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<br />. .-, .." <br /> <br />Baseline values for selected elements in soils in the Western United <br />States are listed in table 7. The upper limit of the expected 95-percent <br />range was compared to measured values in bottom-sediment samples from the <br />study area to identify elevated concentrations. <br /> <br />Water Quality <br /> <br />The concentrations of many constituents increased in a downstream direc- <br />tion at surface-water sampling sites in the study area. Concentrations of <br />several constituents in samples from stream sites are plotted in figure 9. <br />These constituents were selected because they are more concentrated in marine <br />shales than in other rock formations that are exposed in the study area; there- <br />fore, high concentrations in surface water may indicate runoff from land under- <br />lain by marine shale or where 'marine shale is the parent material of the soil. <br /> <br />The patterns of concentrations for sulfate, boron, and uranium are <br />similar. Concentrations were relatively low in the Arkansas River upstream <br />from Pueblo (site AI) near the west end of the study area. Concentrations in <br />the two tributaries were higher but were diluted by the larger volume of <br />water in the Arkansas River, so concentrations increased only slightly in the <br />Arkansas River near Nepesta, Colo. (site A2). Concentrations in the Arkansas <br />River increased substantially at La Junta (site A3) and remained bigh at all <br />sites downstream. Streamflow at site A3 in August was less than 10 percent of <br />streamflow at site A2 (table 8). Most of the flow in the Arkansas River was <br />diverted into irrigation and water-supply canals between these two sites. <br />Water quality of the river downstream from these diversions seems to be <br />affected by runoff from marine-shale areas. This effect is consistent with <br />Cain's (1985) conclusion that much of the flow in the river downstream from <br />La Junta is irrigation drainage during parts of most years. <br /> <br />Table 7.--Geochemical baselines for selected elements in soils in the Western <br />United states (R.C. Severson, U.S. Geological Surveg, written commun., <br />1987; based on data from Shacklette and Boerngen, 1984) <br /> <br />[Baseline is the upper limit of the expected 95-percent range; <br />all values in micrograms per gram] <br /> <br />Element Geometric Geometric Expected <br /> mean deviation 95-percent range <br />Arsenic 5.5 1.98 1. 2-22 <br />Chromium 41 2.19 8.5-200 <br />Copper 21 2.07 4.9-90 <br />Lead 17 1.80 5.2-55 <br />Mercury 0.046 2.33 0.0085-0.25 <br />Molybdenum 0.85 2.17 0.18-4.0 <br />Nickel 15 2.10 3.4-66 <br />Selenium 0.23 2.43 0.039-1.4 <br />Uranium 2.5 1.45 1.2-5.3 <br />Vanadium 70 1.95 18-270 <br />Zinc 55 1. 79 17-180 <br /> <br />30 <br />