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. ~ OTHER WATER QUALITY ASPECTS (Continued) c.o N A In the lower reaches of the Colorado River aquatic plant growths have been associated with fertilization by nutrients discharged to irri- gation drains. A small increase in the nutrient levels in the river has been attributed to heavy recreational activities along the river below Davis Dam. 7. Bacteria The coliform group of bacteria is used as an indicator of pollu- tion. This group is made up of bacteria of diverse origin including that found in the intestinal tract of humans and other warm-blooded animals as well as in the soil and on vegetation. High coliform counts in waters indicate the probable presence of pathogenic organisms where bacterial contamination from sewage or animal wastes appears likely. This, however, is only an indicator. In more recent years, analytical procedures have been developed whereby coliform bacteria of fecal origin can be identified. Fecal coliform tests measure bacteria from both man and animal. All the States of the basin have set standards for fecal coliform as the bacterial indicator of pollution. . High bacterial counts were observed at many locations in the Colo- rado River Basin during the 1966 water quality study. A number of these resulted from raw sewage discharges into a stream and some was because of poor disinfection of the municipal wastewater treatment plant effluents. The raw sewage discharges which were observed during the 1966 survey have been corrected by the addition of ponding or other treatment. Bacteriological pollution has also been observed in popular recrea- tion areas. For example, the fecal coliform densities in Lake Mead have been observed at densities higher than the standards set for body con- tact recreation. Bacteriological pollution has an effect on most of the uses cited earlier. In those cases where it exceeds the criteria set for body contact recreation, it results in the closure of swimming areas. With high coliform counts, the use of water as a public water supply could be impaired. 8. Radioactivity An assessment of the radioactivity in the basin waters should also consider strontium 90 (Sr-90) radionuclides associated with atmospheric fallout ,in addition to radionuclides associated with industrial activi- ties. Strontium 90, like the radionuclide Ra-226, is damaging to human bone cells. The effects of Ra-226 and Sr-90 are additive. . 105