| Summary: | Broken Hills Proprietary Company Ltd. (BHP; now BHP Billiton Inc.,) Ekatia™ Diamond Mine, located approximately 300 km northeast of Yellowknife in Canada's Northwest Territories, began mining operations in the fall of 1998. This diamond mine has a life expectancy of at least 25 years, as more than five diamond bearing kimberlite pipes have been identified on BHP 's claim block. Diamonds are extracted from kimberlite using chemical free processes that reduce the ore to fine particles (≤0.5 mm). Processed kimberlite effluent (PKE), which consists of these fine particles, water used during the extraction process, coagulants, flocculants, and treated sewage effluent, is disposed of in the designated containment facility, Long Lake. The Long Lake Containment Facility (LLCF) has been subdivided into five cells, of which the first four cells were designed to receive PKE and the last cell would act as a final monitoring stage before water was discharged to the downstream aquatic environment. The magnitude of effects PKE had on the ecosystem of the LLCF was evaluated through field assessments of chemistry and zooplankton community structure from August 1998 to August 2000. The toxic components of PKE were determined through a series of baseline toxicity tests and toxicity identification evaluations (TIE) using Ceriodaphnia dubia. The loss of aquatic habitat, higher turbidity and significantly altered water chemistry resulting from PKE addition reduced zooplankton abundance in the receiving cell of the LLCF to less than 10% of values prior to PKE depositions. From August 1998, prior to the addition of PKE, to August 2000 total water hardness, alkalinity, total dissolved solids, pH, and total nitrogen significantly increased in all cells of the LLCF. In the cell farthest from PKE deposition, the concentrations of the following metals increased: barium from 6.6 to 24.3 μg/L; molybdenum from below detection (1.0) to 6.3 μg/L: strontium from 8.1 to 35.0 μg/L, and nickel from below detection (1.0) to 1.2 μg/L. Significantly greater increases in the concentration of these metals were noted in the cell directly downstream of PKE deposition. Processed kimberlite effluent was chronically (7-d reproductive inhibition) toxic to C. dubia at concentrations as low as 12.5% effluent. The following Phase I TIE manipulations did not reduce the toxicity of PKE: ethylenediaminetetra-acetic acid (EDTA) and sodium thiosulfate addition tests, aeration, and solid phase extraction (C-18 column). Toxicity of PKE was significantly reduced with major pH (to 3 or 11) adjustments followed by filtration (0.45 μm). It is believed that the cationic polymer (Magna Floc®) was the toxic component since the estimated final concentration of the polymer in PKE was 1,000 times higher than the estimated 7-day EC50 to C. dubia of 0.014 mg/L. It is postulated that major pH adjustments altered the behaviour and electrostatic charges of the kimberlite minerals in solution in such a manner that excess cationic polymer became bound to the minerals and was no longer able to elicit a toxic response.
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