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1783 :..\~? ~~.~ ~~~~~\f:&%t~:~-~~ ~;1 t=.....: -- " ~-;. ~':":'-" ki .::G~~"ol;f: :1 . ~.... .. ~.~bisct ~C ~','3'11StCn _. - .. Q' ''''-.-':: OR R;::P::A~.;:: '. ''':.' I "OT ........ I.. ... -... .~- : ;"'J'.-;;,.~~, Aporovcl r:;y L..'lrC':'i.~r, Figure 5 near here. Variations in dissolved solids concentrations';':nil~lh9'!~Ij~~ed9~lr'J~/ concentrations of sb: major ions in tbe Arkansas River, Aprth99(t-;Vfarcli 't9!lf I ... Major-ion concentrations in the Arkansas River Basin showed large areal variations (fig. 5). In general, con- centrations for all six major ions increased downstream owing to changing geology, land use, and increased agricul- tural water use. The general downstream pattern was similar to those previously described for total-dissolved-solids concentrations. In the upper basin, calcium and bicarb,:mate were the dominant major ions (fig. 5). Calcium and bicarbonate accounted for about 60 to 70 percent of the cation and anion concentrations, respectively. The down- stream increase in major-ion concentrations in the upper basin was caused by the transition from igneous and meta- morphic geology to sedimentary geology. From Portland to Below Pueblo Reservoir, the water-type transitioned from calcium-bicarbonate to calcium-sulfate as a result of contact with marine shales that occur in the area. In the lower basin, the water-type transitioned from calcium-sulfate at the site Below Pueblo Reser'loir to a sodium-cal- cium-magnesium-sulfate type at Coolidge; sulfate accounted for 82 percent of the anions at Coolidge (fig. 5). . . The principal cations associated with water hardness are calcium and magnesium and are expressed in terms of an equivalent concentration of calcium carbonate. Median hardness decreased significantly from Malta (90 mg! L) to Granite (45 mgIL) because of the inflow of water from Lake Creek. Downstream from Granite, hardness increased steadily but generally remained less than 100 mgIL. In the lower basin, hardness increased 94 percent between Catlin Dam and Las Animas and 165 percent between John Martin Reservoir and Coolidge. The highest median hardness concentration in the basin was 1,505 mgIL ar Coolidge. Seasonally, hardness was smallest during snowmelt runoff and increased as streamflow decreased. Hardness can affect the toxicity of trace metals to aquatic life (Gerhardt, 1993). Most metals are less toxic in hard water (hardness larger than 100 mgfL). In the upper basin, this relation is panicularIy important because of the relative sensitivity of trout to elevated metal concentrations. As such, hardness is an integral part of the Colorado Water Quality Control Commission's stream classifications and water-quality standards for the Arkansas River Basin (Colorado Department of Health, 1994). Many of the trace-element standards (metals) require a hardness con- .enrration as part of the algorithm used [0 calculate a particular in-stream standard. Since hardness is a calculated 13