No abstract

Surface-water dissolved-solids loads for the upper Colorado River basin in westcentral Colorado were used to estimate dissolved-solids-load contributions from the Eagle Valley Evaporite of Pennsylvanian age to the Colorado River. Results indicate that the Eagle Valley Evaporite contributed a total of ϳ800 000 metric tons of salt per year to the Colorado River. Minerals that dissolved from the Eagle Valley Evaporite were contributed to the Colorado River in the following order (by mass): Halite (60%) Ͼ gypsum and/or anhydrite (29%, excluding mass of chemically bound water) Ͼ dolomite (10%) Ͼ magnesite (0.7%). Salts from the Eagle ValleyEvaporite contributed ϳ60% of the total annual dissolved-solids load for the U.S. Geological Survey station Colorado River near Cameo, although the evaporite is exposed or underlies unconsolidated surficial deposits over only ϳ5% of the Colorado River basin upstream from that station.

Discharge of brine with an average dissolvedsolids concentration of about 256,000 milligrams per liter from alluvium in Paradox Valley, a collapsed salt anticline, substantially increases the dissolved-solids load of the Dolores River. In 1996, the Bureau of Reclamation began operation of the Paradox Valley Unit, a series of brine-withdrawal wells completed in alluvium along the Dolores River and a deep-injection well for the brine, to decrease flow of brine into the river. This report presents the findings of a study to determine the effectiveness of the Paradox Valley Unit from 1988 through September 2001. Differences in dissolved-solids load of the Dolores River between two gaging stations, one upstream and one downstream from the Paradox Valley Unit, indicate that an average dissolved-solids load of about 313 tons per day (an annual average of about 115,000 tons) was contributed by brine inflow to the Dolores River before operation of the Paradox Valley Unit began in July 1996. By September 30, 2001, the dissolved-solids load contributed by brine had declined to an average of about 29 tons per daya decrease of about 90 percent. This decrease might have been facilitated by a decrease in precipitation and streamflow into the Paradox Valley during the last few years of the assessed period.

In this report "sea level" refers to the National Geodetic Vertical Datum of 1929 (NGVD of 1929) a geodetic datum derived from a general adjustment of the first-order level nets of both the United States and Canada, formerly called Sea Level Datum of 1929.