The Northern Salinas Valley Ground Water Basin

by: Manuel M. Saavedra

GO-571 Geohydrology

Spring Semester 2007

Earth Science Department

Emporia State University - Emporia, Kansas



Table of Contents


Hydrologic Setting

The Salinas Valley is located in Monterey County on the central California coast, the valley trends northwesterly between the Santa Lucia Range and Sierra de Salinas on the west, and the Gabilan and Diablo Ranges to the east (Figure 1). The Salinas Valley Basin is drained by the Salinas River toward the Pacific Ocean. The Salinas Valley is approximately 240 kilometers long and ranges from 24 kilometers wide at the lower northwestern end along the Pacific Ocean to about 5 kilometers at the narrow upper part of the valley (DWR, 2003).

The Salinas Valley has a Mediterranean climate characterized by year-round moderate temperatures with a short, cool winter rainy season and warm, dry summers. The mean annual precipitation averages between 25 to 41 centimeters with the higher precipitation occurring in the northern areas of the valley. The primary sources for recharge of the Salinas Valley aquifers are from stream infiltration mainly from the Salinas River, Arroyo Seco River and other smaller tributaries, and with a lesser extent from percolation of irrigation return flows and precipitation (MCWRA, 1997).

Figure. 1 Location Map of the Salinas Valley Ground Water Basin.
Source: United States Geological Survey, capp.water.usgs.gov/gwa/ch_b/B-text4.html,
Image taken from capp.water.usgs.gov/gwa/ch_b/gif/b116.gif.

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Hydrologic and Geologic Properties

The Salinas Valley Ground Water Basin is a deep alluvial basin that was formed as the Salinas River meandered across the valley towards the Pacific Ocean. The Salinas River deposited fluvial sediments, and tributary streams that originate in the surrounding mountain ranges deposited alluvial fan sediments. The Pliocene to Holocene water-bearing sediments comprises a sequence of interbedded sands, gravels and clays of at least 650 meters thick (Durbin and Kapple, 1978).

The Salinas Valley overlies a single common aquifer that is divided into four hydrologically interconnected subareas known as the Pressure Area, East Side Area, Forebay Area, and the Upper Valley Area (Figure 2). The Pressure subarea is located near the coast and it covers an estimated surface area of 342 square kilometers. In the Pressure subarea three stratified aquifers exist under confined conditions (Figure 3). These aquifer are known as the Pressure 180-Foot, Pressure 400-Foot and the Deep Zone, and are comprised of permeable sands and gravels separated by confining clay layers that were deposited by the glacio-eustatic sea level fluctuations. Recharge to the Pressure subarea occurs from the unconfined units surrounding the Pressure subarea. The East Side subarea is a semi-confined aquifer consisting of sand and gravel interbedded with fine-grained sediments, and it covers a surface area of approximately 233 square kilometers. The East Side subarea is recharged largely by runoff from the western slopes of the Gabilan Mountains, and precipitation infiltration and irrigation return flows. The Forebay and Upper Valley subareas are unconfined and cover an area of approximately 700 square kilometers. Recharge for the unconfined subareas is directly from the Salinas River and its tributaries, irrigation return flow and precipitation (DWR, 2003). The transmissivity values vary from subarea to subarea. However, available aquifer test data aquifer give transmissivity values averaging 3500 square meters per day, and average storativity coefficients ranging between 0.004 - 0.07 (SGD, 1994).

Figure. 2 Map of the Salinas Valley Ground Water Basin Boundary.
Source: United States Geological Survey, capp.water.usgs.gov/gwa/ch_b/B-text4.html,
Image taken from capp.water.usgs.gov/gwa/ch_b/gif/b118.gif.

Figure. 3 Cross-Section of the confined aquifer portion in the Salinas Ground Water Basin.
Source: United States Geological Survey, capp.water.usgs.gov/gwa/ch_b/B-text4.html,
Image taken from capp.water.usgs.gov/gwa/ch_b/gif/b120.gif.

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Water Resources

The Salinas Valley is an important agricultural area growing a variety of row crops making it a multi-billion dollar industry. The area is strongly dependent on ground water as it accounts for more than 95% of the water used for irrigated agriculture, industrial and municipal purposes (Figure 4). In the northern coastal areas of the valley, ground water extraction occurs primarily from two aquifers zones named the Pressure 180-foot and 400-Foot. Ground water extractions in the Pressure subarea for example during 1999 totaled an estimated 146,629,846 cubic meters or equivalent to 118,873 acre-feet (MCWRA). At the present time, the water needs of the northern Salinas Valley exceed the natural recharge of the underlying aquifer.

Figure. 4 Example of Ground Water Use in the Salinas Valley Ground Water Basin.
Source: United States Geological Survey, capp.water.usgs.gov/gwa/ch_b/B-text4.html,
Image taken from capp.water.usgs.gov/gwa/ch_b/gif/b117.gif.

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Ground Water Contamination

The water quality in the Salinas Valley basin is generally acceptable for most uses with dissolved solids generally less than 800 mg/liter (Planert and Williams, 1995). However, due to intensive agricultural practices and urban growth an imbalance between the demand and supply of water resources has been created. The excess in ground water consumption has overdrafted the aquifer. In particular, pumping in excess of replenishment has gradually lowered the ground water table decreasing the pressure gradient in the confined portion of the aquifer near the coast. The decreased pressure gradient has resulted in a landward hydraulic gradient inducing seawater intrusion degrading the ground waters of the Pressure 180-Foot, and 400-Foot aquifers along the coastal areas of the valley (Figure 5). The high chloride levels have rendered the seawater intruded ground waters too salty for municipal and agricultural use.

Efforts to halt the advancement of seawater intrusion have been implemented by the local water resources agency charged with management of the ground water resources. Some of the measures in place designed to help with the seawater intrusion problem, for example include the implementation during the summer months of scheduled flow releases from two reservoirs located upstream on the Salinas River. The water releases are designed to augment the natural ground water recharge to the aquifers. Also, in lieu of ground water, growers are now using recycled water to irrigate crops farmed near the coastal areas of the valley. The purpose of using recycled water is to reduce or cease ground water pumping near the coast. The reduction in pumping is expected to raise the ground water levels of the aquifer and thus, stop and/or reverse the movement of seawater intrusion by maintaining the ground water hydraulic gradient seaward.

Figure. 5 Ground Water Quality Problems in the Salinas Valley Ground Water Basin.
Source: United States Geological Survey, capp.water.usgs.gov/gwa/ch_b/B-text4.html,
Image taken from capp.water.usgs.gov/gwa/ch_b/gif/b122.gif.

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Conclusions

It appears that the solution to halting seawater intrusion along the coastal Salinas Valley is simply reducing or eliminating ground water pumping to a sustainable basin yield. Accomplishment of this goal will require cooperation of all the water users within the Salinas Ground Water Basin in finding solutions that will help maintain the balance between water supply and demand.

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References:

California Department of Public Works, Division of Water Resources (DWR). 1946a. "Bulletin 52, Salinas Basin Investigation". 246 p.

California Department of Water Resources (DWR). 2003. " Salinas Ground Water Basin Bulletin 118-Basin number: 3-4".

Durbin, T.J., Kapple, G.W., and Freckleton, J.R. 1978. "Two-Dimensional and Three-Dimensional Digital Flow Models of the Salinas Valley Ground-Water Basin, California". U.S. Geological Survey. Water Resources Investigations Report 78-113,134 p.

Fetter, C.W., Applied Hydrogeology, 4th ed., Prentice Hall, Inc. Upper Saddle River, New Jersey, 2001.

Monterey County Water Resources Agency (MCWRA). 1997. "Water Resources Data Report, Water Year 1994-1995". 96 p.

Planert, M, Williams, J.S., 1995, "Ground water atlas of the United States, Segment 1, California-Nevada: U.S. Geological Survey Hydrologic Investigations Atlas 730-B". 28 p.

Staal, Gardner, and Dunne, Inc. (SGD). 1994. "Hydrogeologic Study: Salinas Valley Basin Management Plan Report". 76 p.


Related Links:

Emporia State University
Earth Sciences Department
GO 571-Geohydrology
For more information email: msaavedr@emporia.edu