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- 29 - SUMMARY AND CONCLUSIONS l. The larger portion of flow in the Dolores~San Migu~l River system during this survey came from the San Miguel River. Flows during the second half of the survey, pa~ticularly in the Dolores River, were widely variable due to an intense loca~ rain. The survey period was generally one of low flow when compared to the period of record. 2. Concentrations of sulfate and total dissolved solids in all stream samples were above limits recommended in the U. S. Public Health Service Drinking Water Standards. Water at all stream sampling stations was so hard as to require treatment if used for most domestic and some industrial uses. 3. A sharp increase in the concentration of sodium and chloride ions was noted in the Dolores River below Slick Rock. This is probably due to natural sources between Slick Rock and the confluence of the Dolores and San Miguel Rivers. The concentrations of both ions in the combined Dolores-San Miguel rose sharply over levels in the San Miguel River above. its mouth. High salt concentrations reportedly interfere with irrigation use of the Dolores River in Colorado during low flow seasons. 4. The sum of weight of dissolved iron passing the sampling point above Uravan and weight of dissolved iron added in Uravan mill discharges far exceeded the weight of dissolved iron found in the San Miguel River below Uravan. This was also true for vanadium ions. It seems likely that precipita- tion of iron and vanadium was occurring to a large extent in the first mile and a half of the San Miguel below Uravan. 5. Rather marked discrepancies exist between the weights of sulfate and radium-226 discharges to the San Miguel River by Uravan mill calculated on the basis of stream flows and those calculated on the basis of effluent flows. 6. Attempts were made to strike material balances around the confluence of the Dolores and San Miguel Rivers. These balances resulted in generally good agreement during the dry weather of the first half of the survey. Agree- ment was generally not good during the half of the survey marked by rain. It was not possible to obtain a good balance for radium-226 around the conflu~nce during either half of the survey, perhaps due to natural inflows or mining seepage between some of the sampling stations. 7. An increase in the concentration of dissolved radium-226 was noted in stream samples during the second half of the survey. This has been noted in other similar stream studies and was probably due to redissolution of radium-226 which had been previously associated with settled river sediments. 8. Concentrations of dissolved radium-226 in stream samples were at or above recommended limits of 3 ~~g/l in the Dolores River below Burro Mine at Slick Rock, in the Dolores River just above the entry of the San Miguel River, in the Dolores River at Gateway, and in the Dolores River just above its mouth.