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018~ A total of 21 water samples from surface-water (stream and reservoir) sites and 5 samples from ground-water sites were collected during the study. .he State domestic-water-supply standard of 250 mg/L for sulfate (table 6) was exceeded in 17' of the surface-water samples (tables 8-10) and in 4 of the ground-water samples (table II). The only sites where this standard was not exceeded were near the upstream end of the study area (sites AI, A3, and RI) and the ground-water site at Syracuse (site G3), which may be recharged from the sand-dune area. The State standard and U.S. EPA maximum contaminant level of 10 ~g/L for selenium in domestic-water supply (table 6) was exceeded at three surface-water sites (TI, T2, and A3) and in three ground-water sites (GI, G2, and G4). Two of these surface-water sites (TI and T2) are tribu- taries that contributed little streamflow to the Arkansas River at the time of sampling. Sites A3, GI, and G2 are located in the area between La Junta and John Martin Reservoir, where runoff from marine-shale areas seems to have its most significant effect in the study area. The selenium concentration in surface water from the La Junta area (site A3) was 17 ~g/L (table 8). The concentration in ground water was 12 ~g/L at La Junta (site GI) and 29 ~g/L at Las Animas (site G2), which is about 17 mi downstream from site Gl (table 11). The other well in which selenium exceeded the drinking-water standard was at Lakin (site G4). The concentration of selenium in a sample from that site was 12 ~g/L. The highest concentrations of selenium measured in all the samples during the study were 45 ~g/L at site Tl and 52 ~g/L at site T2 (table 8). These concentrations exceed the U.S. EPA criteria for protection of aquatic life for both chronic and acute exposure (table 5). They also exceed the Colorado standard for chronic exposure but not for acute exposure. The State selenium standards and U.S. EPA criterion for acute exposure were not exceeded at any other surface-water sites (tables 8-10); however, the U.S. EPA criterion for chronic exposure (5 ~g/L) was exceeded at four additional sites on the Arkansas River (sites A2, A3, A6, and A7), at three reservoir sites in June (sites R2, R4, and R5), and at one reservoir site in October (site R5). The only other water-quality standard that was exceeded in any sample was the chronic-exposure aquatic-life standard for mercury (0.1 ~g/L in Colorado and 0.012 ~g/L in Kansas, table 5). The highest mercury concentra- tions were detected at the most upstream sites (tables 8 and 9): Pueblo Reservoir (site Rl, 0.6 ~g/L), the Arkansas River above Pueblo (site AI, 0.9 ~g/L), Fountain Creek (site TI, 0.5 ~g/L), and Lake Meredith (site R2, 0.6 ~g/L). Concentrations decreased downstream but exceeded the standard at all stream sites except the most downstream location (site A7, Arkansas River near Deerfield). The source of mercury seems to be upstream from the study area, and the elevated concentrations probably are not caused by irrigation return flow within the study area. The acute-exposure standard (2.4 ~g/L) was not exceeded at any. site. Concentration of trace elements, other than selenium and mercury, generally were much lower than the most restrictive standards listed in tables 5 and 6. The maximum cadmium concentration was 2 ~g/L, which might have exceeded the aquatic life standard if the hardness was low. The lowest hardness associated with 2 ~g/L of cadmium was 270 mg/L at site A2 (Arkansas River near Nepesta). At this hardness level, the chronic-exposure standard for cadmium is about 2.5 ~g/L. Lead concentrations were less than analytic 35