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~, . i;; ':t.1. " Table 7. Statistical summary of selected major-ion concentrations for stations 09095500 Colorado River near Cameo, 09152500 Gunnison River near Grand Junction, and 09163500 Colorado River near the Colorado-Utah State line, water years 1970-93 {Concenltalions in milligrams per liter; range is mimmum and maximum concentrations) Number Mean Standard Median Range Station Constituent 0' samples deviation 09095500 Calcium 254 62.6 15.3 68 29-100 Magnesium 254 t5.5 4.3 17 6.4-25 Sodium 253 95.2 43.2 t04 t4-200 Sulfate 255 t24 44.8 136 32-220 Chloride 255 t24 60.6 134 11-270 . "--"-:.- .on 10 n ,., ';;O_IIU'l . "............ 09152500 C..lcium 204 100 44.8 91.5 30-240 Magnesium 204 34.2 14.4 32 7.8-68 Sodium 204 60.9 28.6 57 12-130 Sulfate 204 36t t89 330 62-930 Chloride 204 9.8 5.6 8.4 2.3-58 Alkalinily 204 t38 33.3 t37.5 17-22t 09163500 Calcium 210 93.4 32.7 88.5 37-200 Magnesium 210 31.0 11.6 31 9.6-73 Sodium 210 90.6 36.6 93 18-190 SuIrate 209 296 t32 270 67-670 Chloride 210 80.0 35.7 82 11-l70 Alkalinity 210 144 26.6 150 82-220 Tests on individual ions generally had similar trend test results as the unadjusted dissolved-solids concentrations (table 2) for the same stations and periods, except for station 09 I 52500 on the Gunnison River for water years] 986-93. There were upward trends in the four major-ion concentrations for station 09152500 for ]986-93 (table 8). Although the magnitude of the slopes might seem large [such as 25.] (mgIL)/yr for sulfate], the p values were not sig- nificant or were marginally significant. The cumula- tive effect of the upward trends for individual ions manifests itself in a significant upward trend slope of 42.6 (mglL)/yr (table 2) in dissolved-solids concentra- tions for 1986-93 for the Gunnison River. When flow adjusted (from log-log regressions), many major-ion concentrations had significant or highly significant downward trends for certain time periods. All flow-adjusted major-ion trends were either significant or highly significant for water years 1970-93 for all three stations (table 8). By con- trast, many of the trend slopes for 1980-93 were not significanl. Flow-adjusted sodium concentrations had significant downward trends at both Colorado River stations, and flow-adjusted magnesium concen- trations had a highly significant downward trend at station 09095500 in 1980-93. As with dissolved solids, there were differences in the trend results among time periods. 24 Trend Analysts 01 Setectad Water-Quallty Data Asaoclatad With Sallnlty-Control Projects In the Grand Valley, In the Lower Gunnison River Basin, and at Meeker Dome, Western Colorado