The area included in this investigation lies in Eddy County, New Mexico, largely between the foothills of the Guadalupe Mountains on the west and the Pecos River on the east, and extends from Carlsbad southward to Black River. The Pecos River drains the entire area, and in the growing season when water is diverted at Avalon Dam for irrigation its flow in this locality is maintained largely by the numerous springs emerging in the river channel north of Carlsbad. Carlsbad and vicinity depend on ground water fro a domestic water supply as the waters of the Pecos River are too highly mineralized for domestic use. About 1,120 acres of land was irrigation by ground water in the vicinity of Carlsbad in 1940.
Valley fill, of Quaternary age, extends over most of the area, largely as a thin veneer, but it has a maximum known thickness of 256 feet. It is made up largely of clay with lenses of conglomerate, gravel, and sand. The Rustler formation, of upper Permian age, underlies the fills and is composed of gypsum and red beds with one persistent bed of limestone. The Salado formation, which is composed chiefly of halite (common salt), underlies the Rustler formation elsewhere buy it is absent over most of the area described in this paper. This Castile formations, which is predominantly anhydrite, underlies the Salado and overlies the Delaware Mountain group, of middle Permian age, which is deeply buried in most of the area. The upper part of the Delaware Mountain group grades into the Capitan and Carlsbad limestones to the north, east, and west, the latter being exposed near Carlsbad and in the foothills of the Guadalupe Mountain. The Carlsbad limestone in turn grades into the upper part of the Chalk Bluff formation.
Ground water apparently moves eastward from the Guadalupe Mountains through the Carlsbad limestone to recharge the aquifers in the Delaware Mountain, Castile, and Salado units. In these formations the water soon becomes too highly mineralized for domestic or irrigation use. A large part of the water in the Carlsbad limestone emerges in the spring area north of Carlsbad, and a part of it moves into the valley fill in Dark Canyon Arroyo. The water in the fill of Dark Canyon Arroyo moves laterally into the limestone of the Rustier formation. The water moving eastward in the valley fill and Rustier limestone becomes progressively more mineralized and in the farmland area in the Carlsbad Irrigation Dstrict it is unfit for domestic use. In addition, highly mineralized water seeping from the farmlands and canals in the Carlsbad Irrigation District commingles with the water from the west, and the resulting mixture is undesirable even for watering stock, although if necessity it is much used for that purpose.
Water occurs in channels of the Carlsbad limestone, and wells drilled into it generally obtain large yields id hard but potable water. The municipal supply of the city of Carlsbad is derived from 4 wells (1940) in the Carlsbad limestone. The aquifer has a high transmissibility in the vicinity of Carlsbad, as was shown by a test made on one of the wells owned by Southwestern Public Service Co. This well has a specific capacity of 275 gallons per minute per foot of drawdown. The present withdrawal of water from wells penetrating the aquifers in the Carlsbad limestone averages about 4 second-feet (about 2,600,00 gallons a day). The average flow emerging in the spring area in the Pacos River north of Carlsbad is about 60 second-feet (about 40 million gallons a day). It appears that about 12 second-feet of this flow (about 8 million gallons a day) comes from aquifers in the Carlsbad limestone and that the remainder represents leakage from Lake Avalon and the canal system.
The valley fill is less permeable than the Carlsbad limestone but in some places sufficient yields are obtained for irrigation purposes. A test made on a well in the fill just south of Carlsbad showed the aquifer in that locality to have a transmissibility of about 60,000.
Contamination of the household wells in West Carlsbad does not appear to be taking place at the present time although the juxtaposition of cesspools and outhouses on the one hand and poorly cased wells for domestic water supply on the other makes the situation dangerous.