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GROUND WATER IN THE CENTRAL VALLEY, CALIFORNIA A13
<br /> The Central Valley aquifer system contains numerous I thickness of the system. Because of the large percentage
<br /> fine-grained (clay and silt) randomly distributed lenses I of fine-grained sediments, Williamson and others (1989,
<br /> that, in general, constitute over 50 percent of the total table 7) concluded that the part of the aquifer system
<br /> Hydrologic unit used in Layers in digital flow
<br /> Generalized section of geologic units. Reported maximum thickness, many reports such as model(Williamson
<br /> in feet,is in parentheses(adapted from Page, 1986,table 1) Bloyd(1978) and others,1989)
<br /> Flood basin deposits(160)-- Primarily clay,silt and some sand;include Layer 4
<br /> muck,peat,and other organic soils in Delta area. I estrict yield to wells
<br /> and impede vertical movement of water. Most wells tap this
<br /> ncon
<br /> cc River deposits(115)--Primarily gravel,sand,and silt;include minor layer;sto age fined
<br /> amounts of clay. Among the more permeable deposits in valley.
<br /> Continental rocks and deposits(3,000±)--Heterogeneous mix of poorly Layer 3
<br /> sorted clay,silt,sand,and gravel;include some beds of claystone,siltstone, Some wells tap this
<br /> and sandstone. Younger deposits(Pliocene to Holocene)form major aquifer layer;elastic and
<br /> system in valley. Older deposits(Eocene to Pliocene) include breccia,con inelastic confined storage
<br /> glomerate,and some volcanic rocks and deposits. Older deposits close to Unconfined to locally
<br /> land surface on east side are generally small yield aquifers. confined aquifer La er 2
<br /> Y
<br /> AVolcanic rocks and deposits(1,000)--Younger(Pliocene and Pleistocene) No wells; elastic and
<br /> rocks and deposits include tuff and tuff breccia associated with Sutter inelastic confined storage
<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 /> Fto land surface. Layer 1
<br /> No wells;elastic
<br /> confined storage
<br /> A
<br /> Hydrologic unit used in Layers in digital flow
<br /> Generalized section of geologic units. Reported maximum thickness, many reports such as model(Williamson
<br /> in feet,is in parentheses(adapted from Page,1986,table 2) Poland and Lofgren(1984) and others,1989)
<br /> w
<br /> � Flood basin deposits(100) l
<br /> --Primarily clay,silt and some sand;include Upperwater-bearingLayer 4
<br /> muck peat,and other organic soils in Delta area. liestrict yield to wells and zone ,unconfined
<br /> impe�e vertical movement of water. to semiconfined Many wells tap this
<br /> River deposits(100-+ layer;unconfined)-- Primarily gravel,sand,and silt;include minor storage
<br /> Cr amounts of clay. Among the more permeable deposits in valley. Principal Absent
<br /> confining unit
<br /> (modified E clay)
<br /> Lacustrine and marsh deposits(3,600+)--Primarily clay and silt;include Layer 3
<br /> Q some sand. Thickest beneath Tulare Lake bed. Include three widespread Lower water-bearing zone'; Many wells tap this
<br /> clay units A,C and modified E clay. Modified E clay includes the semiconfined to confined. layer;elastic and
<br /> Corcoran Clay Member of the Tulare Formation. Impede vertical movement Extends to base of fresh- inelastic confined storage
<br /> of water. water which is variable
<br /> Continental rocks and deposits(15,000)--Heterogeneous mix ofpoorly �$ Layer 2
<br /> sorted clay,silt,sand,and gravel;include some beds of mudstone,claystone, seo Some wells tap this
<br /> shale siltstone,sandstone,and conglomerate. Form major aquifer system f1f. layer;elastic and
<br /> in valley. inelastic confined storage
<br /> F �8ter
<br /> Marine rocks and deposits-- Primarily sand,clay,silt,sandstone,
<br /> shale,mudstone and siltstone. Locally yield fresh water to wells,mainly on Below the depth of water Layer 1
<br /> :. the southeast side of the valley but also on the west side near Kettleman wells. In many areas,
<br /> F Hills. post-Eocene deposits No wells;elastic
<br /> N
<br /> contain saline waterstorage
<br /> 1 The 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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