Electrical conductivity as a tracer for the migration of lime neutralization sludge in waste rock at the Fire Road Mine, NB

Abstract

Acid Mine Drainage (AMD) is commonly an unfavorable consequence of strip mining in sulfide rich bedrock. Abandoned strip mining operations, such as the Fire Road Coal Mine in Minto New Brunswick, often require lime treatment facilities to neutralize acidic waters originating in abandoned cuts. Lime treatment produces a metal hydroxide precipitate, lime neutralization sludge, which is collected and stored indefinitely in settling ponds. Since 1992, NB Coal has been attempting to reduce AMD production at the Fire Road Mine (and simultaneously eliminate the need for new settling ponds) by pumping sludge from settling ponds back onto the backfilled cut. It was anticipated that the sludge would plug the void space within the sulphide-bearing waste rock backfill, thereby reducing the flux of atmospheric oxygen into the waste rock and subsequent production of AMD. To date, however, there has been no method short of trenching to determine where sludge migrates and ultimately resides after it infiltrates beneath the surface. In 2004, results of a mine-wide EM 31 apparent conductivity survey suggested that the subsurface migration of the sludge material could be identified. It was speculated that elevated apparent conductivities observed in the immediate vicinity of application sites were indicative of sludge material residing within the void space above the water table. Information on the conductivity of lime neutralization sludge was required to further assess this interpretation. This report focuses on the design and interpretation of laboratory column experiments carried out to determine the electrical conductivity of lime neutralization sludge and to investigate variations in sludge conductivity resulting from changes in moisture content. Column experiments were devised to represent conditions below the water table, at the water table surface, and above the water table. Columns consisted of transparent three inch diameter PVC pipe equipped with potential electrodes along their length and current electrode disks at either end. Initial trials failed due to the formation of cracks in the plane containing the potential electrodes causing partial or complete loss of contact between sludge material and the potential electrodes. Columns were reconstructed with removable electrodes eliminating the plane prone to crack formation. Crack formation was still observed but in an orientation that permitted sufficient contact between the sludge and the potential electrodes. Although electrode contact was achievable, crack formation resulted in non-uniform cross sectional areas and disrupted uniform current flow necessary for mathematical interpretation. Removal of sludge from columns was necessary to obtain reliable measurements towards the end of the experiment. Results obtained from the laboratory experiments indicate lime neutralization sludge at Fire Road Mine has a conductivity of approximately 240 mS/m, similar to that of the mine water. Sludge conductivity decreased as moisture content dropped in response to both evaporative drying and draining of the columns. The laboratory findings help to constrain interpretation of EM field surveys by confirming that the presence of sludge in the vadose zone would have a large effect on field measurements of apparent conductivity. In contrast, the presence of sludge below the water table (where it displaces water of similar conductivity) may not be readily detected. The results support the idea that EM field surveys can be used to infer patterns of sludge distribution and migration within the vadose zone, and may therefore help manage future programs of sludge deposition on the waste rock pile.