Decommissioning of South Bay: phosphate rock/brush application in Boomerang Lake; EM34 background Mud Lake survey; seepage Canyon definiton: 1995 final report / submitted to Mr. David Porter (Talisman Energy Inc.)

Abstract

In 1994, the decommissioning activities for the South Bay mine site focused on two main areas. For the overall site the measures taken in 1993 were evaluated (trial phosphate rock application to tailings and Boomerang Lake sediment and ARUM beach installation on Decant Pond). The biological polishing process used in Boomerang Lake was summarized, focussing on contaminant fate.The mine/mill site was surveyed using EM to determine changes brought about by the diversion of the seepage from the Back fill Raise area to Boomerang Lake. A report was issued in December1994 which concluded that the measures for Decant Pond had been successfully curtailing the deterioration of the water quality. The seepage from the mine/mill site have been diverted away from Confederation Lake. However, the contaminant load to Boomerang lake has increased. It was recommended that application of phosphate rock to the lake sediments is required to curtail acidification due to iron oxidation. Due to logistics problems encountered in 1993, the phosphate rock application to Boomerang lake was carried out in 1994. Eighty tonnes of phosphate rock were made into a slurry and applied via barge to the lake sediment in 1994. The tailings have given rise to a deterioration of the water quality in Mud Lake, which was predicted based on the reevaluation of the hydrological conditions in the tailings basin in 1993. Insufficient quantities of water were reporting to the ground water diversion ditch at the southern end of the tailings deposits. In early 1994, the deterioration of Mud lake was documented. Subsequently, activities in 1995 centred around the hydrology of the tailings and drainage basin. The hydrology of the tailings basin was originally defined in 1987, excluding the area around Mud Lake. During 1994, the sampling efforts in and around Mud Lake were intensive in order to define the origin and the pathways of the contaminants moving towards Mud Lake. A report was issued summarizing the historic and current status of the area. Based on a preliminary hydrological investigation focusing on the area between the tailings and Mud Lake, a deep bedrock valley was described, which appeared to define the seepage path underneath Mud Lake, leading to the discharge of the seepage at the north end of Mud Lake. Many hydrological questions were raised by the investigations of 1994, the first year when Mud Lake was contaminated. Thus, to answer key questions, extensive EM surveys were carried out in 1995 to define the contaminant pathway and bedrock conditions. This information wasconsidered essential to any remedial action plan. After completion of the 1994 investigation, it was possible to estimate, utilizing elemental concentrations of surface water in the drainage basin along with flows and precipitation data, a contaminated ground water discharge volume of 0.6 to 1.0 L/s. This volume of contaminated seepage was considered significant, leading to potential contamination of lakes further north. The 1995 work therefore concentrated on defining possible pathways of contaminants to lakes further north of MudLake. The hydrological balance (precipitation/ infiltration/ evaporation) produced an expected surface waterflow volume for the drainage basin containing the tailings and Mud lake. These estimates agreed very well with the measured flow volumes leaving Mud Lake, the ground waterseepage discharge and flows leaving Decant Pond. These results suggested that it might be possible to contain the contaminants in Mud Lake, if the drainage basin provides a solid containment structure. Extensive coverage of the site by EM measurements in the areas surrounding Mud Lake was implemented in early 1995, followed by a hydrogeological investigation where piezometers were installed to define the seepage characteristics. The bedrock topography of the tailings needed to be refined, as containment of the contaminants within the drainage basin could also be assisted through identification of the location where the ground water seepage was leaving the tailings deposit. If such a location could be defined, many alternatives would arise in selecting options to contain contaminants. Therefore, the activities in 1995 focused heavily on collecting hydrological information addressing the spatial distribution of the seepage path and locating the origin of the seepage from within the tailings basin. A summary of the key findings of the hydrology and bedrock definitions is given in Section 2. Although water balance calculations for the Mud Lake/tailings drainage basin agreed with the relatively sporadic flow measurements and the mass balance using conservative elements of in the surface water, the aerial extent of a plume which would be moving below ground out of the drainage basin was not known. In Section 3, electromagnetic survey data are presented which are used to define the extent of the plume in the Mud Lake drainage basin. The EM survey can also be used as a monitoring tool to determine movement of the plume through repeat surveys in subsequent years. From the hydrological understanding, based on the data collected in 1987, it was expected that surface and ground water would leave the tailings basin via Decant Pond outflow and passing through the muskeg between the tailings and Mud Lake. Zinc removal was effective to date with extensive algal growth. Surface water leaving Decant Pond could only improve further upon passage through the muskeg. Contaminants leaving the tailings basin via the ground water was evaluated in 1987 and 1988. Piezometer water quality was used, together with modelling of natural contaminant attenuation, to determine potential problem areas of Confederation lake. The assessment indicated that sufficient attenuation will take place and contaminants would not reach Confederation Lake. At the time this assessment of a reasonable evaluation had to be reconsidered, given the findings in 1994. The 1994 hydrological investigation found the depth of the seepage path at16 m, discharging directly into the north end of Mud Lake with a water quality which suggested no natural attenuation of contaminants. A reassessment of the status of the ground water chemistry, collected sporadically since 1987, was carried out along with an evaluation of seasonal changes in the surface waters, monitored on a more regular basis. The objective of these evaluations, presented in Section 4, was to determine if the ground waterand surface water conditions of the entire drainage basin had undergone significant changes which warranted attention.