Water quality modelling results predicted minor exceedances of the potassium water quality benchmark in the receiving environment downstream of the Ekati Diamond Mine. Concern was raised with the predicted minor exceedances and the protectiveness of the potassium water quality benchmark (i.e., 64 mg/L).
The Dominion Diamond Mines water quality benchmark for potassium was derived in accordance with Canadian Council of Ministers of Environment (CCME) requirements for establishing Type A water quality guidelines, as there were no existing water quality guidelines for this constituent. The derivation used a species sensitivity distribution with long-term toxicity test data from three species of fish, four invertebrates, and three plants or algae.
To provide the regulatory community with additional confidence in the long-term protectiveness of the potassium water quality benchmark (i.e., that adverse effects would not be expected if minor exceedances of the potassium benchmark were to occur) additional longer-duration toxicity tests were conducted. Toxicity tests were conducted to evaluate the chronic effects of potassium on the survival and growth of three laboratory test species in site water; 28-day survival and growth test using the amphipod, Hyalella azteca; 28-day survival and growth test using the fingernail clam, Sphaerium sp.; 32-d survival and growth test using the fathead minnow, Pimephales promelas.
Standardized test methods were used or, in the case of the fingernail clam test, methods that are published in peer-reviewed literature. Potassium chloride was used to introduce potassium into the test solutions. The potassium addition took into account the potassium that was already present in the site water and supplemented to achieve treatment concentrations of 64, 80, 100, and 200 mg/L (as potassium).
Results from the toxicity tests for H. azteca were LC10 (survival) of 143.3 mg/L, and IC10 (weight) of 152.6 mg/L; for Sphaerium sp. were LC10 (survival) of 81.5 mg/L, and IC10 (weight) of >99.6 mg/L; and for P. promelas were LC10 (survival), EC10 (hatch, normal development), and IC10 (biomass, length) of >208 mg/L.
The outcomes from the toxicity tests lend support that the water quality benchmark of 64 mg/L potassium is protective of the test species and continues to be appropriate for use as a water quality benchmark. There were no adverse effects observed in exposure to 64 mg/L potassium with any of the three test species. Further, the lowest tenth percentile point estimate, which is considered a no effect level by CCME was an LC10 of 81.5 mg/L potassium; this value is 27% greater than the potassium water quality benchmark. The most sensitive low effect level from these tests was an LC25 of 88.8 mg/L for survival of Sphaerium sp. over a 28-day period, which is 38% greater than the water quality benchmark.