98 CONSERVATION OF GROUND WATER 



Pipeline Problems 



The pipeline problems are problems chiefly of movement 

 of water within an aquifer, rather than of replenishment to 

 the aquifer. All developed aquifers serve not only as reser- 

 voirs to hold water but also as pipelines to carry water to the 

 wells from the areas where it enters the aquifer (the recharge 

 areas). The perennial yield of a well or group of wells is deter- 

 mined by the quantity of water that can move through the 

 aquifer from the recharge area. If the transmissivity of the 

 aquifer is inadequate, the water levels in wells will decline 

 whether the aquifer as a whole is adequately recharged or not. 



The public has little opportunity to appreciate how slowly 

 water moves through aquifers. Houston, the largest city de- 

 pendent on wells for municipal supply, is served by an aquifer 

 with better-than-average capability for transmitting water. 

 From the recharge area, 10 to 50 miles away, it takes a cross 

 section of the aquifer 45 miles wide and 600 feet thick to trans- 

 mit 80 million gallons a day to the Houston area, at a hydraulic 

 gradient of 10 feet per mile. 37 New York City obtains three 

 times as much water from its Croton Reservoir, 24 miles away, 

 through a pipeline less than 14 feet in diameter. 



Problems of inadequate transmissivity of aquifers have de- 

 veloped in every part of the country. Pumping from a well or 

 a closely spaced group of wells creates a hole, or technically 

 a cone of depression, in the water table or artesian-pressure 

 surface of the aquifer. Generally if withdrawal continues at 

 a constant rate, this cone expands so that the well draws water 

 from a progressively increasing area. The water level in the 

 well continues to drop, but at a decreasing rate as time goes on, 

 until the cone has expanded far enough to reach either an 

 area of natural recharge or an area of natural discharge of the 

 aquifer. Only when the pumping is balanced by increased 

 movement from the recharge area or decreased natural dis- 



37 Guyton, W. F., Application of Coefficients of Transmissibility and Storage 

 to Regional Problems in the Houston District, Texas, Trans. Am. Geophys. 

 Union, vol. 22, p. 767, 1941. 



