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Below John Martin Reservoir and Coolidge, Kans., sites probably due to the prevalence of irrigation return flows in the reach. Temporal variability in nitrate concentrations was apparent, particularly in the lower Arkansas River Basin. Median concentrations were higher during low- flow (October-April) than during either the snowmelt- runoff (May-June) or post-snowmelt runoff (July- September) regime (fig. 26). Low-flow conditions resulted in less dilution in the river, possibly resulting in higber concentrations during this regime. Concen- trations during snowmelt-runoff and post-snowmelt runoff regimes were similar throughout the lower basin. A Speannan's rank correlation test was used to detennine whether nitrate concentrations changed with streamflow. This nonparametric test uses the ranks of the data to determine the strength of the asso- ciation of the variables. Speannan correlation coeffi- cients, or r values, range from -I to + I, and data are highly correlated as these bounds are approached. When r = 0, there is no correlation. The Spearman's rank test indicated there was an inverse relation between streamflow and nitrate concentrations in the lower basin. Main-stem sites from Pueblo to La Junta had r values ranging from --{).68 to --{).89. The nega- tive sign indicates an inverse relation; that is. nitrate concentrations decreased as streamflow increased. All probabilities were significant at the u=0.05 level. Nitrate concentrations in the Arkansas River were low in comparison to stream-water-quality standards and drinking-water standards. The stream- water-quality standard for nitrate is 10 mg/L and was not exceeded in any sample. Although nitrate concen- trations generally increased downstream throughout the basin, concentrations did not exceed stream-water- quality standards at any site. Phosphorus Phosphorus concentrations generally were lower in the upper basin than in the lower basin (fig. 28). Median concentrations in the upper Arkansas River ranged from less than 0.01 mg/L at Nathrop to 0.05 mgIL at Portland. Concentrations in the lower basin ranged from 0.02 mgIL at Pueblo to 0.28 mg/L at Avondale. Concentrations increased significantly (u=0.05) between the Pueblo and Highway 227 sites, probably due to inflow from Fountain Creek' the median phosphorus concentration at the Fou~tain Creek site was 0.96 mg/L. Median concentrations more than doubled in the river downstream to Avon- dale. Median phosphorus concentrations at Highway. 227, Avondale, and Catlin Dam exceeded the EPA' recommendation of 0.1 mg/L for total phosphorus in rivers (U.S. Environmental Protection Agency, 1986a). The median concentration at Las Animas (0.10 mg/L) exceeded the threshold value of 0.05 mg/L as recommended by the Environmental Protec- tion Agency for total phosphorus in a river entering a reservoir (U.S. Environmental Protection Agency, 1986a). There was no distinct temporal pattern in the phosphorus concentrations. Radiochemical Constituents Water-quality samples for radiochemical anal- yses were collected periodically from 1990 through 1992 at eight surface-water sites on the main stem of tbe Arkansas River. The samples were analyzed for natural uranium, gross alpha (expressed as natural uranium), and gross beta (expressed as cesium-137 and strontium-90/yttrium-90). Forty-nine samples were analyzed for dissolved uranium, 4 to 8 samples at each site, and 46 samples were analyzed for gross alpha and gross beta, 5 to 7 samples at each site. Relatively high levels of natural uranium occur in the Rocky Mountain region of the United States (U.S. Environmental Protection Agency, 1986b), and parts of southeastern Colorado have been identified as having consistently high concentrations of dissolved uranium in surface and ground water (Shannon, 1979). Several possible contributing factors for the high concentrations of dissolved uranium include composi- tion bedrock, land use, and concentration of dissolved constituents due to evapotranspiration. Large parts of southeastern Colorado are underlain by marine shale (Pierre Shale) and limestone of Late Cretaceous age (Zielinski and others, 1995). Some sedimentary rocks, especially marine shales, have been shown to have greater than nonnal radioactivity due to uranium, thorium, and potassium (Pettijohn, 1949). Agriculture is the predominant land use along the lower Arkansas River; water initially diverted from the river is used for irrigation, then the return flows are used again for irrigation, and eventually the water returns to the river. This reuse of water increases the amount of time the soil and rock can interact with the water, resulting in further dissolution of minerals and increased ion- exchange interaction, which is indicated by the 56 W.t.r.Quallty Assessment 01 the Ark.nsas River B..ln, Southe..tern Color.do, 1991l-93