COAL MINE SUBSIDENCE AND STREAM CAPTURE PREDICTION USING GIS

Areas undermined for coal using room-and-pillar methods with shallow (<50 ft) overburden are at high risk for subsidence or stream capture into fractures. In cases where surface water is captured into high-sulfur coal mines, water becomes acidified and emerges from nearby or distant mine seeps as acid mine drainage. Source control of mine drainage consists in part, by identifying and closing stream capture features. In cases where the capture is partial, or is under small or ephemeral streams, the capture zones are difficult to identify using field methods. Streams also may be at high risk for capture, but weathering and collapse of support pillars may not yet have occurred. In such cases, identification of high-risk zones is imperative to avoid situating treatment wetlands or retention structures in areas prone to subsidence. A predictive model was developed to map potential capture or subsidence, assuming that overburden thickness is the primary predictor. Using a GIS (ArcInfo, ArcView), surface topography was mapped for the Monday Creek watershed using USGS Digital Elevation Model (DEM) data. The floor of the Middle Kittanning No. 6 coal was represented by kriging 1137 floor elevations obtained from mine maps. Subtracting floor elevations from topographic elevations resulted in a contour map of overburden thickness. This map was intersected with digitized mine extent to create a contour map of subsidence risk, from "no risk" (>200 feet overburden) to "very high risk" (<25 feet overburden). The risk map correlates strongly with observed capture points, and also indicates areas where additional field reconnaissance is needed. The polygonal watershed areas associated with capture points allow quantification of the individual and collective impact of the stream-capture features. Future research will focus on secondary controls on subsidence risk, including age of mines, tectonic fracture orientation relative to mine workings, and overburden lithology.