Development of a pit-lake and fate of contaminants 1992-2001

Abstract

The B-Zone pit was intentionally flooded with water from the adjacent Collins Bay in the winter of 1991/1992. Evidence that contaminants are not being released from the pit walls to the water column and the processes responsible for improvements observed in pit water quality are described. Limnological data gathered for a decade support this finding. Since 1991, the water quality in the pit lake has been rigorously and regularly monitored with close attention paid to the primary productivity (phytoplankton quantification) of the flooded pit. An elemental mass balance has been calculated, based upon materials acquired by suspending sedimentation traps at various depths, and bottom sediment sampling. By 1997, this was indicating that natural cleansing processes were effectively reducing both Ni and As, the contaminants of concern, in the water. The striking singularity in the data showed that concentrations of Ni increased every spring after ice break-up. It was initially believed that Ni was being released by the decomposition of suspended organic matter and recycling. However, by 1998 the dynamics of the lake were well understood and it seemed apparent that the only explanation could be an external source of Ni. That source, a mineralized perimeter road, was identified and in 2001 was eliminated. Since then, the concentrations of Ni in the water column have no longer increased seasonally. The steady state of contaminants in the lake water, and its sediments, provides assurance that no new contaminants are entering the pit. Active contaminant removal by pumping of water from the pit lake to maintain water level was also evaluated. This provided evidence against one of the last arguments that dilution from clean water entering the pit lake was leading to the reductions in As and Ni concentrations and not the natural water cleansing processes. If necessary, these processes could be enhanced further by fertilization. A final decommissioning strategy for the B-Zone Pit has not yet been determined.