Decommissioning of South Bay Mine using ecological engineering.

Abstract

Decommissioning acid generating mining waste generally requires perpetual lime treatment. South Bay Mine, a copper/zinc operation active between 1971 and 1981, generated 0.75 million tonnes of tailings with a pyrite content of 41 YO and a pyrrhotite content of 4 %. It is located 85 km northeast of Ear Falls in northwestern Ontario. Acid generation, based on the sulphur content, is expected to continue for a minimum of 1,110 years and a maximum of 35,742 years. Oxidation rates in the tailings range from 76,000 mol Fe a-’ to 32,452,000 mol Fe a-I. Perpetual lime treatment, although environmentally and economically unattractive, has remained the conventional approach. The option of using Ecological Engineering for the decommissioning of the site was assessed through a feasibility study in 1986. Ecological Engineering uses ecological principles to reconstruct ecosystems within the waste management area. These ecosystems, through their natural water cleansing capacity, transfer the annual contaminant loadings from water to sediments. Hydrogeological studies identified the main ground water flow paths from the tailings to Confederation Lake, and from the minelmill site to Boomerang Lake. Ground water plumes and seepage paths were intercepted with diversion ditches, constructed on both the mine site and from the tailings, and directed to Boomerang Lake. Boomerang Lake was relegated to become the main polishing pond or treatment area for water from the mine site and the southern and western ground water plumes from the tailings. Decant Pond, on the tailings, also became a biological polishing pond. Natural contaminant removal processes used in Ecological Engineering includes biological polishing for zinc and iron. Attached periphyton on brush and other substrates adsorb, co-precipitate, and sequester metals. ARUM (Acid Reduction Using Microbiology) is used for removal of sulphate and acidity. Microbial communities in chemically reducing sediments facilitate this process. ARUM was initiated in Decant Pond in 1992, as both a physical and reducing (redox) barrier on the western beach, the entry point for acidic run-off and contaminated ground water. This process has been quantified for scale-up, through continued joint R&D, supported by CANMET, MEND and other mining companies. Biological polishing was scaled up gradually, by annually increasing the surface area for periphyton growth. In Boomerang Lake this was done through additions of brush cuttings, whereas in Decant Pond, inert demolishing material was used to provide surface area for algal growth. In Mill Pond, the basin which contributes the largest contaminant loadings to Boomerang Lake, organic material was introduced, thereby assisting metal adsorption, ARUM and Biological polishing and overall reducing loadings to Boomerang Lake. From the Biological polishing studies, the contaminant removal rates and estimates of the required surface area for periphyton growth were determined. If a substrate mass with a surface area three times that of the lake bottom was added, biological polishing alone could remove 40 % of the annual loading of zinc, and 100 % of the iron. In Mill Pond, 100 % + of the annual loadings of zinc and iron could be retained, thereby reducing the overall contaminant load to Boomerang Lake. Ecological Engineering measures were implemented gradually and all measures taken at the site have resulted in a average zinc concentration of 7.6 mg L-' in 1992. Using monitoring data for Boomerang Lake, extrapolations indicate that, had no Ecological Engineering measures been implemented, zinc concentrations in the lake would likely have risen well above current levels. Performance characteristics of the Biological polishing system are derived from the growth data, obtained in the field and the laboratory. The ranges in expected performance are large, mainly due to the limitations of methodologies for determining growth rates. In field experiments, growth can only be calculated using linear interpolation between two biomass measurements, which does not represent natural growth patterns. Furthermore, biomass which had accumulated on branches up until the time of sampling does not include that biomass which had sloughed off over the period since the last sampling time. While laboratory experiments examining periphyton growth demonstrated logarithmic growth, it was also apparent from these experiments that the waste water chemistry changes in the experimental vessel. Therefore, there are limitations during projection of laboratory-derived growth rates to arrive at estimates of Biological polishing performance in the field. Decant Pond water quality is variable due to seasonal changes in run-off flow volumes. During periods of exceptionally high precipitation in spring and fall, elevated zinc concentrations are present. The monitoring data suggest that periphyton growing in Decant Pond effectively remove the zinc loading during the summer growing season. Long-term trends in water quality measured in Boomerang Lake suggest that acidity and sulphate have steadily increased. Sedimentation studies indicate that iron, precipitated as solids and settled to the sediment surface, is periodically re-suspended in the lake. In 1992, work addressing the residual contaminant loading in Boomerang Lake, Mill Pond, and Decant Pond was initiated. Processes, including ARUM and Biological polishing, are capable of removing the annual loadings, but cannot remove the entire contaminant loading which has accumulated during the estimated residence time of 3 years in Boomerang Lake. Phosphate rock consumes acidity and precipitates metals. Any remaining (excess) dissolved phosphate, a major plant nutrient, is consumed by the periphyton population. Experimental trials using different grades of this material were first performed in the laboratory, followed by field trials. When phosphate sand (750 kg) was applied to Mill Pond, significant amounts iron and aluminum were precipitated. Phosphate powder (9 tonnes) was applied to areas around the northwest end of Boomerang Lake. While iron and aluminum concentrations in the surface waters were unaffected, concentrations of these elements decreased in bottom water overlying the sediments. The concentrations of metal precipitates increased in these sediments. Based on the increases in metals in the sediments following phosphate rock application, it was estimated that 6 % of the zinc, 74 % of the iron, and 10 % of the aluminum in the lake water were relegated to the sediment. In 1992, five tonnes of coarse phosphate rock was applied to an area of the tailings where AMD was ponding due to the high water levels. In water leaving the area treated with phosphate rock, iron concentrations decreased from 53.5 to 12.9 mg L-' and aluminum decreased from 34.3 to 5.2 mg L-', immediately following phosphate rock application. Full implementation of Ecological Engineering measures has not yet been completed at the site. However, the extensive site-specific data set, and the progress made in technology development, facilitated an overall mass balance of annual contaminant loadings to Boomerang Lake and contaminant removal processes (Biological polishing and ARUM). The annual loadings of zinc, iron, sulphur, and hydrogen ions to Boomerang Lake are 3.9, 2.7, 16, and 0.1 tonnes, respectively. The estimated annual zinc and iron removal capacity, when full implementation of the Biological polishing ecosystem in Boomerang Lake is complete, is 5 tonnes and 2.7 tonnes, respectively. The estimated annual removal capacity of ARUM (in Boomerang Lake sediments), based on sulphate reduction rates and alkalinity generation measured in other field systems, is 15.7 t of sulphur and 0.09 t of hydrogen ions. Ecological Engineering measures for the South Bay site have targeted contaminant generation by the tailings deposit, surface water loadings to Decant Pond, and surface water loadings by the mill site. The potential of the underground workings to produce seepages draining to Confederation Lake during years with high run-off was not addressed until 1992. A large diversion ditch was completed in January 1993. Its performance and the effects on Boomerang Lake will be monitored in 1993. Work addressing ARUM activity in Boomerang Lake sediments and on the tailings beach of Decant Pond will continue. The accumulated contaminant load in Boomerang Lake, Mill Pond and Decant Pond will be addressed using phosphate rock.