Saturation of ecosystems with toxic metals in Sudbury basin, Ontario, Canada

Abstract

Mining and resource recovery activities have not been kind to ecosystems in the Sudbury basin, Ontario. The combination of logging, smelting, fires and erosion resulted in an unusual anthropogenic ecosystem of denuded barren land with lifeless lakes, or a micro-desert. Since the 1970s, however, the concerted efforts made to reduce the emissions and rehabilitate parts of the degraded ecosystem have resulted in improvements in water quality, and recoveries in phytoplankton, zooplankton, zoobenthos and fish communities but have had little impact on toxic metal concentrations in many lakes. We show that most of the catchments in the Sudbury basin have become saturated with Cu and Ni, and some with Zn and Pb. It is estimated that mobilization of metals stored in soils and glacial overburden by surface runoff, groundwater drainage and wind re-working of tailings can sustain the high concentrations of Cu and Ni in many lakes for well over 1000 years. Strategies to immobilize the pollutant metals in the watershed rather than further emission controls may be required for dealing with high levels of toxic metals in surface waters of the saturated ecosystems.

Introduction

Mining and resource recovery activities have not been particularly kind to ecosystems in the Sudbury basin, Ontario. Extensive logging and forest fires and the use of open-bed roasting for ores depleted the vegetation and increased soil erosion (Nriagu et al., 1982; Courtin, 1994). Subsequent installation of smelters with low stacks resulted in extensive acidification and toxification of local soils and surface waters. Careless disposal of fine-grained tailings gave rise to metalliferous wind-blown dusts that were sprayed throughout the area. The combination of logging, smelting, fires and erosion resulted in an unusual anthropogenic ecosystem of denuded barrenland with lifeless lakes, or a micro-desert. From a scientific perspective, the Sudbury basin had become an opportunistic macrocosm (a unique laboratory) for mechanistic research on effects of severe acidification and metal pollution on ecosystems. Data obtained for the deteriorated ecosystems of Sudbury provided the scientific underpinning to much of the early Canadian thinking, debate and activism on long-range transport of pollutants and effects of acid rain.

The defiled ecosystems of Sudbury are now changing in response to public pressure, legislative measures and innovative technology. Since the 1970s, concerted effort has been made to reduce the emissions and rehabilitate parts of the degraded ecosystem. Emission of SO₂ increased from approx. 14 t day⁻¹ in 1950s, peaked around 6000 t day⁻¹ in 1971 and has since declined to approx. 2000 t day⁻¹ in 1980 and further down to <1000 t day⁻¹ in 1995 (Courtin, 1994; Mallory et al., 1998). Between 1973 and 1981, annual emissions from the smelters averaged approx. 670 t year⁻¹ for Cu, 500 t year⁻¹ for Ni, 204 t year⁻¹ for Pb, 125 t year⁻¹ for As and 15 250 t year⁻¹ for total particulates (Chan and Lusis, 1986); current emissions amount to a small fraction of the rates during the 1973–1981 period. Mine tailings have been stabilized, lakes have been limed and land patches revegetated. These control and remedial measures have resulted in improvements in water quality, and recoveries in phytoplankton, zooplankton, zoobenthos and fish communities in many lakes (Griffiths and Keller, 1992; Matuszek et al., 1992; Nicholls et al., 1992; Locke et al., 1994). The Sudbury basin is now becoming less of an environment for the study of adverse effects of acid rain and metal deposition and more of a field laboratory for examining the beneficial effects of emission control and environmental rehabilitation (Gunn et al., 1995; Havas et al., 1995; Keller and Gunn, 1995; McNicol et al., 1995).

The chemistry and dynamics of pollutant metals in surface waters of the Sudbury basin are still not well understood. In 1978 and 1979, we did a detailed study of the distribution and chemical behavior of trace metals in water, seston (suspended particulates) and sediments of selected lakes in the Sudbury basin. These measurements were repeated during 1993/1994. The results of the two sets of measurements are compared in this report to ascertain the effects of various emission control measures on trace metals in lakes during the 15-year interval. The yield of metals from heavily contaminated ecosystems over a relatively long period of time was of major interest in this study.