Geochemical and hydrogeologic evolution of alkaline discharges from abandoned coal mines
Numerous large flow (> 2,000 l/min), historically (pre-1973) acidic, abandoned underground deep mine discharges in southwestern Pennsylvania are now alkaline in character, with circumneutral pH. Recently measured flow rates are consistent with those measured 25--30 years ago; thus the change in chemistry is not simply due to dilution by increased flows of uncontaminated water through the mines. It is likely that flooding of the mines has decreased the extent of acidity enhancing aerobic conditions, and that decades of weathering have reduced the amount of reactive pyrite. However, the mines continue to yield a sulfate-rich, Fe-contaminated (19--79 ppm) drainage. These highly alkaline discharges (up to 330 ppm as CaCO{sub 3}) are accompanied by large concentrations of sodium (up to 700 ppm) and suggest cation exchange with the associated overburden. To assess the hydrogeological conditions that result in the formation of alkaline Fe-contaminated mine discharges, the authors examined all the major discharges from a single synclinal basin. The northeast-trending Irwin synclinal coal basin encompasses 94 mi{sup 2} and was extensively mined by underground methods during the first half of this century. All major streams that arise within or cross the syncline are polluted by mine drainage that ranges from highly acidic Fe- and Al-contaminated discharges in the northern portion of the syncline to highly alkaline, iron and sulfate-contaminated discharges to the south. The hydrology of the basin is controlled by its southern plunging structure, by outcrops or drainage tunnels on the western arms of the syncline, and by several coal barriers. A first-order hydrogeologic model was constructed to evaluate ground water flow into and through the mine complexes found in the basin. The model integrates the basin geometry with structural and mine barrier components to determine groundwater flow paths and estimate residence time. Water quality is related to the cumulative proportion of up-gradient flooded and unflooded mine workings. Small discharges from unflooded, gravity-flow portions of the mined-out portion of the Pittsburgh Coal seam are highly acidic, and large artesian flows of water affected by only a short flow through flooded anoxic mine pools are moderately acidic. Those discharges subjected to increased residence time in flooded anoxic portions of the mines are increasingly alkaline. Refinement of this model could aid in prediction and hydrogeologic manipulation of these high flow Fe-contaminated discharges that are the main pollutant in many streams throughout Northern Appalachia and other coal mining areas throughout the world.
- Research Organization:
- Univ. of Pittsburgh, PA (US)
- OSTI ID:
- 20082232
- Country of Publication:
- United States
- Language:
- English
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