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The major use of surface water in El Paso County is for irrigation of 55,000 to 58,000 acres in the El Paso and Mesilla Valleys in the El Paso County Water Improvement District No.1. During periods when surface water deliveries from Ele- phant But~e Reservoir ~o these Valleys are low, groundwater from the Rio Grande alluvium is used as a supplementary water supply. In 1974, approximately 173,502 acre-feet of Rio Grande Project surface water was used to irrigate 55,195 acres in the two Valleys. In 1964, during a period of low surface water delivery, 120,303 acre-feet of grou~dwater was pumped while only 20,378 acre-fee~ of Project surface water was used. Groundwater from the Hueco Bolson deposits is being "mined" in El Paso County and the City of Juarez area. It has been estimated, Lhrough the use of an aquifer model, that in 1903, about 11.45 million acre-feet of fresh groundwater was in storage in the Hueco Bolson deposits in El Paso County. In 1973, ~he amount in storage had been reduced to 10.64 million acre-feet. One application of ~he model indicates tha~ by 1990 storage will be decreased to about 9.84 million acre-feet. Depletions in storage are causing saline water encroachment from aquifers and degradation of groundwater quality. The amounts of fresh to slightly saline water that can be removed under "safe yield" conditions has not as yet been determined. In 1974, about 99,700 acre-feet of water was pumped from the Hueco Bolson deposits in El Paso Coun~y and by the City of Juarez in Mexico. Studies in- dicate that.the aquifer received about 6,000 acre-feet per year of natural recharge and about 21,000 acre-feet per year of water from induced recharge (leakage) from the Rio Grande alluvium. Induced recharge or leakage from the alluvium is slowly degrading the quality of groundwater pumped from the Bolson deposits. Water quality degradation will also occur due to lateral and vertical encroachment of saline water from adjacent saline water sands in the Bolson deposits as the fresh to slightly saline water in storage is depleted. The E1 Paso area has surface water supply and quality degrada- tion problems also. In the last 20 to 30 years, ElephanL Bu~te Reservoir has been able ~o deliver about 65 percent of ~he water originally planned for .the Rio Grande Project. At the El Paso Gaging Station, Lhe rate of annual depletion in discharge of the river averaged about '16,400 acre-feet over a 65 year period 0907- 1972). These ~ypes of water 'shortages coupled with agricultural, municipal, and industrial return flows have caused and will continue to cause water quality degradation in ~he area. 68 00:.116