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- 9 - Flows during the two cycles were lower in each case than the lowest mean discharge for August during the period of record. Flows in cycle II have a wider range than those of cycle I and are 80 to 90 percent of the lowest mean August discharge over the periods of record. August is usually a low flow month in this area as weather is warm and much water is used for irrigation upstream. RESULTS ANJ DISCLSSION Chemica.1 Chemical analyses were performed by the laboratory of the Field Operations Section, R. A. Taft Sanitary Engineering Center, Cincinnati, Ohio. Results are presented for the Slick Rock area in Table V and for the Uravan area, including the Dolores River below the mouth of the San Miguel River, in Table VI. Dolores River - Slick Rock Area Even in this period of low flow, sufficient dilution was provided by the Dolores River to reduce concentrations of some constituents in Burro Mine effluent and Slick Rock plant seepage to values within the limits of the Public Health Service Drinking Water Standards (3). This was true of selenium, arsenic, and iron in the mine effluent. According to present knowledge, 'selenium is toxic both to humans and some animals in low concentrations; in the Drinking Water Standards, a concentration of 0.01 mg/1 is grounds for mandatory rejection of the source for public water supply. Selenium in the stream was below limits at all river sampling stations, but there is a danger of its being present in unsafe concentra- tions for direct consumption at low flows if there should be any large increase in concentration from either effluent. It may be pointed out that the probability of direct human consumption at such times is remote. There was an increase in average nitrate concentration from 0.14 to 2.5 ~g/l as the river passed from above Slick Rock to below the upgrading plant. Sulfate and total dissolved solids concentrations were above limits recommended in the Drinking Water Standards at all river stations in the Slick Rock area. Sulfate values ranged from 680 to 3160 mg/l; total dissolved solids concentrations as measured ranged from 1940 to 2240 mg/l (TDS values not reported for all stations). During cycle II, the concentrations of sodium and chloride in the Dolores River rose sharply between Slick Rock and the confluence of the Dolores and the San Miguel Rivers. Below Slick Rock plant, t3ese concen- trations were 400 and 93 mg/l respectively, whereas they were 5800 and 5100 mg/1 just above the entry of the San Miguel River. This rise was even more marked during cycle I. A similar occurrence was noted in a set of grab samples taken by the U. S. Geological Survey in August 1947 (4). Sodium and chloride concentrations then at Slick Rock were 53 and