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~ .11.'. 'i,"'i&. . The step-trend results of multiple t-tests on chlo- ride and sodium concentrations and on ranks of the data (Wilcoxon rank-sum test) among the various periods for each station are summarized in table 9. The t-tests on concentrations (the T-group results in table 9) test for differences in means between data sets; t-tests on the ranks (the W-group results) test for differences in medians between data sets. An alpha level of 0.01 was selected as the significance level for the t-tests. At the background station 09304200, chloride and sodium have not changed, except for the anomalous fluctua- tions after completion of well plugging in 1981, and the TRANS period had significantly higher concentra- tions than the other three periods. The cause of the higher concentrations during the TRANS period at C't'lt;n.n n01.M"J(V) ic nn....lpgr hilt fhp R.ru~ {10Sl"ih\ . .. reported that there might have been a short-term change in the ground-water flow system after the well plugging that caused discharge of some saline water into the White River near station 09304200. At stations 09304600 and 09304800, mean chloride and sodium concentrations were almost equal for the PRE and TRANS periods, and those periods had signifi- cantly greater concentrations than the post-project peri- ods (table 9). All step-trend results were the same among stations and time periods when using the para- metric or non parametric method. Based on step-trend tests, significant decreases in chloride and sodium con- centrations occurred in the While River after comple- tion of the well plugging at Meeker Dome in 1981. Mean chloride and sodium concentrations at station 09304800 for the POST I and POST2 periods were not statistically different (table 9). Also, the chloride and sodium concentrations in the three sets of samples collected in water year 1995 at station 09304800 (fig. 12) were similar to concentra- tions in the POST2 period, and the 1995 data collected at station 09304600 were similar to the concentrations from the POSTI period for that station. The chloride and sodium data for the POSTI and POST2 periods and the new data collected in 1995 indicate that the well plugging at Meeker Dome has remained intact because there is no indication that chloride and sodium concentrations have increased in the White River since April 1982. The chloride and sodium data collected in water year 1995 at the four gaging stations on the White River (fig. 2) were used to calculate dissolved-solids, chloride, and sodium loads for comparison of loading along the White River. The gain in dissolved solids between stations 09304200 and 09304600 primarily \ would be from ground-water discharge from Meeker Dome (natural discharge plus perhaps some small leakage in the wells) and from Coal Creek. The dissolved-solids loading into the White River between stations 09304200 and 09304600 accounted for about 24 percent of the dissolved-solids load, for about 75 percent of the chloride load, and for about 38 percent of the sodium load at station 09304800 for the three sets of samples. The chloride loading indi- cates that some ground water continues to discharge from the Meeker Dome area into the White River. The step-trend results were based on chloride and sodium concentrations, and t-tests were not done on flow-adjusted concentrations. As in other drainage basins in the Upper Colorado River Basin, stream discharge was above normal in water years 1983 and 1984 in the White River Basin. The instantaneous stream discharges measured when chloride and sodium samples were collected indicated that the samples from the POSTI and POST2 periods generally were col- lected at higher stream discharges than the samples col- lected during the PRE period. However, the chloride and sodium data for stations 09304600 and 09304800 indicale that ion concentrations were much lower in the POSTI and POST2 periods compared to the PRE period for samples collected at approximately the same stream discharge. To more rigorously confirm that observation, ESTREND was used to determine flow-adjusted trends for chloride and sodium data during the intensive data-collection period (July I978-September 1984). The well plugging would have had no measurable effect on streamflow in the White River; therefore, the flow-adjustment proce- dure was used to remove the variance in concentrations caused by changes in streamflow. The mean chloride and sodium concentrations of samples for each month were used for the monotonic trend tests because of the weekly sampling frequency. The flow-adjusted chlo- ride and sodium trends for stations 09304600 and 09304800 are highly significant (p value less than 0.01) and indicated decreasing concentrations (table] 0). LOWESS smooth curves were examined for flow- adjusted chloride and sodium concentrations, and the curves (not shown) indicated that flow-adjusted concentrations for stations 09304600 and 09304800 decreased after 1982. Therefore, the trend tests on flow-adjusted concentrations also confirm that signifi- cant decreases in chloride and sodium concentrations occurred in the White River downstream from Meeker Dome after completion of well plugging in 1981. 34 Trend Anelysls of Selected Wetsr-Quallty Date A.socleted With Sellnlty-Controt Prolectstn the Grsnd Velley, In the Lower Gunnison River Basin, and at Meeker Dome, Western Colorado