Mud Lake report, South Bay Project

Abstract

As a part of 1993 scope of work, the hydrogeological conditions of the tailing site were re-evaluated. It was concluded that some groundwater is not reporting as expected to the diversion ditch and, therefore, may be leaving the tailings drainage basin in the N-W direction towards the Mud Lake. Mud Lake had been monitored outside the regulatory requirements intermittently since 1988. Generally, the pH was around 7.5, the conductivity of 430 pS.cm-‘ and the water free of contaminants. Given the disparity in the water balance for the tailings drainage basin, in March of 1994 water samples were collected in Mud Lake through the ice. The pH had dropped to 5.3 and the conductivity had risen to 1,430 pS.cm” (Table 1). No surface flow of contaminated water has been located. These results confirmed that contaminated groundwater was entering Mud Lake. Upon completion of spring run-off, the dissolved iron in Mud Lake water had oxidized and hydrolysed, and the pH of Mud Lake dropped to pH 3.1 by April, 1994. In August, 1994, the pH was 2.7. The data collected during the March 1994 sampling campaign clearly indicated water quality problems in Mud Lake. A second field trip was organized in April, 1994. Top and bottom water samples were taken in Mud Lake. The data indicated that the entire water body was contaminated with zinc. The conductivity of bottom water samples were generally higher than surface water samples. Given that Mud Lake is shallow, and well mixed by wind-driven circulation during the ice-free season, this suggested that the seepages are emerging from the bottom of the lake through the sediments. This report documents the background information on Mud Lake and the investigations performed in 1994 so to gain an overall understanding of the seepage path. This site specific information is then placed in the broader context of the decommissioning objectives for the South Bay Mine site. Ecological Engineering, a low-maintenance approach to close-out, was chosen for this site, as the tailings represents an environmental liability for a very long period. Since the results from ongoing research and the gradual scale-up were promising, the property has been returned to the Crown with the provision that no contaminated seepages discharge into Confederation Lake. In Section 2 of this report, the historic data of the Mud Lake drainage basin are summarized, and contaminant loadings from the basin are estimated. In Section 3, the seepage pathways are defined through preliminary geophysical surveys and test drilling. In Section 4, the various approaches to achieving the overall objective are outlined to provide a framework for selecting the implementation steps for the decommissioning approach. The rationale for the decommissioning approach will be complete upon, first, review of these approaches; second, selection of the final approach; third, development of an action plan for the steps towards implementation.