Hypersaline Groundwater Flow In the Paradox Valley, Montrose County, Colorado

Salinity of the Colorado River is a major concern in both the United States and Mexico because the Colorado River and its tributaries provide municipal and industrial water to about 33 million people and irrigation water to about 4 million acres in the United States and about 3 million people and 500,000 acres in the Republic of Mexico. The salt load of the Colorado River is derived from various geologic sources in the basin through natural processes, primarily discharge of saline groundwater (brine) and dissolution of minerals in sediments eroded from rocks, and through water-use processes, including irrigation, reservoir evaporation, and municipal and industrial wastewater discharges. Discharge of less than 1 cubic foot per second of sodium-chloride brine to the Dolores River in the Paradox Valley, Montrose County, Colorado, contributes about 6 percent of the dissolved-solids load of the Colorado River at Lees Ferry, Arizona. The U.S. Department of Interior Bureau of Reclamation has operated the Paradox Valley Unit, a salinity-control project, since 1996, to capture brine before it discharges to the river.

The Paradox Valley is about 30 miles long and 3- to 5- miles wide and was formed by the collapse of a salt anticline. Groundwater in the valley occurs in three types of deposits: Quaternary alluvial deposits, fractured Mesozoic and Paleozoic sedimentary rocks, and collapse breccia that formed as salt dissolved from the underlying salt anticline. Concentrations of dissolved solids in groundwater from the Paradox Valley range from less than 500 to more than 365,900 milligrams per liter. Groundwater in the valley is density stratified, with relatively fresh groundwater overlying the brine. Brine discharge to the river varies seasonally, as density and hydraulic gradients vary as a result of stream-aquifer interaction and operation of the salinity-control project.