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....... en c..'1 I' .-, '-.- GROUND-WATER CONTRIBUTION TO THE SALINITY OF THE UPPER COLORADO RIVER BASIN By James W. Warner, Frederick J. Heimes, and Robert F. Midde1burg ABSTRACT A reconnaissance level study was conducted to estimate the ground-water contribution to the salinity of streamflow in the Upper Colorado River Basin. Salt-load estimates were derived from a mass balance using measurements of the quantity and quality of base flow. Ground-water inf10w,was considered to represent the bulk of the streamflow during the winter months of low flow. A one-time sampling of the base flow of streams in the Upper Colorado River Basin was conducted in December 1977 and January 1978. Data on, discharge and specific conductance and samples for chemical analysis were collected at 142 sites in the Upper Colorado River Basin upstream from the confluence of the Colorado and Green Rivers. Available data were used for other areas in the Upper Colorado River Basin. In some of these areas, data were obtained from local and regional studies. Elsewhere, data were obtained from records from streamflQw-gaging stations operated by the U.S. Geological Survey. The study area was divided into two major regions; the Green River basin (referred to here as the Green River region) and the Colorado River basin upstream from the confluence with the Green River (referred to here as the Colorado River region). These two regions were divided into nine subregions. The annual salt load contributed to streams by ground water in these subregions ranged from 30 to 93 percent. In general, the salt load contributed by ground water was larger in the Colorado River region than in the Green River region. The Colorado River region had an overall average ground-water salt-load contribution of 69 percent of the total compared with 38 percent for the Green River region. The estimated total base-flow salt load of the Upper Colorado River Basin above the confluence of the Colorado and Green Rivers was 3.B million tons per year. This is about 55 percent of the total annual salt load. Diffuse ground-water discharge to streams accounts for most of the base-flow salt load. However, significant increases in the salt load along fairly short reaches in certain locations result from the surface-water solution of salts in the Upper Cretaceous shales, mostly the Mancos Shale; ground-water discharge from highly saline formations, such as the Paradox Member of the Hermosa Formation of Pennsylvanian age; and from p.oint sources, such as the highly saline mineral springs near Glenwood Springs, Colo., and Dotsero, Colo. 1