<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 />
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