Limnology of two proximal pit lakes after twenty years of intense flooding

Abstract

Two adjacent pit-lakes, Caland and Hogarth, formed after the flooding of open pit iron mines near Atikokan, Ontario, Canada, were examined for their water quality relationships and potential to support aquaculture. Detailed sampling for physical and chemical parameters took place during 1998.1999. and early 2000. Since mining terminated in 1979. both pits have continued to fill with water from rainfall, snowmelt. and groundwater seepage, and. by 2000. had water depths in excess o f 160 m. Water quality in both pit-lakes was largely dependent on proximal waste rock composition and surficial geology of the area. Limestone and dolostone deposits at the site countered production of acid from waste rock and resulted in neutral water pH’s in Caland and Hogarth pit-lakes. Drainage was a major factor in distinguishing the water quality between the two pit-lakes. Inflow of freshwater was much greater into Caland and resulted in meromictic conditions with a well defined mixolimnion, chemocline and anoxic monimolimnion. Hogarth pit-lake had less inflow and the entire water column was sulfate-saline and aerobic. Statistically significant variations occurred between the two pit lakes for metal, anion, cation, conductivity, hardness, and dissolved oxygen concentrations. Variations in precipitation and evaporation between the two sampling years resulted in increased or decreased ion concentrations for the same seasons or depths within the two pit-lakes. Caland pit-lake has supported an aquaculture operation for the production o f rainbow trout (Oncorhynchus mykiss) since 1989. Production increased from 20,000 kg in 1998 to 163,325 kg in 1999. Nutrient loading from excess fish food and waste increased phosphorus and ammonia levels during 1999 and 2000. The expanding fish form also decreased the volume of usable (>6.0 mg 1) dissolved oxygen. Hogarth pit-lake was devoid of aquatic life. Using water from Hogarth, standard LC50 tests showed 100% mortality of Daphnia magna at full strength concentrations. None of the tested metal parameters had levels that would be considered toxic and elevated salinity was suspected as the cause of mortality. Any future aquaculture in Hogarth pit-lake w ill require the use of species tolerant to such saline conditions.