58 CONSERVATION OF GROUND WATER 



to El Paso — highly mineralized water in the shallow allu- 

 vium of El Paso Valley which is recharged chiefly from the 

 Rio Grande, and the more desirable water obtained at 

 depths greater than 500 feet in that valley as well as under 

 the mesa to the north. Daily pumpage of this good water for 

 El Paso and adjacent Ciudad Juarez increased progressively 

 from 1 million gallons a day in 1906 to 25,500,000 gallons 

 a day in 1943. The average replenishment is estimated to 

 be about 15 million gallons a day. As a result, water levels 

 in the wells in the mesa well field dropped as much as 15 

 feet between 1936 and 1949. There has been no sustained 

 downward trend in artesian pressures of the deep wells in 

 El Paso Valley. However, mineral content has increased 

 steadily since 1935 in several of these deep wells and in some 

 wells on the mesa near the rim, probably because of percola- 

 tion from the shallow alluvium in the valley. 



Since 1943 the demand for water in the area has increased 

 still further, and by 1948 more than 28 million gallons a 

 day was being pumped. However, the pumpage from the 

 deep valley wells and from the mesa has been reduced to 22 

 million gallons a day, thanks to the completion in 1943 of a 

 plant capable of treating as much as 10 million gallons a 

 day of the mineralized water of the Rio Grande and adjacent 

 shallow walls. Experiments have shown that it is feasible to 

 recharge the deep valley wells with treated surface water 

 from this plant and thus reduce or halt the encroachment 

 of salt water. 



Another form of progressive deterioration in quality of 

 ground water occurs in reservoirs that are replenished in large 

 part from water that has already been used and has acquired 

 soluble contaminating material. Pumpage from these reser- 

 voirs is a contributing factor to the extent that the reservoir 

 is emptied sufficiently to induce replenishment by the con- 

 taminating water. But the basic cause of contamination is the 

 development and use of the water before it reaches the ground- 

 water reservoir. The Gila River Basin in Arizona is an out- 



