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<br />Wo.er flows from Ihos well, and piezom,Hr;c
<br />"Pielo . surface is d,own down os show"
<br />rn.,,,c 1\1rf(lce"\
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<br />fect), but m~t wells drilled deeper than 2.000
<br />feet find little water. The pores and cracks in
<br />the rorks al great depth!' arc dmt'd up ocrausr of
<br />the "right of O\'erlying rocks.
<br />The flow of ground \,'ater from rocks onto the
<br />land surfan', or diuhaT!:t', takes place in "('\'eral
<br />way~. Wate-r may seep into a Hre-am through the
<br />bed and bank~ it may emerge as a ~pring. or it
<br />may '('('p out to the surfaCe in a swampy area.
<br />Tht" are-a where lhe aquifer i~ ruharged with
<br />watn i" hig:her than the area of di.-'char~e. w Ihat
<br />waler moves Ihrough the aquifer by Iht' force of
<br />gra\'ity (fig. 6). Tht' recharge area j" usually
<br />where a laycrof penneable material is do.<;t' to the
<br />land "urfare. The average amount of recharge
<br />is an important ronsidcralinn in lhe u.;e of ground
<br />waler. Thl.'rl.' arc areas in Ihe \\'estern States
<br />where pumpa.l?;t" ~reatly t'xcrl.'ds natural repleni~h-
<br />rnent (fig. 7).
<br />\\'h('n rain falls, water rnters the ground and
<br />the water table- rl"CS: in times of drought. the
<br />water tahle declines hecau"t of drainage to the
<br />natural outlets. Ikcausc of thi~ fluctuation, a
<br />well that is not drilled \'ery far below the current
<br />water lable may inlerminently "go dry" when
<br />the "-aler table faiL-..
<br />The words aquifrr and grot/nd-waln rnt'n'oir
<br />are sometimes lL'-f'd interchangeably, but gt"nerally
<br />a ground-wauT rC2iel"\'oir is under.-tood to mean
<br />
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<br />,IIJlI,
<br />II I II/I
<br />I Red,orgeoreo
<br />. 'i'
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<br />well 10 this
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<br />Wot.., "l"S ,n
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<br />the whole "zonl.' of saturation" from the water
<br />table to the depth where openings in the rocks dis--
<br />appear. Ground-water reservoirs prm'ide water
<br />for wells and spring:o; and alw supply \\'ater to
<br />stream.~ in rainle..s periods. We do not know
<br />exaclly how much the..<.c ground-water rescl"\'oirs
<br />hold throughout the United States, but it is un-
<br />doubtedly '<("\"CrOll times as much as all our lakes
<br />and surface resel"\'oirs put together.
<br />Though ~imiliar in function. a ground-water
<br />rt.....el"\,oir is obviously not quite the same thing as
<br />a surran."-water re5('I"\'oir. For one thing, the
<br />surfare rcsavoir is ul'ed to regulate the flow of
<br />strt"ams. '\'ater in the ground-watt"r resel"\'oir,
<br />on the other hand, is not so easily regulated.
<br />HOhever, the rate of movement is so slow, com-
<br />pared to that of >.trrams, thaI for all practical
<br />purp~cs the-c ground-water resel"\'oirs may be
<br />considered long-term media of water storage.
<br />Ground-water resel"\'oirs have certain practical
<br />ad\'amage- over surface re.<,er\'oirs: they do not
<br />lose great quantities of water through e\'3pora-
<br />tion, nor do they fill up with sediment.
<br />Be-ides mo\'ing horizontally and downhill.
<br />ground waleI' ran move upward when it is con-
<br />fined und,'r prcs-sure bl"tween two tight layers of
<br />impcnncable rock such as clay or shale. You
<br />know that if the water is. turned on and you acci-
<br />dentally puncture the g-arden hOf>e, water will
<br />
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<br />FICCR!': 6--.-Irt";lan aqwfrr and Tuhargr arra.
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