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- 16 - uranium ranged from 1.1:1 to 1.7:1. As discussed before, rain fell in the survey area at the beginning of cycle II. Stream flows rose as a consequence of the rain and the turbidity of the rivers also increased. The increase in turbidity at anyone point was due partly to natural runoff and partly to resuspension of sedi- ment which had settled on the stream bed above that point. It is possible that radium-226 and uranium from upstream sources of radio- active contamination were redissolved during the period of high streamflow after having been previously associated with settled sediment in the upstream channel. This could have occurred in the following manner. During the previous low flow period, radium-226 and uranium from upstream sources which were .associated with the suspended sediment in the river settled to the stream bed. When the stream flow rose rapidly, as was the case during the survey, the greater turbulence of swiftly moving water scoured the settled sediment from the channel bottom. There 'was much more intimate contact between the mass of resuspended sediment, its associated radioactivity, and the liquid phase, than would be the case had there been no increase in stream flow. This more thorough cont.actbetween solids and water increased the chances for dissolution of the previously settled radium-226 and uranium, resulting in a corresponding increase of concentrations of these constituents in the stream. This phenomenon has been reported in other similar studies (1) (11) . Burro Mine effluent contained an arithmetic average of 47 ~~g/l of radium- 226 and raised the concentration of radium-226 in the Dolores River at its entry .by at least .a factor of si,,". The average radium-226 concent.rationin the Dolores River above the mine was < 0.55 ~~g/l; below the mine but above Slick Rock plant, it was 3.3 ~~g/l.Onthe-other hand, radium-226 levels in the Slick Rock plant seepage were essentially the same as in the Dolores River below the plant, just less than 1 ~~g/l. The level of activity in the plant seepage may be accounted for by. two factors: (l)In-plant leaching is carried out on the II sands II fraction of ore particles only. Since only a portion of the ore is leached, radium-226 .'levels in the effluent to the tailings pile would be lower than if all ore fed to the plant had been leached; (2) There is apparently a reductioninradiuIIl-226 cori- tent of. the..mill efflUent liquor due to an ion exchange or similar mechanism occurring in the soil through which the liquor passes before reaching the river as seepale. Handbook 69 of the National Bureau of Standards (6) gives the maximum 168 hour ~ccupational exposure limits to radium-226 and uranium in water as 10-7 and 2xlO- IJ.c/cc respectively. The InternationalCollllIlission on Radiological Pro- tection recommends (7) a maximum of 1/30 of the continuous occupational exposure; or 3.3 IJ.lJ.g/l for radium-226 and 20,000 ~g/l for uranium for lifetime consumption ' by the general population. In the U. S. Public Health Service Drinking Water Standards, an annual average of greater than 3.0 ~lJ.g/l of dissolved radium-226 in water constitutes ground for rejection of the supply as a source of drinking water. At the tow flows encountered during this survey, concentrations of radium-' 226 in the Dolores River were at or above these levels in three locations in the survey area; below Burro Mine (average 'of 3.3 ~~g/l), above the confluence of the San Miguel River (3.0 and 3.8 ~lJ.g/l), and at Gateway (arithmetic average of 5.5 IJ.lJ.g/l). The effects of radium-226 levels above 3.0 IJ.lJ.g/l have not been evaluated for the inhabitants of Slick Rock who may use Dolores River water for domestic purposes. Neither have the effects caused by irrigation of crops in this area