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hardness increased steadily but remained less than 100 mg/L as CaC03. Hardness values continued to increase downstream from Portland and throughout the lower basin. From Catlin Dam to Las Animas and from John Martin Reservoir to Coolidge, Kans., hard- ness increased 94 and 165 percent, respectively. The highest median bardness concentration in the river was 1,505 mg/L as CaC03 at Coolidge. Temporally, bard- ness was lowest during snowmelt runoff and increased as streamflow decreased. The toxicity of trace elements to aquatic life is affected by the hardness of the water; most metals are less toxic in water exceeding 100 mg/L as calcium carbonate (Gerhardt, 1993). In the upper basin, this relation is particularly important because of the rela- tive sensitivity of trout to elevated metal concentra- tions. Hardness is an integral part of the Colorado Water Quality Control Commission's stream classifi- cations and water-quality standards for the Arkansas River Basin (Colorado Department of Health, 1994). Many of the water-quality standards for trace elements (metals) require a hardness concentration as part of the algorithm used to calculate a particular in-stream stan- dard. Because bardness is a calculated value. that requires tbe chemical analysis of calcium 1litd magne- sium, it is useful to estimate hardness using an easily obtainable measurement such as specific conductance. Regression models for estimating hardness using specific conductance were developed for 15 main-stem sites and one tributary site using an ordi- nary least-squares regression technique. A summary of the regression coefficients and various statistical parameters is presented in table 3. All the regressions showed statistically significant (p<0.001) linear rela- tions for specific conductance and hardness. Trace Elements Selected trace elements were analyzed and eval- uated in different parts of the Arkansas River Basin on the basis of potential geologic and land-use effects on water quality. In the upper basin, the trace elements selected for analysis are affected mainly by local Table 3. Least-squares regression equations for hardness and specific conductance for selected sites in the Arkansas River Basin, April 199O-March 1993 [HARD, hardness. as milligrams per liter of calciwn carbonale; semI measured specific conductance, in microsiemens per centimeter at 25 degrees CelsIUs; <, less than] Number Regression coefficients Sland.rd Site name In lh. equ.llon Coefflcl.nl 01 (...I.bl. I) 01 paired HARD = . + b(SCml determination error of p-value values coefficient a b Leadvi lie 26 0.4 0.46 0.97 0.017 <0.001 Malta 26 3.1 .42 .94 .023 <.001 Granite 26 -5.7 .48 .98 .013 <.001 Buena Vista 27 -5.6 .48 .95 .023 <.001 Nathrop 25 -4.8 .46 .97 .016 <.001 Wellsville 26 -5.6 .47 .97 .016 <.001 Parkdale 26 -D. I .42 .97 .016 <.001 Portland 26 -2.3 .41 .98 .012 <.001 Below Pueblo Reservoir 25 9.0 .40 .96 .018 <.001 Pueblo 26 -1.5 .41 .98 .011 <.001 Fountain Creek 25 -38.9 .34 .94 .017 <.001 Highway 227 24 47.1 .31 .96 .013 <.001 Avondale 24 19.8 .36 .97 .013 <.001 Catlin Dam 24 ~21.8 .42 .98 .012 <.001 Las Animas 24 19.5 .38 .98 .010 <.001 Below John Martin Reservoir 24 -129.6 .45 .94 .023 <.001 14 W.ler-Quallty Assessm.nt 01 lhe Ark.ns.s Rlv.r B.sln, Soulheasl.rn Colorado, 1991Hl3