Ground water in the San Juan Basin, New Mexico and Colorado

Principal aquifers in the San Juan Basin of New Mexico and Colorado are the Entrada Sandstone, Westwater Canyon Member of the Morrison Formation, Gallup Sandstone, several sandstones in the Mesaverde Group above the Gallup (Dalton Sandstone, Point Lookout Sandstone, Menefee Formation, Cliffhouse Sandstone), and sandstones of tertiary.

Most ground water flows from topographically high outcrop areas toward the San Juan River and Rio Grande Valley. Much of the water may move through confining layers to other aquifers or to the land surface rather than discharging directly to the streams.

Transmissivities of the sandstones range from 50 to 300 squared feet per day. Lowest dissolved-solids concentrations occur in or near outcrops of the sandstones and :Increase in the direction of ground-water flow. Concentrations range from less than 500 milligrams per liter to more than 30,000 milligrams per liter.