WSP01461 - Laserfiche WebLink

Home Browse Search

I I I I I I I I I I I I I I I I I I I L ~ o referred to as the Roswell Basin, are among the most productive and impor- tant in the entire Pecos basin. It is from this area of the valley, repre- sented by the reach of the Pecos River from Acme, New Mexico, downstream to Lake McMillan, that much of the River's base flow is derived. The San Andres Limestone is the principal aquifer in the Roswell Basin. Leakage from older formations recharges the San Andres along with direct precipita- tion and downward leakage from surface streams. Total recharge is about 235,000 acre-feet annually [15]. Down dip, the Greyberg and Queens Formations confine the water in the San Andres and serve as aquitards. Overlying the aquitards is the shallow Seven Rivers Formation and the Pecos River alluvium aquifer. Under natural conditions, upward leakage from the confined aquifer, direct precipitation and percolation of irrigation water have recharged the shallow aquifer with the total recharge from precipita-. tion being about 30,000 acre-feet per year. [15,21]. In recent years, however, pumpage from the artesian aquifer has lowered piezometric levels sufficiently (about 80 feet between 1905 and 1970, and up to 225 feet in the southern part of the Basin) to reverse the direction of ground water leakage. This, coupled with considerable pumpage from the shallow aquifer which has lowered the water table 40 feet since 1940, has reduced the base flow contribution to the River and has allowed saline water to encroach into the fresh water aquifer east and north of Roswell. The potential yield of wells in this region is very high; yields in excess of 9,000 GPM have been measured in wells in the artesian aquifer with 2,000 to 3,000 GPM being average. Wells in the shallow aquifer yield about 1,000 GPM. The confining layers are capable of yielding 200 to 300 GPM [21]. The ground water hydrology from Lake McMillan southward to the Texas state line is the most complex of the Pecos River basin in New Mexico. Near Lake McMillan, shallow and artesian ground water is discharged out of the Roswell Basin to the Seven Rivers Formation. Leakage from Lake McMillan also contributes to ground water in the Seven Rivers Formation. For about a third of the length of the valley from Lake McMillan southward to Carlsbad, the Seven Rivers Formation is the principal aquifer and well yields of a few thousand gallons per minute are produced [21]. Further southward, toward Carlsbad, ground water occurs in the Tansill and Yates Formations and in the Capitan Limestone (collectively called the limestone aquifer). Recharge is through solution channels in the limestone and from leakage from Lake Avalon. 30