Mine Water Quality Prediction at Surface Coal Mines in the United States

This paper briefly summarizes the contents of a recently completed book on the subject of predicting water quality at surface coal mines. The book was prepared by a segment of the Acid Drainage Technology Initiative, (ADTI) and is available at nominal or no cost. ADTI is a joint venture of the U.S. government and industry dedicated to the development and use of best science applications to the problem of acid mine drainage. Several significant recommendations are made. First, if adjacent surface mines are geologically similar to the site to be mined, the water quality at such sites provide the best indication of the water quality likely to be encountered at the new mine. Little or no additional overburden sampling may be necessary. Second, when sampling is necessary, an absolute minimum of three, and more likely six or seven holes, are needed per 40 hectares (100 acres) if one hopes to capture the geological variability of the site. Site-specific criteria might dictate that sampling density be increased beyond this range. Third, the effect of weathering must be accounted for during sampling; the top 6 or 7 m of a site is likely to be leached of carbonate minerals and/or pyrite. Fourth, the effectiveness of the most widely used overburden analysis method, acid-base accounting, has been improved by modifying the determination of neutralization potential (NP) to eliminate the effects of siderite and the subjectivity of the fizz test. Fifth, the confidence one should have in the results of such an analysis is high if one has a NP less than 10 or greater than 21 tons per 1000 tons, or a net neutralization potential less than 0 and greater than 12 tons per 1000 tons. The lower range will most likely produce acid drainage, while the higher rage will most likely produce alkaline drainage. For strata lying between these values (in what is conventionally referred to as the gray zone), kinetic tests (humidity cell or leaching tests) might be useful. These values are based on the previous incarnation of acid-base accounting; it is anticipated that as the modified method becomes more widely used, the gray zone will shrink.