An ecological risk assessment for acid mine drainage from copper-nickel mining in the kawishiwi watershed, minnesota

Abstract

The mining of heavy metals such as copper and nickel has been identified as a potential source of acid mine drainage (AMD), which is a consequence of the oxidation of sulfide minerals that produces sulfuric acid due to mine wastes being exposed to air and water. Contamination of AMD to surface waters may pose detrimental effects on aquatic organisms. Accidental releases of process or seepage waters are likely to occur by mining ventures. The Kawishiwi watershed lies over a layered mafic intrusion known as the Duluth Complex, a mineral deposit rich in copper and nickel. Twin Metals Minnesota has renewed the mineral leases looking to develop an underground mine operation adjacently to Birch Lake and the South Kawishiwi River. Therefore, an aquatic ecological risk assessment is necessary to access and protect ecosystems that may be exposed to AMD. My study examined four potential contamination scenarios (A-D) in Birch Lake, Kawishiwi Watershed, Minnesota, based on hypothetical spills and metal concentrations. Concentrations reported in seepage waters from mines or from local sources that share the same mineral deposit or rock characteristics, were chosen to represent realistic scenarios. The concentrations varied for copper from 11 to 0.1 ?g/L and for nickel from 1,580 to 9 ?g/L. The scenarios also accounted for a determined size of spill, which varied from 10 to 0.3% of the total volume of the contact water pond 1. This pond is designated in the Twin Metal Minnesota operations plan to collect the waters from direct precipitation or stormwater that comes in contact with ore or tailings. The purpose of my assessment was to estimate the risks of this contamination to fish species living in the lakes. I used these final concentrations from the proposed scenarios to evaluate if exposure would exceed toxicity endpoints. The risk was assessed using the risk quotient (RQ) method, where the RQ is the ratio of exposure to effect. Risk quotient values < 0.5 are consider below regulatory levels of concern. My results suggest that the toxicity endpoint was exceeded for one of the four contamination scenarios by the most sensitive species: embryos of rainbow trout and bluegill. The contamination scenarios representing the two lowest concentrations for copper and nickel did not suggest levels of concern for either of the fish species. Based on my results, the water volume of Birch Lake would provide sufficient dilution in pollution events that are as much or less than ones represented by Scenarios C and D. Conversely, a spill representing Scenario A would pose risks to fish species exceeding levels of concern, especially to the most sensitive species. My assessment may be applicable when considering the risks that aquatic species could face in the event of a leak or spill of AMD of a sulfide mine in the Kawishiwi Watershed.