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N tv W en minor amounts of potassium, silica, fluoride, carbon- ate, and nitrate, For most natural waters, alkalinity is a measure of dissolved carbon dioxide in water and, in the study area, almost aH the alkalinity in the Colorado and Gunnison Rivers is from bicarbonate ions. A sta- tistical summary of major-ion and alkalinity concentra- tions for samples collected during water years 1970--93 at the three gaging stations for the Grand Valley Unit is in table 7. A large proportion of the gain in dissolved solids in the Colorado River from irrigation in the Grand VaHey is from calcium, magnesium, sodium, and sulfate. That conclusion was determined using approximate mass-balance calculations on major- ion loads for the three gaging stations and using chemical data for irrigation-drainage samples from the Grand VaHey and Uncompahgre Project (Butler and others, 1994). The mean chloride concentration for station 09163500 (table 7) can be derived directly from chloride loads for stations 09095500 and 09152500 and inclusion of some consumptive water loss in the Grand Valley. The differences in mean alkalinity concentrations among the three stations are smaH (table 7). Alkalinity concentrations probably - are controlled by chemical equilibria of carbonate minerals. Therefore, monotonic trend tests using the seasonal Kendall test were done on calcium, magne- sium, sodium, and sulfate concentrations because these major ions would most be affected by irrigation practices and potentially by salinity-control projects. Trend tests were done for three time periods: 1970--93, 1980--93, and 1986-93. Only 5 of 36 trend tests on unadjusted major-ion concentrations were significant or highly significant (table 8). The only highly significant trends in major- ion concentrations were for magnesium concentrations at station 09095500 for water years 1986-93 and for sodium concentrations at station 09152500 for 1970--93. The trends in major-ion concentrations for 1986-93 were either zero or increasing, except for the magnesium concentrations for station 09095500, which seems to be anomalous. The upward trends for concentrations during 1986-93 were not unexpected because the beginning of the period generally had low concentrations at the end of the high-flow years, which then were followed by several dry years when concen- trations of major ions were expected to increase. 300,000 , , , , , , , (J) z g ~ ci 200.000 - 9 (J) 0 ::; 100,000 - 0 "I 0 w :i 0 (J) 0 (J) i5 ;;! :J Z z -100,000 <( 0 w t; :J ..., -200,000 ~ ;;: 9 LL , , "" " """" """ " ................ "" .............\,.. "~.......... " ....:..:..... .............----, " .......-....""'.............-.........,............. ',,,,/ ,...... '---'-.......... """ .. "\\\ \ \ - \\ , , , 1976 , 1978 -300.000 1968 1970 1972 1974 STATION 09095500 STATION 09152500 STATION 09163500 - - , 1980 , 1986 , 1990 1992 1994 1982 1984 1988 WATER YEAR Figure 8. LOWESS smooth curves of flow-adjusted annual dissolved-solids loads for stations 09095500 Colorado River near Cameo, 09152500 Gunnison River near Grand Junction, and station 09163500 Colorado River near the Colorado-Utah State line, water years 1970-93. 22 Trend Analysis of Selected Water-Quality Deta Assocleted With Sallnlty-Control ProJects In the Grsnd Valley, In the Lower Gunnison River B8sln, and at Meeker Dome, Western Colorado