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~ ..." (\ .":0 oJ-' land surface W O'e"oble .~ -- -... ~. ----- O<~" ~ , 'I .~ /L ---------- /L /,,' __/'~Sol' -- -...- - __ Fre~h wo~ - ...allH NA rURAL CONDITIONS Pumped well " ~~ ........ --,., _Fr.~"" wot..., ~-"L ,1\' " ---, ~ ,'--- -...... '--- - l' ,"'-- ~,' - .. Soh wat.. ~/ "-- - - SALT INTRUSION Pumped well Recharge well ~_Ill~b\e - ~;:- ___/j\. .A, ~ -"1"'- / '--'it..: '-.-/ 'j Fresh I water V,' "- ~", :~ /,~ ... Soil , Soil........ ' _~ woler ,,\, J ----.. Oce;on ~ water - A REMEDY flC:t"RL 10---Rrfaliol1 of frrsh <<'0/1'1 10 salt u'o/a il1 o cOQ5/alarta. 18 ing persistently_ Unfortunately, many of th~ areas are in the dl)' Southwest, just where water is scarcest (fig_ i). Where the water table lowers persistently, it meam thaI more water is ~ing taken out of the ground-water re.er\'oir than is being returned to it from precipitation or streamflow. In some plan~s ground water is removrd so much faster than it is replenished that' the process can be called u'ater mining. Ob\'iomly, the rates of pumping and of precipitation art: both important in IhL~ connection, If there are many .....ells in one area, all pumping at a H'I)' high rate, and the rainfalI i~ scanty or nonexL~lent o\'er long periods of time, then the water lable will be: low- ered. Climale tOO is important. In many wel1s, water le\'el~ decline when the weather is dl)' and rL<;C when it is ....-et. Another factor is the rate of mo\"Cment through the aquifer. If Ihe rate of mo\'ement is ~Iow because Ihe rocks are not per- meable, not much water will come into the well \'{~ry quickly---e\'en though a great quantity of it may be available. Aquifen; are recharged by rain percolating downward from the ~urfaC(" or by ~t"page from a lake or a streanl. The relation of a lake to ground v,ater is like that of a leaky di.~h or sie\'e set into a sponge. In m0t (a,;.c~, the aquifer will be re<:barged where the permeable formalion is near the land surface. The an':a of recharge, where rainwatf'r or !'.('epage actually enlers the aquifer, may be miks from the ,~'dls themselves, and '~'aler mo\"(~s Hry slo,\'ly underground. Or, e\'en if recharge occur.;; locally, it may occur at a "Cr)' low ratc. :\'atural rcfillin~ of ground-water ~r- \'oin; thus may be a \'el)' slow process. It haJ'l been estimated that if the ground-water rtSCf'\'oir of Ihe High Plain" of Texas and Xew ~fexico .....'ere emptied. it would take many centuries to refill at the pre-sent ~timated rate of rechar~. Recharge generally occurs according to the SC3..<;,()ns.. At different times, became of unusually hea\"y rainfall combined with reduced e\'apora- tion. or of ~no....m('lt. there L" enoug:h water to ~t- urate the !'Oil and reach the water table. In a large part of the country. this usually hapJ>t'ns in the ~pring. at the s..'lme time that enough water is a\'ailable to cause Aoods in the streams (fig. 14-).