Relations of Main-Stem Reservoir Operations and Specific Conductance in the Lower Arkansas River

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These differences in specific conductance were statis- tically significant for January, March —June, and September— December (table 5). On a seasonal basis, the median specific conductance increased from 460 gS /cm in 1966 -74 to 465 gS /cm in 1975 -94; the increase was not statistically significant (p =0.51) during the growing season. In the winter - storage season, the median specific conductance decreased significantly (p< -0.01) from 720 gS /cm in 1966 -74 to 575 gS /cm in 1975 -94. Historically, before the construction of Pueblo Reservoir, specific conductance was highest during low streamflow in the winter. With completion of the dam, the high- specific- conductance inflow (October —April) began to mix with and be diluted in the reservoir by lower specific - conductance water derived from snowmelt runoff, resulting in decreased specific- conductance water flowing out of the reservoir during low flow. A decrease in the specific conductance of the reservoir inflow, as indicated by the 19- percent decrease in the median specific conductance at Canon City, also contributed to the decreased specific conductance at station 07099400. Additionally, before the completion of Pueblo Dam, the annual minimum specific conduc- tance occurred during high flow, May through August. Following the closure of Pueblo Dam, the specific conductance at station 07099400 increased during high flow probably because of mixing of low- specific- conductance inflow from snowmelt runoff with higher specific- conductance water in the reservoir (fig. 11). Estimates were made of the frequency of occurrence of specific- conductance values at station 07099400 and at the diversion points for the domestic water supplies for the Pueblo Board of Water Works and the St. Charles Mesa Water District. This information and the observed relations between specific conductance and dissolved - solids concentration were used to estimate the percentage of time the dissolved - solids concentration exceeded the SMCL for drinking water (500 mg/L). A compar- ison of specific - conductance data that have been collected at station 07099400 with data that have been collected at the diversion points for the Pueblo Board of Water Works and the St. Charles Mesa Water District indicates that specific conductance increases about 3 percent per mile in that reach. An estimate was made, based on this relation, of the specific conductance at the diversion points for the two domestic water supplies (fig. 12). These estimates were made by increasing the specific conductance at station 07099400 by 12 percent for the diversion for the Pueblo Board of Water Works and by 27 percent for the diversion for the St. Charles Mesa Water District. Table 5. Step -trend results on specific conductance at station 07099400 (Arkansas River above Pueblo) between 1966 -74 and 1975 -94 [µS /cm, microsiemens per centimeter at 25 degrees Celsius; N, number of values; p value is the significance level of the test; <, less than; NS, trend not statistically significant; I, statistically significant increasing trend; D, statistically significant decreasing trend] to statistically significant trend was defined as having a p value less than or equal to 0.05. RELATIONS OF STREAMFLOW AND SPECIFIC - CONDUCTANCE TRENDS TO RESERVOIR OPERATIONS 21 IN THE ARKANSAS RIVER 1966 -74 1975 -94 Median Median Month specific specific p value Significance conductance N conductance N (µS /cm) (µS /cm) January 673 9 574 4 0.04 D February 718 9 604 3 .14 NS March 790 8 559 11 <.01 D April 625 13 572 12 .05 D May 404 12 557 13 <.01 I June 258 12 375 13 <.01 I July 380 9 377 15 .86 NS August 340 9 408 12 .34 NS September 593 9 451 12 .05 D October 748 9 514 12 <.01 D November 716 10 557 4 .03 D December 710 8 549 6 .02 D to statistically significant trend was defined as having a p value less than or equal to 0.05. RELATIONS OF STREAMFLOW AND SPECIFIC - CONDUCTANCE TRENDS TO RESERVOIR OPERATIONS 21 IN THE ARKANSAS RIVER