Airborne Electromagnetic Surveys over the Kempton Mine Complex

As a legacy of past mining operations, approximately 450 abandoned mines underlie western Maryland. A portion of these mines have been responsible for land surface subsidence, acid mine discharge (AMD), and flooded mine tunnel blowouts. AMD is produced by a complex series of chemical weathering reactions that are spontaneously initiated when mine spoil material is exposed to oxygenated water typical of rainfall and stream infiltration. Until oxidizing conditions are reversed through environmental restoration efforts, these reactions will continue to release damaging quantities of acidity, metals, and other soluble components into the environment. After abandonment, surface water infiltration may gradually fill and flood portions of the mine tunnel network. For the mine in the project area, the flooded tunnel network, or “mine pool,” is estimated to contain more than one billion gallons of water. The hydraulic stress due to the mine pool may exceed the strength of the surrounding rock, resulting in tunnel blowouts and sudden releases of acidic mine water.

To characterize AMD and flooded mine tunnels, a helicopter electromagnetic survey was conducted over the 31 square kilometers of the Kempton/Coketon abandoned mine complex straddling the Maryland/West Virginia border. The electromagnetic survey measured the electrical conductivity contrasts in vertical profiles at closely spaced horizontal coordinates along each of the 124 east–west survey lines, resulting in more than one million data records. Spatial trends derived from the survey data were correlated with available geologic, topographic, and environmental sampling information. High-conductivity data signatures consistent with large AMD sources were found to correlate with abandoned surface mines positioned along topographic highs at the headwaters of two major watersheds.

Data signatures strongly suggest that AMD infiltration from the surface mines is flowing down gradient toward the flooded mine tunnel network. Although the flooded mine tunnels were found to be beyond the maximum depth of exploration given the survey methods and site conditions, problem AMD source areas were clearly identified for prioritizing future mine restoration efforts. Coupled with recent improvements in equipment and sensors, airborne electromagnetic surveys are an increasingly valuable tool for large-scale mine characterization and environmental restoration studies.