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<br />- 21 - <br /> <br />Partial causes for this may be inflow of the Dolores River at the confluence <br />of the Dolores and San Miguel Rivers and seepage from mines along the Dolores <br />River, both above and below Gateway; and in Cycle II, natural runoff during <br />the period of rain. Results have not yet been reported from sediment samples <br />taken recently in the stretch of the Dolores River below Gateway to clarify <br />the second hypothesis raised above. <br /> <br />From the foregoing discussion, it is apparent that several questions <br />unsolved by this survey will not be answered without further work. <br /> <br />Table XI presents data on radioactivity -in sediments from sampling point <br />located upstream of man-made sources of contamination (8). These data indicate <br />generally the level of radioactivity to be found in naturally occurring Upper <br />Oolorado River Basin sediments. Table XII presents similar data obtained in <br />the survey area for comparison. Eighteen of 20 samples in this survey area <br />contained higher concentrations of radium-226 than the average radium-226 <br />concentrations in the background samples, i.e., l.3 ~g/gm of dry sediment. <br /> <br />TABLE XI <br />BACKGROUND LEVELS OF RADIOACTIVITY IN RIVER SEDIMENTS(a) <br /> <br /> Location Gross a Gross fl Radium-226 <br />Colorado River at Silt, Colorado 6.5 78 1.7 <br />Yampa River at Juniper Springs, Colo. 4.0 50 <1.0 <br />Green River near Dutch JohTh Utah 8.6 45 <1.0 <br />San Juan River above Farmington, N.M. 5.3 2l 0.9 <br />Animas River above Durango, Colorado 11 29 1.3 <br />Dolores River above Slick Rock, Colorado l4 43 2.2 <br />San Miguel River above Naturita, Colorado 9.5 38 1.0 <br />Tomichi Creek above Gunnison, Colorado 11 47 1.2 <br />Gunnison River above Tomichi Creek 9.0 52 1.6 <br /> <br />(a)All analyses in ~~c/gm of dry sediment <br /> <br />As would be expected, the data show an increase in radioactivity immedi- <br />ately below uranium mills. Gross alpha and radium-226 concentrations rose <br />gradually as the Dolores River passed from above to below Burro Mine to below <br />the Slick Rock upgrading plant. Levels of these constituents just below Slick <br />Rock were one and one-quarter to two times as high as those just above Slick <br />Rock. Radioactivity levels in sediment taken just above Uravan were better <br />than twice as high as the average corresponding levels in the background samples, <br />reflecting the presence of a source of radioactivity upstream from Uravan. <br />