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The pattern of selenium concentrations shown in figure 9 is different from the other constituents. The selenium concentration was low at the upstream site and high in the tributaries, and the same dilution effect occurred at site A2. The concentration increased at site A3, although proportionally less than for the other constituents. The concentration then decreased downstream from John Martin Reservoir (site A4) and increased only slightly at the downstream sites. Selenium concentration may be affected by some process occurring in John Martin Reservoir. The geologic sources of selenium may not be as extensive downstream from the reservoir. The patterns of sulfate, boron, and uranium concentrations also were similar at the reservoir sites (fig. 10). All the reservoirs, except Pueblo Reservoir (site R1), near the upstream end of the study area, seem to be affected by runoff from marine-shale areas. Lake Meredith (site R2) and Lake McKinney (site R5) seem to be affected to the greatest extent. Concentrations in Lake Meredith were substantially higher than in the Arkansas River near Nepesta (site A2), which is near the point of diversion for inflow to the lake. Some other source, perhaps runoff from irrigated areas near the lake, may be contributing constituent loads to the lake. A similar situation occurs at Lake McKinney and may be contributing to concentrations that are somewhat larger than inflow values. Little direct irrigation runoff accumulates in the Great Plains Reservoirs (site R3) or John Martin Reservoir (site R4). Concen- trations of sulfate, boron, and uranium at site R4 were similar to those at stream sites A3, upstream, and A4, downstream from the reservoir. Selenium concentrations were low (1-10 ~g/L) in all reservoir samples. As indicated by the data upstream and downstream from John Martin Reservoir, some processes may affect selenium concentration in reservoirs. If sllch a process is occurring, dissolved selenium may be removed from the water and possibly stored in the biota or bottom sediment. Oremland and others (1989) have demonstrated the possibility of such a process involving bacterial reduction of selenate under anoxic conditions. The concentration patterns of all four constituents were similar to each other in samples from the ground-water sites (fig. 11). Unlike the surface- water sites, the highest concentrations occurred at the more upstream ground- water sites. The sites that seem most affected by recharge from marine-shale areas are at La Junta (site G1) and at Las Animas (site G2). These sites are downstream from the Fort Lyon storage canal diversion and upstream from John Martin Reservoir, which is the area where the largest increases in concentra- tion were detected in the Arkansas River. This area also is near extensive outcrops of the Carlile Shale and the Niobrara Formation, which contains seleniferous marine shale. Constituent concentrations at site Gl were similar to those in the Arkansas River at La Junta (site A3); but at site G2, concen- trations were much larger than at site A3. Concentrations were low at Syracuse (site G3). This site is south of the Arkansas River in an area that may receive dilute recharge water from precipitation on sand dunes. At Lakin (site G4) and Deerfield (site G5), concentrations were slightly larger than at site G3 but, except for selenium, were less than those in the Arkansas River near Deerfield (site A7). Sites G4 and G5 also may be receiving recharge from a source more dilute than seepage from the river. 32