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REGIONAL AQUIFER-SYSTEM ANALYSIS-CENTRAL VALLEY, CALIFORNIA
<br />GROUND WATER IN THE CENTRAL VALLEY, CALIFORNIA-
<br />A SUMMARY REPORT
<br />By GILBERT L. BERTOLDI, RICHARD H. JOHNSTON, and K.D. EVENSON
<br />ABSTRACT
<br />The agricultural productivity of the Central Valley depends on
<br />irrigation. Half of the 22 million acre-feet of irrigation water applied
<br />annually is ground water. The valley is a long, narrow structural trough
<br />filled with about 32,000 feet of sediment in the south and as much as
<br />50,000 feet in the north. Nearly all the fresh ground water is contained
<br />in the continental rocks and deposits younger than Eocene age.
<br />Streamflow, an important factor in recharging the aquifer system, is
<br />influenced by precipitation in the mountains surrounding the valley.
<br />The majority of recharge from infiltration of streamflow occurs on the
<br />east side of the valley.
<br />Ground-water pumpage, which greatly exceeds the natural re-
<br />charge rate, has dramatically altered the ground-water flow in the
<br />Central Valley. During the 1960's and 1970's, the recharge rate was
<br />more than five times that of the predevelopment period and was largely
<br />derived from percolation of imported surface water or recirculated
<br />pumped ground water rather than precipitation and recharge from
<br />streams. Prior to development, most ground water was discharged as
<br />evapotranspiration; however, in recent years, most discharge has been
<br />well pumpage. Computer simulation of the Central Valley aquifer
<br />system suggests that the total flow through the system has increased
<br />from about 2 million acre-feet per year to nearly 12 million acre-feet per
<br />year. The vertical movement of ground water has been artificially
<br />enhanced by many of the 100,000 irrigation wells that contain long
<br />intervals of perforated casing. When unpumped, these wells permit
<br />vertical flow between permeable layers within the aquifer system.
<br />The total fresh ground water presently (1986) in storage in the
<br />upper 1,000 feet of the aquifer system is about 800 million acre-feet.
<br />During the 1960's and 1970's, ground water in storage was depleted at
<br />an average rate of 800,000 acre-feet annually.
<br />In the San Joaquin Valley from the 1940's to the late 1960's,
<br />substantial withdrawals of ground water were accompanied by hun-
<br />dreds of feet of head decline. This head decline caused inelastic
<br />compaction of fine-grained beds, resulting in land subsidence that is
<br />unequaled anywhere else in the world. More than one-half of the San
<br />Joaquin Valley (or about 5,200 square miles) underwent subsidence of
<br />more than 1 foot. In one location, subsidence exceeded 29 feet. Within
<br />the areas of heavy withdrawals, subsidence is greatest where the
<br />aquifer system contains thick sections of montmorillonite clay. Land
<br />subsidence created engineering and economic problems, including
<br />damage to canals and drainage systems, and loss of irrigation wells
<br />caused by casing failure.
<br />More recently (since the drought of 1976-77), surface-water
<br />imports have increased, ground-water pumpage has decreased, and in
<br />Manuscript approved for publication May 22, 1987.
<br />places, ground-water levels have recovered. Land subsidence has
<br />virtually ceased; however, it could resume with increased pumpage, if
<br />water levels decline below previous lows.
<br />Ground-water quality in the Central Valley is generally influenced
<br />by the water from streams that are a major source of recharge. In
<br />general, water on the east side of the valley and from east-side streams
<br />contains low concentrations of dissolved solids compared to water on the
<br />west side and from west-side streams. Concentrations of dissolved
<br />solids in ground water generally are lower in the northern part of the
<br />valley than in the southern part. There are, however, localized
<br />exceptions in many places through the valley. Local concentrations of
<br />boron, chloride, and nitrate in the ground water of the Central Valley
<br />are large enough to be a problem either to crops or humans.
<br />Human activities have some influence on the concentration and
<br />location of water-quality problems in the valley. Significant increases in
<br />concentrations of dissolved solids and, specifically, dissolved nitrate
<br />indicate that ground-water quality is degrading as a result of increasing
<br />application of fertilizer in agricultural areas and the growth of urban
<br />population. Pesticides such as dibromochloropropane (DBCP) as well as
<br />selenium and other trace elements in agricultural drainage water cause
<br />ecological and health problems in the San Joaquin Valley.
<br />INTRODUCTION
<br />In 1978 the U.S. Geological Survey began a series of
<br />ground-water investigations, the Regional Aquifer-Sys-
<br />tem Analysis Program (RASA), as described in the
<br />"Foreword." The aquifer system in the Central Valley of
<br />California is one of 28 major aquifer systems in the
<br />country. It was selected for study because of the valley's
<br />long history of ground-water development and the im-
<br />portance of the area's agricultural production to the
<br />national economy (Bertoldi, 1979). Information needed
<br />for effective management of this aquifer system in the
<br />future includes (1) hydrogeologic framework of the val-
<br />ley, (2) hydraulic characteristics of the porous media
<br />(alluvium) through which ground water flows, (3) under-
<br />standing of the ground-water-flow system including sour-
<br />ces and rates of recharge and discharge, (4) chemical
<br />character of the ground water, (5) processes that control
<br />ground-water chemistry, and (6) effects of past and
<br />current human activities on the aquifer system.
<br />The 5-year Central Valley aquifer study included the
<br />collection, analysis, and evaluation of data and prepara-
<br />Al
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