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GROUND WATER IN THE CENTRAL VALLEY, CALIFORNIA A13
<br />The Central Valley aquifer system contains numerous
<br />fine-grained (clay and silt) randomly distributed lenses
<br />that, in general, constitute over 50 percent of the total
<br />thickness of the system. Because of the large percentage
<br />of fine-grained sediments, Williamson and others (1989,
<br />table 7) concluded that the part of the aquifer system
<br />Generalized section of geologic units. Reported maximum thickness,
<br />in feet, is in parentheses (adapted from Page, 1986, table 1)
<br />Hydrologic unit used in
<br />many reports such as
<br />Bloyd (1978)
<br />Layers in digital flow
<br />model (Williamson
<br />and others, 1989)
<br />Flood basin deposits (160) Primarily clay, silt, and some sand; include
<br />muck, peat, and other organic soils in Delta area. Restrict yield to wells
<br />and impede vertical movement of water.
<br />River deposits (115) Primarily gravel, sand, and silt; include minor
<br />amounts of clay. Among the more permeable deposits in valley.
<br />Continental rocks and deposits (3,000 ± ) Heterogeneous mix of poorly
<br />sorted clay, silt, sand, and gravel; include some beds of claystone, siltstone,
<br />and sandstone. Younger deposits (Pliocene to Holocene) form major aquifer
<br />system in valley. Older deposits (Eocene to Pliocene) include breccia, con-
<br />glomerate, and some volcanic rocks and deposits. Older deposits close to
<br />land surface on east side are generally small yield aquifers.
<br />Volcanic rocks and deposits (1,000) Younger (Pliocene and Pleistocene)
<br />rocks and deposits include tuff and tuff breccia associated with Sutter
<br />Buttes; locally important source of water. Older (Miocene and Pliocene)
<br />volcanic rocks and deposits include andesite, obsidian, pumice, tuff, volcanic
<br />breccia, gravel, sand, volcanic mud flows, and some basalt. The rocks and
<br />deposits are important aquifers in the northeast part of valley where close
<br />to land surface.
<br />Unconfined to locally
<br />confined aquifer
<br />Layer 4
<br />Most wells tap this
<br />layer; unconfined
<br />storage
<br />Layer 3
<br />Some wells tap this
<br />layer; elastic and
<br />inelastic confined storage
<br />Layer 2
<br />No wells; elastic and
<br />inelastic confined storage
<br />Layer 1
<br />No wells; elastic
<br />confined storage
<br />Generalized section of geologic units. Reported maximum thickness,
<br />in feet, is in parentheses (adapted from Page, 1986, table 2)
<br />Hydrologic unit used in
<br />many reports such as
<br />Poland and Lofgren (1984)
<br />Layers in digital flow
<br />model (Wilfiamson
<br />and others, 1989)
<br />ernary
<br />*>
<br />39
<br />£3
<br />1
<br />3or
<br />Tertiary anc
<br />f
<br />H
<br />Flood basin deposits (100) Primarily clay, silt, and some sand; include
<br />muck, peat, and other organic soils in Delta area. Restrict yield to wells and
<br />impede vertical movement of water.
<br />amounts of clay. Among the more permeable deposits in valley.
<br />Lacustrine and marsh deposits (3,600+) Primarily clay and silt; include
<br />some sand. Thickest beneath Tulare Lake bed. Include three widespread
<br />clay units - A, C, and modified E clay. Modified E clay includes the
<br />Corcoran Clay Member of the Tulare Formation. Impede vertical movement
<br />of water.
<br />Continental rocks and deposits (15,000) Heterogeneous mix of poorly
<br />sorted clay, silt, sand, and gravel; include some beds of mudstone, claystone,
<br />shale, siltstone, sandstone, and conglomerate. Form major aquifer system
<br />in valley.
<br />Marine rocks and deposits Primarily sand, clay, silt, sandstone,
<br />shale, mudstone, and siltstone. Locally yield fresh water to wells, mainly on
<br />the southeast side of the valley but also on the west side near Kettleman
<br />Hills.
<br />Upper water-bearing
<br />zone 1 ; unconfined
<br />to semiconfined
<br />Principal \ AU *
<br />(modified E clay)/
<br />/
<br />Lower water-bearing zone 1 ;
<br />semiconfined to confined.
<br />Extends to base of fresh-
<br />water which is variable"X,
<br />^\
<br />Below the depth of water
<br />wells. In many areas,
<br />post-Eocene deposits
<br />contain saline water
<br />Layer 4
<br />Manv wells tap this
<br />layer; unconfined
<br />storage
<br />Layer 3
<br />Many wells tap this
<br />layer; elastic and
<br />inelastic confined storage
<br />Layer 2
<br />Some wells tap this
<br />layer; elastic and
<br />inelastic confined storage
<br />Layer 1
<br />confined storage
<br />upper and lower water-bearing zones are undifferentiated where the modified E clay (includes Corcoran Clay Member of the Tulare Formation)
<br />is absent.
<br />B
<br />FIGURE 9. Geologic and hydrologic units and equivalent layers in digital flow model. A, Sacramento Valley. B, San Joaquin Valley.
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