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~ . predictable, depending on climate, reservoir morphometry, the type of discharge release, and the distance the releases travel downstream. Coal mlnlng. Overburden on coal deposits in the west are typically low in pyritic minerals and rich in shale and carbonates. These carbonates buffer surface waters and streams that flow through coal mines on the Colorado Plateau may not suffer the dramatic reduction in pH observed in eastern streams (Canton and Ward 19B1). Uranium mining. Because anthropo~enic groundwater pollution exerts long-term effects on water qual,ty, pollution has become a regional concern, particularly where uranium mining is taking place. The Rio Puerco originates in the Chuska and Zuni Mountains in northeastern Arizona and drains a significant portion of the Navajo Indian Reservation, as well as Petrified Forest National Monument before entering the Little Colorado River. Originally an ephemeral stream, the Rio Puerco began receiving perennial effluent from uranium mines in the 1950's. The groundwater quality of the Rio Puerco in the Little Colorado River drainage is generally high in total dissolved solids (ca. 700 mg/L), and is characterized by high alkalinity, sodium, sulfate, iron, manganese and strontium. In July, 1979, a tailings-pond dam at United Nuclear Corporation's Church Rock Mill failed, pouring 3.6 x 108 L (94 million gallons) of radioactive effluent into the Rio Puerco. Although just one of numerous water quality violations in the Rio Puerco drainage in recent years, the Church Rock spill was the largest radioactive contamination accident in the United States' history. However, 2 years after the spill, well water quality analyses in the vicinity of the spill conducted by the U.S. Geological Survey failed to show increased groundwater radioactivity (Webb et al. 19B7b). Movement of contaminants into the local aquifers is undoubtedly slow (Webb et al. 19B7b). Bacteriology. Water quality, including bacteriology, was examined in the Grand Canyon reach from 197B to 19BO (Tunnicliff and Brickler 19B1). They found that the water leaving Glen Canyon Dam was stable in quality and relatively free of enteric contaminants. Although hypolimnetic release kept the water below 180e through the warmest summer months and retarded bacterial growth, various natural and anthropogenic sources of bacterial contamination occurred with increasing distance downstream. They found elevated concentrations of bacteria particularly in fine sediments which were subject to re-entrainment in the water column by discharge fluctuation, wave action and recreational activities. Drinking water quality standards for total coliforms were often exceeded in the mainstream, with up to 13,000 FC/100 ml entering from flooding tributaries, such as the Paria and Little Colorado rivers. Bacterial content in benthic mud at numerous tributary . . . . . . . . 28 .