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N f"i Table 8. Monotonic trends in periodic calcium, magnesium, sodium, and sulfate concentrations for gaging stations 09095500 ,.p,. Colorado River near Cameo, 09152500 Gunnison River near Grand Junction, and 09163500 Colorado River near the. <::> Colorado-Utah State line, water years 1970-93--Continued Station Period Unadjusted concentration Flow-adjusted concentration Slope Percent p value SL Slope Percent p value SL Sulfate 09095500 1970-93 0.00 0.00 0.963 NS -D.46 -D.37 0.020 S 1980-93 1.15 .97 .023 S -.55 -.46 .147 NS 1986-93 .00 .00 .875 NS -3.96 -3.21 <.001 HS 09152500 1970-93 -4.02 -1.12 .055 MS -3.19 -.89 <.001 HS 1980-93 3.25 1.03 .500 NS -.25 -.08 .92t NS 1986-93 25.1 7.41 .067 MS -3.17 -.94 .395 NS 09163500 1970-93 -1.33 -.45 .165 NS -1.96 -.66 <.001 HS 1980-93 4.10 1.47 .1I0 NS -.70 -.25 .448 NS 1986-93 5.93 2.02 .594 NS -5.90 -2.01 .009 HS The LOWESS smooth curves for flow- adjusted major-ion concentrations based on water years 1970--93 data are shown in figures 9 and 10. The LOWESS smooth curves show downward trends in all four major-ion concentrations at the Colorado River stations after about 1985 or 1986, which is substanti- ated by the trend results for 1986-93 (table 8). Except for calcium concentrations at station 09163500, the flow-adjusted trends in all four major-ion concentra- tions for stations 09095500 and 09163500 for 1986-93 are highly significant, and all trend slopes indicate decreasing major-ion concentrations with time (table 8). The smooth curves indicate that much of the decrease in flow-adjusted major-ion concen- trations during 1970-93 in the Gunnison River (station 09152500 in figs. 9 and 10) occurred during about 1974-81. The trends in flow-adjusted major-ion concentrations for station 09152500 for 1980--93 and for 1986-93 were not significant or were marginally significant (table 8). Reasons for the downward trends in major-ion concentrations in the Gunnison River during 1974-81 are not known, but might be related to the streamflow regulation by Blue Mesa Reservoir. The LOWESS smooth curves for the Colorado River stations do not mimic this apparent trend for the Gunnison River (figs. 9 and 10). Many of the flow-adjusted trend results for periodic calcium, magnesium, sodium, and sulfate concentrations were similar to the flow-adjusted trends in the dissolved-solids concentrations for water years 1970--93, 1980--93, and 1986-93 for all three gaging stations. The largest trend slopes for the Colorado River stations (09095500 and 09163500) were for 1986-93 (table 8), which was during con- struction of major parts of the salinity-control project in the Grand Valley and start-up of salinity-control work in the Lower Gunnison Basin Unit. In terms of the slope expressed as a percent rate of change, calcium concentrations changed less in 1986-93 than did the other three major-ion concentrations tested. The trend slopes (flow-adjusted data) for the two Colorado River stations were almost always in agreement in terms of the direction of the trend (upward or downward) and in significance level. As with dissolved-solids trends, the similarity of the trend results for the major ions for both Colorado River stations complicates the issue of how much of the observed trend at station 09163500 at the State line was related to salinity-control projects and how much was caused by climatic or other factors. That issue is discussed in more detail in the following section. 26 Trend Analyala of Selected Water-Quality Data Asaoclated With Salinity-Control Projects In the Grand Valley, In the Lower Gunnison River Basin, and at Meeker Dome, Western Colorado