Hydrodynamic and water quality modelling of a shallow lake influenced by stormwater discharge from an active goldmine

Abstract

Several mine-influenced lakes have become meromictic in Northwestern Ontario, a state in which complete turnover of the water column does not occur. Whether or not a lake experiences complete turnover has a significant impact on dissolved oxygen, nutrient cycling, available fish habitat and the production of potentially toxic compounds. Lower Unnamed Lake is the stormwater receiving body for Barrick Gold’s Hemlo Gold mine. The mine-influenced water discharged into Lower Unnamed Lake is laden with a high concentration of total dissolved solids. Rather than freely mixing with the lake water, the mine influenced water sank to the bottom of the lake as a result of the density difference between the discharge and the lake water in the receiving body. It was of concern that stratification, induced by the density gradient, caused by the concentration of total dissolved solids had stabilized the lake and prevented turnover, causing Lower Unnamed Lake to become meromictic. This study used CE-QUAL W2 to create a hydrodynamic and water quality model of Lower Unnamed Lake, in order to evaluate mixing trends. Full turnover was predicted in the fall and two additional mixing events were produced by the model. The mixing events are evident in field observations of the lake outflow total dissolved solids, temperature and in-lake profiles of temperature, total dissolved solids and dissolved oxygen. [...]