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Table 2. Least-squares regression equations for dissolved-soiids concentration and specific conductance for selected sites in the Arkansas River Basin, April 199o-March 1993 [DS, estimated dissolved solids. in milligrams per liter; SCm. measured specific conductance, in microsiemens per cenlimeter at 25 degrees Celsius; <, less than] Number of Regression coefficients In paired the equation Coefficient of Standard Site name error 0' p-v8lue (see table 1) measurements DS = a . b(SCm) determination coefficient a b Leadville Mine Drainage Tunnel 15 45.9 0.65 0.78 0.096 <0.001 Leadville 26 18.7 .51 .93 .030 <.001 California Gulch 15 -166.9 .93 .96 .053 <.001 Malta 26 5.6 .55 .83 .051 <.001 Granite 26 -15.0 .71 .92 .043 <.001 Buena Vista 27 ~.5 .63 .83 .058 <.001 Nathrop 25 -3.9 .60 .88 .045 <.001 Salida 26 -11.6 .66 .96 .028 <.001 Wellsville 25 -7.0 .62 .95 .031 <.001 Park dale 26 -1.5 .59 .97 .020 <.001 Callon City 26 ~.6 .61 .90 .043 <.001 Ponland 26 -25.1 .68 .93 .038 <.001 Below Pueblo Reservoir 24 -37.5 .71 .93 .042 <.001 Pueblo 26 -92.8 .81 .99 .020 <.001 Fountain Creek 25 -122.7 .78 .97 .026 <.001 Highway 227 24 ~1.0 .74 .98 .024 <.001 Avondale 24 -121.3 .83 .98 .025 <.001 Nepesta 24 -30.3 .73 .87 .059 <.001 Catlin Dam 24 -12Q.6 .82 .97 .029 <.001 Timpas Creek 18 -280.1 .95 .98 .032 <.001 La Junta 24 -224.0 .92 .99 .018 <.001 Las Animas 24 -260.0 .91 .92 .057 <.001 Purgatoire River 24 --415.2 1.02 .99 .026 <.001 Below John Martin Reservoir 24 -298.8 .94 .97 .034 <.001 land use, and increased agricultural water use. The general downstream pattern was similar to that previ- ously described for dissolved-solids concentrations. In the upper basin, calcium and bicarbonate were the dominant major ions (fig. 5). Calcium and bicarbonate accounted for about 60 to 80 percent of the cation and anion concentrations, respectively. The downstream increase in major-ion concentrations in the upper basin likely was caused by the transition from igneous and metamorphic rock to sedimentary rock, and from the . increased use of water for municipal and agricultural purposes (Crouch and others, 1984). Downstream from Portland, the water changed from a calcium bicarbonate type to a calcium sulfate type, probably as a result of contact with marine shales that occur in the area. In the lower basin, the water changed from a calcium sulfate type to a mixed water type. While no dominant cation was detected at the Coolidge site, sulfate accounted for about 82 percent of the anions (fig. 5). The concept of water hardness dates back many centuries but, in modem times, has been associated with the presence of calcium and magnesium (Hem, 1985). Hardness generally is defined in terms of these two constituents and is expressed in terms of an equiv- alent concentration of calcium carbonate. In the upper Arkansas River, median hardness values decreased between Malta (90 mgIL as CaC03) and Granite (45 mg/L as CaC03) because of dilution from Lake Creek inflow. Downstream from Granite to Parkdale, 12 Water-Quality Assessment of the Arkanses River Besln, Southeastern Colorado,199lHl3