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0172 Climate The semiarid climate of the study area is characterized by low to moderate precipitation, substantial evaporation, low humidity, moderate to intense winds, and a large daily range in temperature. At Pueblo, the mean annual temperature is 52.7 of, the mean January temperature is 30.0 of, and the mean July temperature is 76.5 of. About 250 mi to the east at Garden City, the mean annual temperature is 54.7 of, the mean January temperature is 25.1 of, and the mean July temperature is 79.0 of. The average time between killing frosts ranges from 173 days at Pueblo to 184 days at Garden City. The last killing frost generally occurs in late April, and the first killing frost generally occurs in mid-October. Mean annual precipitation ranges from 11.8 in. at Pueblo to 17.5 in. at Garden City. About 75 to 80 percent of the annual precipitation falls as rain during the growing season. Throughout the area, potential evapotranspiration greatly exceeds precipitation. Mean pan evaporation during April-October is about 60 in. near Pueblo and increases to about 67 in. at John Martin Reservoir and about 77 in. at Garden City (Farnsworth and Thompson, 1982). Geology The headwaters of the Arkansas River are in an area of mineralized igneous and metamorphic bedrock around Leadville. Drainage from active and abandoned mines in this area contains a variety of trace elements, particu- larly cadmium, copper, iron, manganese, and zinc (Kimball and Wetherbee, 1989). Cretaceous marine shales and limestones are exposed extensively within the study area (fig. 3). Outcrops of the Pierre Shale are common in the Fountain Creek basin in Colorado, north of Pueblo. The Niobrara Formation (shale and limestone) is exposed north of the Arkansas River from the vicinity of Fowler, Colo., east to the State line. The Niobrara Formation and the Carlile Shale are exposed south of the river from Pueblo Reservoir to La Junta. In the Kansas part of the study area, the Graneros Shale and Greenhorn Limestone are exposed along the north edge of the Arkansas River valley. The Tertiary Ogallala Formation (loose to well-cemented sand and gravel) outcrops extensively in the eastern part of the study area. Other formations exposed at the surface include Cretaceous sandstones; Jurassic, Permian, and Pennsylvanian sedimentary rocks; and Precambrian igneous rocks. In shale and limestone, concentrations of many elements, particularly arsenic, boron, selenium, and sulfur can be enriched relative to concentra- tions in other rock types (Turekian and Wedepohl, 1961). In the Pierre Shale, arsenic, cadmium, molybdenum, selenium, and uranium are highly concentrated (Schultz and others, 1980). Elevated selenium concentrations also are present in the Niobrara Formation near the study area (Hatten, 1969). 9