Effects of coal mine drainage on the water quality of small receiving streams in Washington, 1975-77

Drainage from abandoned coal mines in western and central Washington has minimal environmental impact. Water quality characteristics that have the most significant environmental impact are suspended sediment and turbidity. Water quality data from 51 abandoned coal mines representing 11 major coal bearing areas indicate that less than 1% of the mine drainage has a pH of 4.5 or less. Fifty percent of the drainage is alkaline and has pH 7.0 and greater, and about 95% of the drainage has pH 6.0 and greater. Less than 2% is acidified to a pH of 5.6, a point where water and free (atmospheric) carbon dioxide are in equilibrium. The area where pH 5.6 or less is most likely to occur is in the Centralia/Chehalis mine district. No significant difference in diversity of benthic organisms was found between stations above and below the mine drainage. However, within the 50-ft downstream reach ostracods were more abundant than above the mine drainage and mayflies, stoneflies, and caddisflies were less abundant than at the control site. Correlations to water quality measurements show that these faunal changes are closely associated with iron and sulfate concentrations. (USGS)