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N h) f.... (.)t Trend Analysis of Selected Water-Quality Data Associated With Salinity-Control Projects in the Grand Valley, in the Lower Gunnison River Basin, and at Meeker Dome, Western Colorado ByDavid L. Butler Abstract To decrease salt loading to the Colorado River from irrigated ligricuIture, salinity-control projects have been under construction since 1979 by the Bureau df Reclamation and the U.S. Department of ~griculture in the Grand Valley and since 1988 in the lower Gunnison River Basin of western Colorado. In 1980, a salinity- control project was initiated at Meeker Dome, which involved plugging three abandoned oil wells that were disclmrging saline water to the White River. Trend linalysis was used to deter- mine if the salinity-control projects had affected salinity in the Colorado and White Rivers. The mean annual dissolved-solids load in the Colorado Rivet near the Colorado-Utah State line for water y\:ars 1970-93 was about 3.32 million tons. About 46 percent of that load was from the Colorado River upstream from the Grand Valley and about 38 percent was from the Gunnison River. About 16 percent of the dissolved-solids load in the Colorado River near the State line was discharged from the Grand Valley, and most of the Grand Valley dissolved- solids load was from irrigation-induced sources. Monotonic trend analysis of dissolved- solids and major-ion data for the Colorado and Gunnison Rivers was used for determining if salinity-control projects had affected salinity (dissolved solids) in the Colorado River. Data col- lected in water years 1970-93 at gaging stations on the Colorado River-<>ne near Cameo and the other near the Colorado-Utah State line, and at the station on the Gunnison River nt~ar Grand Junction-were analyzed for trends. A computer- ized procedure developed by the U.S. Geological Survey that uses the nonparametric seasonal Kendall test with adjustment for streamflow was used for trend analysis of periodic and monthly data, and linear regression was used for trend anal- ysis of annual data. Three time periods were tested, including periods that were concurrent with work on salinity-control projects. Many of the trends in unadjusted concentration and load data were not statistically significant. There were downward trends in flow-adjusted dissolved- solids and major-ion concentrations and in monthly dissolved-solids loads for all three sta- tions in the 1970's, prior to the salinity-control projects. The two stations on the Colorado River also had significant downward trends in flow- adjusted concentrations and loads for water years 1986-93. The cumulative effects of salinity- control projects in the Grand Valley and in the lower Gunnison River Basin on salinity in the Colorado River would have become more substan- tial after the mid-1980's. Part of the decrease in dissolved solids in the Colorado River near the State line probably was related to salinity-control projects; however, there apparently arc other fac- tors that are affecting dissolved solids in the upper Colorado River in addition to salinity-control projects. Abstract