Abstract
Disposing of lime neutralization sludge from an acid mine water neutralization facility back into the acid generating waste rock could provide several benefits for reclamation of abandoned coal mines. These include: a low cost final disposal area for the sludge; reduced diffusion of oxygen into the waste rock; reduced personal liability by eliminating sludge ponds, and less disturbance of land for sludge disposal purposes. NB Coal had been depositing lime neutralization sludge from its acid mine water treatment plant back onto the waste rock at the backfilled Fire Road strip mine since 1992. Chemical investigations have identified a decrease in the mine water acidity and iron and aluminum concentrations, which may be in part due to the application of the sludge. Electromagnetic (EM) surveys were first used over parts of the Fire Road mine in 2000 to identify subsurface accumulations of acid mine drainage (AMD). One unexpected result was the observation of anomalous electrical conductivities in an area that had previously been used for the disposal of sludge. The possibility of using electrical conductivity as a tracer to track sludge migration within the waste rock motivated NB Coal to sponsor the acquisition of an EM apparent conductivity survey over the entire backfilled pit. Results of that survey, conducted in 2004, show that distribution of electrical conductivity over the mine is highly variable. A long, linear conductivity high, located along the high wall of the mine, is attributed to pooling of mine water and higher porosities (higher water contents) in that zone. Other conductivity highs, however, are clearly associated with historical patterns of sludge application and its subsurface migration. The presence of moist, conductive sludge filling a portion (or all) of the void space in the waste rock above the water table may explain this association. If so, then apparent conductivity maps may be useful as a management tool to decide which parts of the mine site would benefit most from the application of sludge for purposes of reducing the infiltration of oxygen and production of AMD.
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