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