The San Juan Basin in the southeastern part of the Colorado Plateau, southwest of the San Juan Mountains, broadly includes the Acoma and Gallup Sags in its southern part and the small Chama Basin in its northeastern part. Regionally, the water-yielding strata (aquifers) dip inward toward the center of San Juan Basin or toward the axes of the adjoining structural sags. Aquifers exposed or at shallow depths along the margin of the basin are deeply buried in the center of the basin.

The occurrence and movement of ground water is strongly influenced by the structural configuration of the basin; fractures along joints and faults, particularly the Puerco fault belt; distribution and lithology of the rock strata, and the relationship of the uplands recharge areas of the aquifers to the lowland areas where the most ground-water discharges.

The aggregate thickness of sedimentary rocks is more than 10,000 feet in the deepest part of the basin. In order of decreasing abundance, these rocks consist of mudstone, claystone, siltstone, sandstone, silty sandstone, coal, limestone, conglomerate, and gypsum. The main aquifers consist of sandstone or sandstone containing lenses of conglomerate except for the San Andres Limestone.

The principal aquifers, listed in descending stratigraphic order, are the Tertiary Cuba Mesa and Llaves Members of the San Jose Formation and the Ojo Alamo Sandstone; the Pictured Cliffs, Cliff House, Point Lookout, Gallup, and Dakota Sandstones; the Westwater Canyon and Salt Wash Sandstone Members of the Morrison Formation, the Cow Springs Sandstone, the Zuni Sandstone, and the Entrada Sandstone; and the San Andres Limestone, Glorieta Sandstone, and De Chelly Sandstone. These aquifer are separated by formations, that do not readily transmit water between the aquifers.

Regional movement of ground water is mainly to the San Juan and Chaco s Rivers, the Puerco River, the Rio Puerco, and their main tributaries. Ground water from aquifers overlying the Dakota Sandstone discharge within the confines of the San Juan hydrologic basin. Part of the ground water in the Dakota Sandstone, Morrison Formation, and underlying formations moves across the interbasin divide into the Black Mesa, the Blanding Basin, or to the Rio Grande Trough.