Modesto Subbasin: A California Aquifer Holding Its Own

Modesto Subbasin
Located within Stanislaus County, California
(Courtesy of Oakdale Irrigation District)
Richard L. Gunther

May 1, 2005

Professor Marcia Schulmeister

GO 571 Introduction to Hydrogeology

The Big Picture: Water from the West

California's Two Seasons
The Tale of Two Seasons

California's seasonal water delivery system is dependent on air pressure and moist air masses. A high air pressure is located between California and Hawaii. The high pressure shifts location seasonally. In the winter the high pressure diminishes and recedes toward Hawaii. When this occurs the storm door is open for air masses off the north Pacific to bring precipitation to the state. In the summer the high pressure grows larger and approaches the coast of California. The high pressure deflects approaching moist air masses into Washington and/or Oregon.

For roughly five months each year the storm door is open. California receives 85% of its precipitation from November through April. During the long, cool winter moist air masses sweep in from the north Pacific bringing rain to the Coast Range and Central Valley. Precipitation falls primarily as snow in the Sierra Nevada. Every three to eight years El Nino events bring air masses from the south-central Pacific. These air masses are much warmer and bring heavy rain throughout the state.

California's dry season lasts approximately seven months. From May through October the Pacific High moves closer to the coast and prevents moist air masses from entering Califonia. In the summer the storm door is closed in California. The dry season is also California's warm season.

This pattern of cool, moist winters and warm, dry summers define California's climate as being of the Mediterranean type. Native plants have adapted to growing seasons with little or no precipitation. Modern agriculture in California is all about making an endless supply of water accessible to introduced plants that are not adapted to the warm, dry summers.

Transerve section through California
California Water Cycle

The California water cycle has its origins far to the west over the Pacific Ocean. Out in the Pacific huge air masses absorb enormous volumes of water. The air masses are delivered to California by the prevailing westerly winds. These moist air masses first meet resistance to their easterly flow when they come in contact with the Coast Range. As the air masses move up slope precipitation occurs. In normal years the Coast Range receives up to 40 inches.

After releasing water to the Coast Range the air masses move eastward to the Central Valley. The air masses quickly pass over the Central Valley and release very little water. Precipiation in the Central Valley is quite meager. During an average year the City of Modesto receives 12 inches of precipitation.

Continuing eastward air masses collide with the Sierra Nevada where significant orographic precipitation occurs. In normal years precipitation reaches 80 inches. Primarily received as snow in the winter months the snow slowly melts during the spring and summer. A gradual snowmelt is most beneficial because this allows reservoirs to serve a dual purpose. Sierra reservoirs are used to prevent flooding and as storage for irrigation water.

Formation of the Modesto Subbasin Aquifer

Sediments are deposited in the Central Valley
An Aquifer is Born

The development of the aquifer began millions of years ago with the uplift of the Sierra Nevada and the accretion of the Coastal Ranges. Between the two ranges a large depression formed that is today's Central Valley. For millions of years sediments were deposited in the depression from two very different sources.

One of these sources of sediment was the Pacific Ocean. The Pacific Ocean invaded and then retreated from the ancestral Central Valley many times. This pattern continued until the final retreat between two and three million years ago. The oceanic invasions left behind a thick layer of salty marine sediments. Marine sediments up to twenty-thousand feet thick lie under the Modesto Subbasin.

The other source of sediment has come from the Sierra Nevada and the Coast Range. Weathering of igneous and metamorphic rock produced sediment that was carried by rivers from the mountains and deposited in the Central Valley. Sand and gravel carried by flowing water formed thick alluvial deposits. These sediments account for a thickness of two-thousand four hundred feet. The sediments of continental origin rest upon the marine sediments. The upper 800 feet of continental sediments are the primary source of ground-water in the Modesto Subbasin.

Today's Modesto Subbasin Aquifer

Eastern Stanislaus County, CA
Eastern Boundary

This pastoral scene exists in eastern Stanislaus County south of the City of Oakdale. This picture was taken looking from the grassland toward the oak woodland. The grassland/woodland boundary is delineated by precipiation. The beginning of the Sierra foothills is high enough to capture the minimum required precipiation (20-25 inches) for the oak trees to survive and reproduce. The approximate elevation of the eastern boundary of the Modesto Subbasin is 250 feet.

Geologically significant in this area is the appearance at the surface of the Mehrten formation. The Mehrten formation is composed of andesitic fragments. This formation dips to the southwest and is frequently tapped by wells drilled in the eastern part of the subbasin. The Mehrten formation is easily identified by well drillers because of its characteristic black sands.

Stanislaus River, Riverbank, California
Three Rivers

Riparian habitat grows along three rivers that make up the remaining boundaries of the Modesto Subbasin. The northern boundary is traced by the Stanislaus River. The southern boundary is determined by the Tuolumne River. The western boundary is marked by the San Joaquin River. The elevation along the western boundary is approximately 25 feet. The San Joaquin River flows northward into the delta where it merges with the Sacramento River for the final push to the Pacific Ocean by San Francisco.

Each of these rivers support fall migration and spawning of salmon from the Pacific Ocean. The salmon start and end their lives just a few miles upstream of this picture. To begin their life cycle, young salmon journey downriver with the spring floods to the Pacific Ocean where they mature. After several years of surviving in the ocean the adult salmon return in the fall to spawn in gravel beds. After spawning the salmon die. After decades of neglect, the three boundary rivers are being restored and water flow better managed so that salmon can complete their life cycle in greater numbers.

Cropland south of Oakdale, California
Recharge of Aquifer

In predevelopment times the aquifer was recharged by direct precipitation and snowmelt that came via the rivers from the Sierra Nevada. Most of the recharge occurred along the perimeter of the Modesto Subbasin.

Today, the primary source of recharge is excess irrigation water. Irrigation water that is not absorbed by plants or evaporated into the sky percolates down into the zone of saturation. The primary area for recharge is a large strip of agricultural land that extends south of Oakdale all the way to the Tuolumne River. The role played by the rivers with relationship to the aquifer is not clear at this time.

Primary recharge zone and Mehrten Formation
Specific Area of Recharge

The prime area of aquifer recharge is indicated by the diagonal slashes in the illustration. This area south of Oakdale and west of the Mehrten formation is heavily agricultural in nature. Field crops such as grain and corn are grown in this area. Much of those crops go directly to dairies for feed. A majority of this milk is sent to large chocolate manufacturing plant in Oakdale.

The extra water infiltrates down to the water table. Ground-water in this area generally flows in a westward direction toward the San Joaquin river.

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Hydrologic and Geologic Properties

Cross section through Modesto Subbasin aquifer
Modesto Subbasin: Multiple Aquifers

There are several aquifers that comprise the Modesto Subbasin. First, the Mehrten formation wedges under the subbasin from east to west. This is a confined aquifer with a thickness of 800 feet. Second, the forebay aquifer is located directly on top of the Mehrten formation beween Oakdale and Modesto. This unconfined aquifer is composed of unconsolidated, coarse sediments of alluvial origin. Third, the deep aquifer located beneath the confining Corcoran clay layer. The deep aquifer is confined and reaches down to the Mehrten formation which is 400 feet deep under the City of Modesto. Fourth, the shallow aquifer that rests on the Corcoran Clay. This shallow unconfined aquifer varies in thickness. The shallow aquifer ranges from 130 to 220 feet down depending on the elevation of the clay.

The Mehrten formation contains a confined aquifer with the following properties. Wells drilled into the upper layer have found coarse-grained sediments. The wells produce 300 to 2,800 gallons per minute. Hydraulic conductivity in this layer is from 0.01 to 67 feet per day. Transmissivity ranges from 11,000 to 32,000 gallons per day per foot.

The forebay and shallow aquifers are unconfined. Wells drilled in these aquifers yield an average of 1,900 gallons per minute. Hydraulic conductivity ranges from 27 to 54 feet per day. Transmissivity in this layer is from 60,000 to 80,000 gallons per day per foot. Specific yield is this aquifer is 13%.

The deep aquifer located under the Corcoran clay is confined. Transmissivity in this aquifer varies from 28,000 to 35,000 gallons per day per foot. Storativity is estimated to be between 0.0001 to 0.000001.
Eastern boundary of the Corcoran Clay layer
Corcoran Clay: A Confining Layer

The Corcoran Clay is a layer and a boundary. It is a layer of diatomaceous lake clay. This regional layer is composed of well sorted clay with no sand content. Drillers hit this layer in a range from 130 to 220 feet beneath the City of Modesto. The Corcoran clay acts as a boundary. The eastern edge marks the beginning of two aquifers that lie in the western part of the subbasin. The western area has two aquifers, the shallow and deep.

Before modern times the Corcoran clay acted as an aquitard. Today with many wells penetrating the clay layer water is more readily exchanged between the shallow and deep aquifers.

Modesto Subbasin
(Courtesy of Oakdale Irrigation District)
Groundwater Elevations in the Modesto Subbasin

This map shows the ground-water elevation in the Modesto Subbasin. Ground-water flows in the upper unconfined aquifer in a westward direction. This flow eventually creates a problem for farmers on the west side of the subbasin.

The problem on the west side is with the zone of saturation. So much water is flowing in the ground from the eastern region that the zone of saturation moves up into the root zone. The land on the west side could not support crops except for special pumping that Modesto Irrigation District (MID) performs. MID has installed shallow wells to pump the water table down. This pumping makes sure there is a zone of aeration for plants to have their roots. Water from this pumps is used for irrigation or released into the San Joaquin river.

Groundwater levels beneath Modesto
Years: 1969 through 2003
(Courtesy of Modesto Irrigation District)
Water Table Recovers!

Data collected from two wells show how the water table in the City of Modesto have fluctuated. Heavy municipal pumping from 1969 until 1993 lowered the water table. The water table dropped from twenty to sixty feet below ground level during those years. From 1994 until 2003 the water table moved up from sixty feet to twenty-five feet.

The turning point for the recovery of the Modesto water table was the opening of the Modesto Irriagation Districts surface water treatment plant in 1994. The City of Modesto contracted with Modesto Irrigation District to supply drinking water to its customers. Modesto Irrigation District draws surface water from the Tuolumne River and treats the water in a plant near Modesto Reservoir. The delivery of clean surface water allowed the City of Modesto to turn off many of its ground-water pumps. With many pumps turned off the aquifer has recovered significantly.

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Aquifer as a Water Resource

Canal south of Oakdale, California
Concrete River

Irrigation began in the early 1900's. Modesto Irrigation District and Oakdale Irrigation District provide surface and well water to farmers in this area. A network of miles of canals radiate from the upper reaches of the Stanislaus and Tuolumne Rivers. The primary canals are lined with concrete so little water is lost by percolation. Water flows passively through the canal system utilizing gravity to propel water throughout the system.

Strawberry field east of Riverbank, California
Bounty of California's Central Valley

Seventy percent of the surface area of the Modesto Subbasin contains irrigated crops. The dominant crops in this area are almonds, walnuts, peaches, grapes, grain, pasture and corn. The average size farm in the Modesto Subbasin is 20 acres.

Ground-water Contamination

Breakdown on MCL exceedances for public supply wells
Water Quality

The circle graph is representative of the main groups of contaminants in the San Joaquin aquifer. This shows that roughly one quarter of tested wells exceed MCL's in one or more areas.

In the Modesto Subbasin primary contaminants include nitrates, uranium, and dibromochloropropane. Nitrates have entered ground-water from human and animal waste products and excess fertilizing. Uranium is a natural part of the alluvial sediments from the Sierra Nevada. The pesticide, dibromochloropropane (DBCP), was used as a soil fumigant to kill roundworms. DBCB is unstable when exposed to oxgyen so if it remained near the surface of the soil it broke apart. However, DBCP was carried down into the aquifer with percolating water and is quite stable in an anaerobic environment.

Nitrates enter the ground-water from septic tanks, sewage, runoff and leaching of fertilizer, and from natural deposits. Pregnant women who consume water with nitrates in excess of MCL may experience anemia. Infants less than six months in age become seriously ill is they drink water with nitrates in excess of MCL. The babies experience shortness of breath and blue baby syndrome

Uranium entered the Central Valley by the erosion of natural deposits in the Sierra Nevada. Drinking water in excess of the MCL for uranium for many years may have an increased risk of getting cancer.

Dibromochloropropane (DBCP) was used as a soil fumigant. DBCP which is now banned was used to treat field for growing soybeans, cotton, grapes, tomatoes and fruit trees. Exposure to DBCP in excess of the MCL over many years may lead to reproductive problems and increased cancer risk.

Riverbank Army Ammunition Plant
Superfund Information Systems
Superfund Site in Riverbank

The Riverbank Army Ammunition Plant has been used for a variety of manufacturing purposes since 1942. During that time a variety of wastes were not disposed of properly. Consequently, hazardous materials entered the soil and percolated down into the aquifer. These hazardous materials include chromium and cyanide. Since 1980, the Army has been working to clean up the site. Clean up has cost millions of dollars. The clean up process is an ongoing activity and will be for some time to come. The Environmental Protection Agency is responsible for making the Army clean up this Superfund site.

While casually taking pictures of the plant I was approached by security officers and told to stop taking pictures. They said I could not take any more pictures for security reasons. I told them that I was a school teacher and doing a report on the local aquifer.

Modesto, California
Looking southeast from downtown
Future of the Aquifer

The Central Valley of Caliornia is experiencing tremendous growth. The 2000 census set the greater Modesto areas population at 446,000. This represented an increase of twenty percent since 1990. Rapid growth has been primarily in the form of urban sprawl. Housing developments, shopping centers, and industrial parks are all effectively sealing off the earth's surface. The impact on the aquifer will be significant. Fortunately, environmental laws have been passed and should protect the aquifer.

I would like to thank the following three individuals for their time and expertise in helping me understand the Modesto Subbasin Aquifer.

Kevin King, Supervisor of Water Operations, Oakdale Irrigation District; Michael Niemi, Water Resources Specialist, Modesto Irrigation District; Doug Tackett, City of Riverbank

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  • Ahrens, D. 2000. Meteorology Today. Brooks/Cole Thomson Learning, Pacific Grove, California, 527 p.

  • Buron,K.,Shelton,J.,Hevesi, and Weissmann,G. 2004. Hydrologic Characterization of the Modesto Area, San Joaquin Valley, California.Scientific Investigation Report 2004-5232, U.S. Department of the Interior, U.S. Geologic Survey, Reston, Virginia, 54 p.

  • Swartz,R. and Hauge,C. 2003. California's Groundwater, Bulletin 118. Department of Water Resources, State of California, 265 p.

  • Stanislaus and Tuolumne Rivers Groundwater Basin Association, 2004.Integrated Regional Groundwater Management Plan for the Modesto Subbasin (DRAfT) 212 p.

  • United States Geological Service. Ground Water Atlas of the United States, World Wide Web homepage URL: USGS Retrieved on April 1, 2005

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