas Sands, Terrace Deposits, Quaternary alluvium, and Dune Deposits (Johnson and others, 1988). The Purisima Formation mainly is marine in origin, and contains a thick sequence of highly variable sediments ranging from shale beds near its base to continental deposits in its upper portion (Johnson and others, 1988). The sediments primarily are poorly consolidated, moderately permeable gravel, sands, silts, and silty clays (Johnson and others, 1988). The Aromas Sands Formation is composed of friable, quartzose, well-sorted brown to red sands that generally are medium-grained and weakly cemented with iron oxide (Johnson and others, 1988). This unit ranges in thickness from 100 ft above sea level in the foothills, to nearly 900 ft below sea level near the mouth of the Pajaro River (Allen, 1946). The Aromas Sands, considered the primary water-bearing unit of the basin, consists of upper eolian and lower fluvial sand units that are separated by confining layers of interbedded clays and silty clay (Johnson and others, 1988). The Terrace Deposits consist of unconsolidated gravel, sand, silt, and clay overlain by alluvium. The alluvium is composed of Pleistocene terrace materials, which is overlain by Holocene alluvium, consisting of sand, gravel, and clay deposited by the Pajaro River, and dune sands, with an average thickness of 50-300 ft (Johnson and others, 1988). A 400-ft deep, inland-projecting buried paleodrainage of the Salinas River acts as the southern subbasin boundary and restricts flow into the 180/400-Foot Aquifer Subbasin.
South of the Pajaro Valley Basin lay the 180/400-Foot Aquifer and Langley Area Subbasins. The 24-mi2 Langley Area Subbasin is a series of low hills composed of the following formations, from oldest to youngest: the Pliocene to Pleistocene Paso Robles Formation, the Pleistocene Aromas Sands, Quaternary terrace deposit, Holocene alluvium, and sand dunes (California Department of Water Resources, 1977). Outcrops of the Aromas Sands compose most of the subbasin, but exposures of Quaternary terrace deposits and Holocene alluvium along creeks form a small portion of the southeast subbasin. The lower portion of the Aromas Sands interfingers with the upper portion of the Paso Robles Formation to form the 400-Foot Aquifer to the west in the Salinas Valley 180/400-Foot Aquifer Subbasin.
The 180/400-Foot Aquifer Subbasin contains three water-bearing units, the 180-Foot, the 400-Foot, and the 900-Foot Aquifers, named for the average depth at which they are found. The confined 180-Foot Aquifer occurs only in this subbasin, as its confining blue clay layer thins and disappears east of the subbasin. The 180-Foot Aquifer consists of interconnected sand, gravel, and clay lenses, and ranges in thickness from 50 ft near Salinas, to 150 ft near Monterey Bay (Durbin and others, 1978). The 180-Foot Aquifer is separated from the 400-Foot Aquifer by a zone of lesser aquifers and aquitards that range in thickness from 10 to 70 ft. The 400-Foot Aquifer consists of sands, gravels, and clay lenses, with an average thickness of 200 ft (Durbin and others, 1978). The upper portion of the aquifer may be correlative with the Aromas Sands and the lower portion with the upper part of the Paso Robles Formation (Montgomery-Watson Consulting Engineers, 1994). The 900-Foot Aquifer is present in the lower Salinas Valley. It consists of alternating layers of sand, gravels and clays with a total thickness of up to 900 ft thick and is separated from the 400-Foot Aquifer by a blue marine clay aquitard.
To the east of the 180/400-Foot Aquifer is the Eastside Aquifer Subbasin. This 90-mi2 subbasin contains the same water-bearing units as the 180/400-Foot Aquifer Subbasin. However, the blue clay layer that confines the 180-Foot Aquifer does not extend into the Eastside Aquifer Subbasin.
To the south of the 180/400-Foot Aquifer Subbasin are the Seaside Area and Corral de Tierra Area Subbasins. These subbasins contain water-bearing units that include, from oldest to youngest: the Miocene and Pliocene Santa Margarita Formation, the Pliocene Paso Robles Formation, the Pleistocene Aromas Formation, and Pleistocene and Holocene age alluvial deposits (Muir, 1982). Although the aggregate maximum thickness of these units is more than 1,000 ft, surface outcrops are limited to alluvial sand and terrace deposits (Muir, 1982). The Santa Margarita Formation has a maximum thickness of 225 ft, and is a poorly consolidated marine sandstone (Muir, 1982). The Paso Robles Formation is the primary water-bearing unit in the area and consists of sand, gravel, and clay interbedded with some minor calcareous beds (Muir, 1982). The Aromas Formation is grouped with the dune sand deposits within this subbasin due to their similarities. These units consist of relatively clean red to yellowish-brown, well sorted sand and are estimated to range in thickness from 30 to 50 ft near the coast to up to 200 ft inland (Muir, 1982).
The Carmel Valley Ground-Water Basin is a small intermontane basin that lies along the Carmel River south of the Seaside Subbasin. The basin contains younger alluvium and river deposits, and older alluvium and terrace deposits, underlain by Monterey Shale and Tertiary sandstone units. The younger alluvium comprises the main water-bearing units and consists of boulders, gravel, sand, silt, and clay, with a thickness between 30 to 180 ft (Kapple and others, 1984).
The Salinas Valley (MSSV) study area (figs. 2 and 5) includes the following ground-water subbasins of the Salinas Valley basin: the Forebay Aquifer and the Upper Valley Aquifer, as defined by the DWR Bulletin 118 (2003). For the purposes of this study, these subbasins were combined into the MSSV study area on the basis of similar geology of the upper and central Salinas Valley. The MSSV study area’s northern boundary is shared with the 180/400-Foot Aquifer and Eastside Aquifer Subbasins. To the west, the MSSV study area is bounded by the Sierra de Salinas and Santa Lucia Ranges, with altitudes up to 4,850 ft, and to the east, it is bounded by the Gabilan Range. The southern boundary, at the constriction of the Salinas Valley where Sargent Creek joins the Salinas River, is shared with the Paso Robles Area Subbasin and separates the upper and lower Salinas River drainage basins.
Mean annual precipitation at Salinas is 15 in. and mean annual temperature is 58ºF, based on a 50-year record from NCDC. The MSSV study area is drained by the Salinas River and its tributaries. Sources of ground-water recharge include river and stream runoff infiltration and applied irrigation water.
The MSSV study area covers approximately 300 mi2 of the central Salinas Valley. The main water-bearing units of this subbasin are unconsolidated to semi-consolidated and interbedded gravel, sand and silt, alluvial-fan, and river deposits (Durbin and others, 1978). These deposits form the 180-Foot and 400-Foot Aquifers that are mentioned previously in the MSMB study area description. The northern boundary of the MSSV study area marks the southern boundary of the confining conditions for the 180-Foot Aquifer, while just south of Arroyo Seco in the center of the MSSV study area (the southern boundary of the Forebay Aquifer subbasin), marks the southern boundary of the confining conditions above the 400-Foot Aquifer. In the Forebay Aquifer Subbasin, ground water is found in the lenses of sand and gravel that are interbedded with massive units of finer grained material (Durbin and others, 1978). In the northern Forebay Aquifer subbasin, the unconfined 180-Foot Aquifer varies in thickness from 50 to 150 ft, with an average of 100 ft, and is separated from the 400-Foot Aquifer by a zone of discontinuous sands and blue clays called the 180/400-Foot Aquiclude. The aquiclude ranges in thickness from 10 to 70 ft, above the 400-Foot Aquifer, which has an average thickness of 200 ft (Durbin and others, 1978). To the south, the Upper Aquifer Subbasin, a lateral equivalent to the 180/400-Foot Aquifers, includes unconsolidated to semi-consolidated and interbedded gravel, sand, and silt of the Paso Robles Formation alluvial fan and river deposits, but the 400-Foot Aquiclude is absent in this portion of the valley.
An additional deeper aquifer consisting of alternating layers of sand-gravel mixtures and clays, the 900-Foot Aquifer, is present in the Forebay Aquifer Subbasin of the Salinas Valley, but does not extend into the Upper Valley Aquifer Subbasin owing to the southward shoaling of the basement complex (Durbin and others, 1978).
The Paso Robles (MSPR) study area (figs. 2 and 6) lies within the Paso Robles Area Subbasin of the Salinas Valley Basin, as defined by the DWR Bulletin 118 (2003). For the purposes of this study, the Quaternary alluvium that fills the valleys in this subbasin is designated as the MSPR study area (fig. 6), which excludes the higher altitude Quaternary-Pleistocene deposits. The MSPR study area is bounded to the east by the Temblor Range, to the south by the La Panza Range, to the west by the Santa Lucia Range, and to the north by the Upper Salinas Valley Aquifer Subbasin (California Department of Water Resources, 2003).
Mean annual precipitation at Paso Robles is 13 in. and mean annual temperature is 60ºF, based on a 50-year record from NCDC. Sources of ground-water recharge include infiltration of precipitation, return flow from irrigation, and seepage from rivers and streams.
The MSPR study area covers approximately 300 mi2 of valley sediments in the low-lying areas along the San Antonio and Nacimiento Rivers in the west, the Salinas River and Huerhuero Creek in the south, the Estrella River in the center, and the San Juan Creek to the southeast (fig. 6). These rivers and their tributaries drain the MSPR study area. Water-bearing formations in this study area include the Quaternary alluvium, which consists of unconsolidated, fine- to coarse-grained sand with pebbles and boulders up to 130 ft thick near the Salinas River (California Department of Water Resources, 1999).
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