WATER SUPPLY 229 
and metamorphic rocks is insignificant; but their joints 
may hold useful supplies, especially in the uppermost 10 or 
20 feet (e.g. Wisconsin, Geol. Surv. Bull. 35, 1915, p. 350). 
The renewal of water in wells depends on the rate at which 
water percolates through rocks; and the movement is usually 
very slow. It may be a few yards a day through gently 
dipping sand, or 5 feet a day through a sandstone with a 
slight dip. The Dakota Sandstone in the west central part 
of the United States receives much water from rivers that 
rise in the Rocky Mountains, and it feeds wells far to the E. 
In part of the area the water flows through the sandstone 
from one to two miles a year (N. H. Darton, 1897, 18th Ann. 
Rep. U.S. Geol. Surv., pt. iv, p. 609; also Prof. Pap., No. 
32, 1905) ; in Wisconsin, according to Weidman and Schultz 
(Wisconsin Geol. and Nat. Hist. Surv. Bull. 35, Econ. Ser., 
1915, p. 50) the rate seldom reaches half a mile a year, is 
often only a quarter of a mile a year, or less that 4 feet a 
day, and may be slower. The hydraulic gradient varies with 
the permeability of the rocks. Thus in one section the 
descent of the water-table in the first 6} miles is 74 feet a 
mile; in the next 16} miles it is 10 feet per mile; in the 
next 9 miles it is 2 feet per mile; during the last 10 miles 
near the Mississippi the fall again steepens to about 6 feet 
per mile. 
As water percolates underground it usually’ undergoes 
chemical changes by loss of its oxygen and carbon dioxide, 
and the solution of material from the rocks. It may become 
“hard” by solution of bicarbonate and sulphate of lime, 
or salt by dissolving common salt, or alkaline by dissolving 
soda and potash; it may also become charged with iron, 
magnesia, silica, sulphuretted hydrogen, sulphides, sulphates, 
ste. 
Subterranean waters may become too salt for domestic 
or agricultural use, but are usually preserved from organic 
pollution by the purifying action of the soil. The living 
soil acts as a filter which absorbs the organic matter in water 
and destroys noxious germs. If the soil is pierced by a 
pit or cesspool, water may carry germs into an underlying 
sheet of sand and gravel; all the water may be infected 
and a widespread epidemic ensue. The disposal of sewage 
by cesspools, percolation-wells or dumb-wells is therefore