The report contains information about the occurrence, quality, and use of ground water in the Lower Rio Grande Valley area which consists of Cameron, Hidalgo, Starr, and Willacy Counties in southern Texas.

The principal use of water in the area is for irrigation. The principal irrigated crops are cotton, winter vegetables, and citrus fruits. In southeastern Starr County, southern Hidalgo County, and western Cameron County, the main source of water is the Rio Grande. The greatest development of ground water in this area was after 1948 when ground water was needed to supplement water from the river.

The Lower Rio Grande Valley area has four major ground-water reservoirs. Because of the uncertainty in mapping the stratigraphic units and because some of the ground-water reservoirs are composed of parts of two or more formations, three of the ground-water reservoirs have been given names in this report. The major ground-water reservoirs are: the Oakville sandstone, an important source of water for industrial use in northeastern Stair County; the Linn-Faysville ground-water reservoir, which supplies irrigation water in the Linn-Faysville area in central Hidalgo County; and the Rio Grande ground-water reservoir and the Mercedes-Sebastian shallow ground-water reservoir, both of which supply considerable irrigation water in southeastern Starr, southern Hidalgo, western Cameron, and southwestern Willacy Counties.

The quality of water differs considerably from place to place in the Lower Rio Grande Valley area. In most of the area, water is available that can be used for domestic or public supply, but it generally is slightly saline. In most of the area, the ground water is unsuitable for irrigation .particularly if used exclusively. Water of the best quality in the area is from the Rio Grande groundwater reservoir near the Rio Grande at depths of less than 75 feet in southeastern Stair County, between 50 and 250 feet in southern Hidalgo County, and between 100 and 300 feet in western Cameron County. At progressively greater distances from the Rio Grande, the ground water at these depths tends to be more mineralized. Also at some places at depths greater than those indicated, the water tends to be more mineralized. In the Linn-Faysville area the ground water from the Linn-Faysville ground-water reservoir is moderately mineralized and ranges from fair to unsuitable for irrigation.

In western Cameron County, water levels in some wells tapping the Rio Grande ground-water reservoir declined about 10 feet from 1954 to 1957. In 1959 the water levels stood higher than in 1954. The water levels in most wells tapping the Linn-Faysville ground-water reservoir declined 10 feet or more from 1948 to 1958. In some wells the decline was more than 15 feet.

The available information indicates that in some localities the Rio Grande ground-water reservoir may be nearly filled to capacity, and waterlogging will occur during periods of above-normal precipitation. During protracted periods of below-normal precipitation, the available water in the ground-water reservoir may be depleted.

Further studies should be made in the area to correct important deficiencies in available information. A continuing program is recommended because information such as fluctuations in water levels and the amount and distribution of pumping can be obtained only on a current basis.