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<br />basin occurred in the vicinity of Buena Vista before <br />increasing because of increased bicarbonate sources <br />downstream. In the upper basin, the highest median <br />pH occurred at Portland. In the lower basin, the <br />median pH of the Arkansas River ranged trom 8.2 at <br />Coolidge, Kans., and the site Below John Martin <br />Reservoir to 8.5 at Highway 227. The pH of the <br />Arkansas River typically was lower during the snow- <br />melt-runoff regime because of the lower pH of snow- <br />melt runoff. Instantaneous pH values for main-stem <br />sites generally were within the acceptable range (6.5- <br />9.0) for in-stream water quality set by the State of <br />Colorado (Colorado Department of Health, 1994). <br /> <br />Dissolved Solids and Major Ions <br /> <br />Water samples were collected and analyzed for <br />dissolved-solids concentrations at all sites in the <br />Arkansas River Basin. Analysis for concentrations of <br />major ions was done for main-stem sites, but most <br />tributary sites were not sampled for major ions. <br />Calcium, magnesium, sodium, bicarbonate, sulfate, <br />and chloride were the major ions analyzed. Potassium <br />and fluoride were not analyzed because historical data <br />indicated that the concentrations were not substantial <br />in the basin. <br />Dissolved-solids concentrations in the Arkansas <br />River are affected by many factors, including runoff <br />trom snowmelt or rainfall, geology, land and water <br />use, mine drainage, and inflow trom ground water <br />(Crouch and others, 1984). These factors resulted in <br />distinct spatial variations of dissolved-solids concen- <br />trations in tbe river (fig. 4). Dissolved solids were <br />slightly elevated at Leadville and Malta because of <br />mineralized drainage from mines and mine tailings in <br />the Leadville area. Downstream trom Malta, <br />dissolved-solids concentrations decreased as flow <br />from Lake Creek (pI. I) diluted the dissolved solids in <br />the river. The lowest median main-stem concentration <br />in the basin (62.5 mg/L) occurred just downstream at <br />Granite. A gradual increase ofless than I mg/L per <br />mile occurred from Granite downstream to Caiion <br />City; dissolved solids generally doubled along this <br />!03-mi reach. The increase was due to decreased <br />precipitation and runoff, changes in geology and <br />chemical composition of rocks, and increased water <br />use (Crouch and others, 1984). Measured tributaries <br />along this reach contributed about 50 percent of the <br />measured dissolved-solids loads. Dissolved-solids <br /> <br />concentrations nearly doubled again along a 16-mi <br />reach from Caiion City to Portland (ahout 5.5 mg/L <br />per mile) because of a change in geology from igneous <br />and metamorphic rock to sedimentary rock and <br />increased return flows (Miles, 1977). Only about <br />35 percent of the dissolved solids along this reach <br />could be attributed to the two measured tributaries in <br />the reach. Inflow from ground water and unmeasured <br />tributaries and surface return flows may account for <br />the remaining dissolved-solids load. In the Pueblo <br />area, median concentrations in the river increased by <br />about 15 percent due to tributary inflow from Fountain <br />Creek. Downstream from Pueblo, concentrations <br />increased steadily to Catlin Dam (about 5 mg/L per <br />mile). A 7 I-percent increase in median concentration <br />occurred between Catlin Dam and La Junta (about <br />16 mg/L per mile) where irrigation return flows are a <br />large portion of the streamflow. Between La Junta and <br />Las Animas, the dissolved-solids concentration in the <br />river increased slightly. Median dissolved-solids <br />concentrations increased about 183 percent (about <br />30 mg/L per mile) trom Las Animas to Coolidge, <br />Kans. The highest median concentration in the basin <br />was 3,825 mgIL at Coolidge. Concentration of <br />dissolved salts resulting trom the use and reuse of irri- <br />gation water throughout the lower basin resulted in a <br />greater than tenfold increase in dissolved solids in the <br />lower basin. <br />Dissolved-solids concentrations in the upper <br />basin were lowest during snowmelt runoff and highest <br />during low flow. Dissolved-solids concentrations <br />generally were as much as 2.5 times larger during the <br />low-flow regime than during snowmelt runoff. Thus, <br />dissolved-solids concentrations were inversely propor- <br />tional to streamflow in the upper basin. <br />In natural water, specific conductance is a good <br />estimator of dissolved-solids concentrations (Hem, <br />1985). Regression models of specific conductance and <br />dissolved-solids concentration were developed using <br />least-squares techniques for 24 sites in the Arkansas <br />River Basin where 15 or more paired measurements <br />were available. A listing of the sites, regression coeffi- <br />cients, and various statistical qualifiers is in table 2. <br />All the regressions showed statistically significant <br />(p<0.001) linear relations for specific conductance and <br />dissolved-solids concentration. <br />Major-ion concentrations in the Arkansas River <br />Basin showed large spatial variations (fig. 5). In <br />general, concentrations for all six major ions increased <br />downstream, probably because of changing geology, <br /> <br />10 W.ler-Qu.llty A...ssment 01 the Ark.ns.. River B..ln, South...t.rn Color.do, 199~3 <br />