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PART IV COLORADO RIVER WATER QUALITY ASSESSMENT e State standards are concerned, but the program has helped identify some areas of concern. There is some concern about secondary drinking water quality criteria. The Colorado main stem is high in dissolved salts, particularly sulfates. At Parker Dam, average concentrations of sulfates and TDS exceed the U.S. Public Health Service drinking water recommendations. The threat of trihalomethane production when Colorado River water is chlorinated may require changes in the treatment proc- ess for the drinking water systems within Arizona, which plan to utilize Colorado River water. Another area of concern is the high levels of fecal coliform bacteria found in some areas along the river. The high bacte- rial levels were probably caused by high river flows which caused the overflow of septic tanks. Although this situation may only be temporary, there is concern that sewage disposal problems will increase as development and recreation increase. E. Nevada e The Colorado River met water quality standards, provided for pro- tection and propagation of fish and wildllfe, and allowed recreational activities in and on the water. The high water level in Lake Mead con- tinues to contribute to improved water quality by diluting the high pollutant loads entering Lake Mead via Las Vegas Wash. Since July 1981, the municipal dischargers in the Las Vegas area have installed chemi- c~l addition, reducing the phosphate load to the wash by 90 percent. Attainment of this phosphate reduction has resulted in the associated reduction of biochemical oxygen demand (BOD) and suspended solids loading from these municipal sources to the wash and the lake. Monitor- lng conducted during 1983 indicated high concentration of chlorophyll-a in the inner Las Vegas Bay despite cutbacks in the phosphorus loading. The Virgin River, a tributary to Lake Mead, exhibited poor water quality in terms of bacteria, esthetics, and solids, and very minimal fish life. Phosphorus is of concern, chiefly, with respect to present and future domestic use and, secondarily, with respect to recreation and fisheries. Studies have indicated that in-lake concentrations greater than 0.013 mg/L as phosphorus will produce algae concentrations which will have adverse. effects on recreation, whereas other studies have im- plied that more phosphorus is necessary for fisheries. In view of this, Nevada adopted and implemented the requirements of no more than 1 mg/L as phosphorus for all point sources. The major point sources are the three large municipal facilities along the Las Vegas Wash. e TDS, hardness, sulfates, and chlorides are of concern with respect to domestic use of water from Lake Mead. The present levels appear to be accepted by the public, although an economic impact is felt as a 20