BARRIER LEAKAGE AND FLUID BALANCE MANAGEMENT IN FLOODED BELOW-DRAINAGE MINES (Invited Presentation)

Large contiguous below-drainage underground mines have closed and flooded in the Pittsburgh coalbed in the last fifty years. Post-closure, there is potential for discharge to streams and aquifers, and seepage into adjacent operating mines. We examined a case history from West Virginia of late-stage flooding and fully flooded conditions, including spatial and temporal barrier leakage, barrier properties, flooding history, and fluid budget. This flooded mine complex is characteristic of underground mining and closure practices prior to enactment of environmental monitoring standards in 1977.

The Fairmont WV complex has eight contiguous underground mines separated by barriers of varying thickness. Barriers restrict water flow between adjacent mines. Flooding history and fluid balance were reconstructed from water level measurements, barrier dimensions, seepage calculations, pumping records and precipitation data. Seepage calculations included inflow and outflow for the entire complex and transfer among mines for late-stage active flooding and post flooding steady state conditions. A control water level was established to prevent discharge to surface waters. Mines closed at various dates over a 35-year period and cover about 21,000 Ha.

Infiltration for the entire complex is about 11 m³/minute. During late-stage flooding, extractive pumping was the largest fluid budget component, followed by barrier leakage, with some water entering storage. When flooded to control elevation, steady state conditions were maintained by approximately equal amounts of barrier leakage outflow and pumping extraction.

Barrier properties strongly affected hydraulic head and rate of head change during flooding, Intact barriers maintained hydraulic head differences between adjacent mines. If a barrier was breached between mines, water levels were very similar across the adjoining mines. Post flooding, barrier properties control steady state head and seepage transfer among underground mines and surrounding strata. A combination of pumping and barrier leakage is maintaining approximate steady state conditions and preventing uncontrolled discharges from the Fairmont complex. Understanding barrier properties is a key element of post-flooding mine water control.