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<br /> <br />.~ ,. ~, ~ <br />~v.: - <br /> <br />continued long, u.<.(' than an unconfined supply, <br />which is recharged more locally and ea.~ily. <br />~Iany people wonder what cau<;cs "prings. <br />From ..,..hat he haH said ahout rising: and falling <br />water table;, it i~ ohvious that occa..ionally the <br />water labk mmt riS{' high enough to intersect low <br />places on the land. :\ sprin~ is a place where <br />there is natural discharge at the land surfa(e of <br />water from a ground-water reservoir thaI is filled <br />to overflowing. There arc diffnent kinds of <br /><;prin~. They may he c1a'iSificd according to the <br />Iypc of geologic formation from which they come, <br />such as limestone springs Of la\'a-rock spring.'i-, <br />Or tht'y may he c1a.<;,~ified according to the amount <br />of water they discharge {large or small), or ac- <br />cording to the temperature of the water (hot, <br />warm. or cold), or by the forces causing the <br />springs (gravily or artesian flow). <br />Thnmal springs arc the same a.. ordinary <br />springs except that the water is warm, or, in some <br />places. actuall)' hot. ~Ian)' occur in volcanic <br />regions. and arc fed by ground water that is <br />heated by contact with still-cooling rock.'. far <br />helow the surface, lnerc art' many of these ther- <br />mal springs in Yellowstone National Park. Even <br />where there has heen no recent volcanic action, <br />the rock... become warmer with increasing d('pth. <br />In som(' such ar('a.., water may de;cend slowly <br />to con..iderable depth, getting warmf'r a~ it d('- <br />!'Cends. If it then reuhes a large crevice in which <br />it can ri~ more quickly than il descended, it <br />.....ill nol han time to cool off completely hefoft' <br />it emerges. and ,...e have another kind of themlal <br />~pring. The famous Waml Springs of Georgia <br />and Hot Springs, I\rk.. are of this lyJX. <br />Cepers arc thermal 5-prings which erupt inter- <br />mittently. Some ~pollt only a few f{'ct, olhers a <br />hundrro ft'ct or more. They may erupt at in- <br />tt'rvals of a fe....' <;('cond~, minul~, hours, days. or <br />t'\'en wt't'ks. Some erupt at intt'ryal.. of years. <br />ThO'(' t'ruplions may occur at mort' or 1("S... regu- <br />lar intervals or quitt' irrc.gularly. :\ ft'w ~t'~'scrs <br />erupt into pool~ of their own makin~, which are <br />heautifully adorned hy d('~il" of silica. Tht' <br />mm:t famou$ ~ey"'Cr of them all. Old Faithful at <br />Yellowstone :"\ational Park, u"t"d to erupt about <br />t'\"{'ry 66 minutt':'1, to a heig:hl of 110 to 160 fetl. <br />But, since the tarthquake in Yellowstone Park, <br />in 1959, its eruptions han bcrome more <br />irrt'!:~ular. <br />Ground water .;.\owly di5.<;Qlves rock.<- and min- <br />erak mo.<<t conspicuously in limestone. dolomite, <br /> <br />10 <br /> <br />and marhIe. Water percolating through these <br />rocks gradually diS..,:ok~ them and "fonn.. ca..-ern~, <br />Somt of these, such as Carlsbad Caverns in New <br />:\lexico or ~lammoth Ca\'e in Kentucky, are <br />spectacularly large. The underground streams <br />that peaplt so often picture in connection with <br />ground water occur only in such ca"tms. Else- <br />h'here ground water percolates slowly through <br />the rock.... <br />You may have wondered how the strangely <br />dramatic stalagmites and stalactit~ .....ere formed <br />in these grt'at cavt'rns, Dripping wattr in the <br />caves deposits mintrak Where drops fall from <br />the roof of Ihe cave, part of the water evaporatts <br />and lhe dt'fM)liits lelt behind form Iong mineral <br />"icicles." Those that grow downward arc cal1t'd <br />stalactites; those fanned whert' the drops splash <br />on tht' ca"e floor and grow upward are called <br />slalagmitl!S. Sometim~ two of the formations <br />will grow together and fonn a column. <br />Where i.. ground .....ater found in the United <br />States? It accun ntarl)' e,.t'f)'where. hut the ac- <br />companying map (fig. 8) shows area.. where aqui- <br />feN typically will yield more than 50 gallons of <br />w'ater per minute to a well. Thrte general type'! <br />of ground-waler areas are shown: (I) a peren- <br />nial stream .....ith the adjoining and underlying <br />water-saturated d<'I)(~il.., ShO\'.rJl in solid color; <br />(2) 100S(' sandy and gra\"ClIy watt'r-bearing ma- <br />uriah:., including the producti\"C aquifef'j of the <br />Coa~tal Plain, the High Plains, and western ,,'al- <br />leys. shoh'n dotted; (3) c_onsolidaled wat(':r-bear- <br />ill.~ rock, of which lime;tone, ba...alt, and sand- <br />!itone art' th(' most imponam, shown by diagonal <br />lines. The blank area.. on the map are of course: <br />not completel)' empty of ground water. Amounts <br />adequate for domt5tic use can be obtained in <br />most of Ihem. though not all. <br /> <br />HOW GOOD IS GROU)';D WATER? <br /> <br />Ideal ,,'ater, for most people, i!'- spring water- <br />clt'ar, cold, pure, and tast)'. Ground water, on <br />the ....hole, is cleaner and purer than mos.t surface <br />water. The soil and ror:k..~ through which it JXr. <br />colates screen out bacteria. But this does not <br />reall)' mean that the water is completely pu~. <br />.-\pJX'arance isn't t'\.t'f).thing: the tm"Ct'n qualitits <br />aTe mOTt: important. \\.e cannot see them with <br />the naked eye, but the delicious spring water may <br />contain many mineral.., which gi,'e it the tangy <br />ta~tt' wt' like so much. Without the mineral~. <br />it would t;bte flat and imipid. Some !'pring and <br /> <br />