Decommissioning of South Bay Mine using ecological engineering.

Decommissioning acid generating mining waste generally requires perpetual lime \ntreatment. South Bay Mine, a copper/zinc operation active between 1971 and 1981, \ngenerated 0.75 million tonnes of tailings with a pyrite content of 41 YO and a pyrrhotite \ncontent of 4 %. It is located 85 km northeast of Ear Falls in northwestern Ontario. Acid \ngeneration, based on the sulphur content, is expected to continue for a minimum of \n1,110 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. \nPerpetual lime treatment, although environmentally and economically unattractive, has \nremained the conventional approach. The option of using Ecological Engineering for \nthe decommissioning of the site was assessed through a feasibility study in 1986. Ecological Engineering uses ecological principles to reconstruct ecosystems within the \nwaste management area. These ecosystems, through their natural water cleansing \ncapacity, transfer the annual contaminant loadings from water to sediments. Hydrogeological studies identified the main ground water flow paths from the tailings \nto Confederation Lake, and from the minelmill site to Boomerang Lake. Ground water \nplumes and seepage paths were intercepted with diversion ditches, constructed on both \nthe mine site and from the tailings, and directed to Boomerang Lake. Boomerang Lake \nwas relegated to become the main polishing pond or treatment area for water from the \nmine site and the southern and western ground water plumes from the tailings. Decant \nPond, on the tailings, also became a biological polishing pond. Natural contaminant removal processes used in Ecological Engineering includes \nbiological polishing for zinc and iron. Attached periphyton on brush and other \nsubstrates adsorb, co-precipitate, and sequester metals. ARUM (Acid Reduction Using \nMicrobiology) is used for removal of sulphate and acidity. Microbial communities in \nchemically reducing sediments facilitate this process. ARUM was initiated in Decant Pond in 1992, as both a physical and reducing (redox) \nbarrier on the western beach, the entry point for acidic run-off and contaminated ground \nwater. This process has been quantified for scale-up, through continued joint R&D, \nsupported by CANMET, MEND and other mining companies. Biological polishing was scaled up gradually, by annually increasing the surface area \nfor periphyton growth. In Boomerang Lake this was done through additions of brush \ncuttings, whereas in Decant Pond, inert demolishing material was used to provide \nsurface area for algal growth. In Mill Pond, the basin which contributes the largest \ncontaminant loadings to Boomerang Lake, organic material was introduced, thereby \nassisting metal adsorption, ARUM and Biological polishing and overall reducing \nloadings to Boomerang Lake. From the Biological polishing studies, the contaminant removal rates and estimates of \nthe required surface area for periphyton growth were determined. If a substrate mass \nwith a surface area three times that of the lake bottom was added, biological polishing \nalone could remove 40 % of the annual loading of zinc, and 100 % of the iron. In Mill \nPond, 100 % + of the annual loadings of zinc and iron could be retained, thereby \nreducing the overall contaminant load to Boomerang Lake. Ecological Engineering \nmeasures were implemented gradually and all measures taken at the site have resulted \nin a average zinc concentration of 7.6 mg L-' in 1992. Using monitoring data for \nBoomerang Lake, extrapolations indicate that, had no Ecological Engineering measures \nbeen implemented, zinc concentrations in the lake would likely have risen well above \ncurrent levels. Performance characteristics of the Biological polishing system are derived from the \ngrowth data, obtained in the field and the laboratory. The ranges in expected \nperformance are large, mainly due to the limitations of methodologies for determining \ngrowth rates. In field experiments, growth can only be calculated using linear \ninterpolation between two biomass measurements, which does not represent natural \ngrowth patterns. Furthermore, biomass which had accumulated on branches up until \nthe time of sampling does not include that biomass which had sloughed off over the \nperiod since the last sampling time. While laboratory experiments examining periphyton \ngrowth demonstrated logarithmic growth, it was also apparent from these experiments \nthat the waste water chemistry changes in the experimental vessel. Therefore, there \nare 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. \nDuring periods of exceptionally high precipitation in spring and fall, elevated zinc \nconcentrations are present. The monitoring data suggest that periphyton growing in \nDecant Pond effectively remove the zinc loading during the summer growing season. \nLong-term trends in water quality measured in Boomerang Lake suggest that acidity \nand sulphate have steadily increased. Sedimentation studies indicate that iron, \nprecipitated as solids and settled to the sediment surface, is periodically re-suspended \nin the lake. \nIn 1992, work addressing the residual contaminant loading in Boomerang Lake, Mill \nPond, and Decant Pond was initiated. Processes, including ARUM and Biological \npolishing, are capable of removing the annual loadings, but cannot remove the entire \ncontaminant loading which has accumulated during the estimated residence time of 3 \nyears in Boomerang Lake. \nPhosphate rock consumes acidity and precipitates metals. Any remaining (excess) \ndissolved phosphate, a major plant nutrient, is consumed by the periphyton population. \nExperimental 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 \nPond, significant amounts iron and aluminum were precipitated. \nPhosphate powder (9 tonnes) was applied to areas around the northwest end of \nBoomerang Lake. While iron and aluminum concentrations in the surface waters were \nunaffected, concentrations of these elements decreased in bottom water overlying the \nsediments. The concentrations of metal precipitates increased in these sediments. \nBased on the increases in metals in the sediments following phosphate rock \napplication, it was estimated that 6 % of the zinc, 74 % of the iron, and 10 % of the \naluminum in the lake water were relegated to the sediment. \nIn 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 \ntreated with phosphate rock, iron concentrations decreased from 53.5 to 12.9 mg L-' \nand aluminum decreased from 34.3 to 5.2 mg L-', immediately following phosphate rock \napplication. \nFull 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 \ntechnology development, facilitated an overall mass balance of annual contaminant \nloadings to Boomerang Lake and contaminant removal processes (Biological polishing \nand ARUM). \nThe annual loadings of zinc, iron, sulphur, and hydrogen ions to Boomerang Lake are \n3.9, 2.7, 16, and 0.1 tonnes, respectively. The estimated annual zinc and iron removal \ncapacity, when full implementation of the Biological polishing ecosystem in Boomerang \nLake is complete, is 5 tonnes and 2.7 tonnes, respectively. The estimated annual \nremoval capacity of ARUM (in Boomerang Lake sediments), based on sulphate \nreduction rates and alkalinity generation measured in other field systems, is 15.7 t of \nsulphur and 0.09 t of hydrogen ions. \nEcological Engineering measures for the South Bay site have targeted contaminant generation by the tailings deposit, surface water loadings to Decant Pond, and surface \nwater loadings by the mill site. The potential of the underground workings to produce \nseepages draining to Confederation Lake during years with high run-off was not \naddressed until 1992. A large diversion ditch was completed in January 1993. Its \nperformance and the effects on Boomerang Lake will be monitored in 1993. Work \naddressing ARUM activity in Boomerang Lake sediments and on the tailings beach of \nDecant Pond will continue. The accumulated contaminant load in Boomerang Lake, Mill Pond and Decant Pond will be addressed using phosphate rock.

Récupéré en direct depuis OpenAlex et désinversé. Les résumés ne sont pas conservés dans cette base de données : les index inversés représentent 8,6 Go des 9,3 Go de texte de la base, et le serveur dispose de 13 Go libres.