The physical, chemical and biological dynamics in two contrasting pit lakes: Implications for pit lake bio-remediation

Abstract

The Main Zone and Waterline pit lakes at the Equity Silver Mine near Houston, BC (Canada) are the sites of an ongoing 3-yr government- and industry-funded research program. The pits exhibit considerable contrasts in their physical, chemical and biological limnology. The Main Zone pit is deep (~120 m), maintains permanently oxygenated bottom waters, and hosts a modest level of phytoplankton productivity. In contrast, the Waterline pit is shallow (~40 m), exhibits negligible plankton productivity, and is characterized by permanent stratification and bottom water suboxia below a water depth of ~10 m. In order to assess the feasibility of various bio-remediation strategies, field-scale manipulations were conducted using limnocorrals. Two manipulation strategies were tested: 1) addition of algal nutrients (phosphate and nitrate) to Main Zone Pit surface waters; and 2) addition of nutrients (surface waters) and dissolved organic carbon (deep waters) to the Waterline Pit. Nutrient addition was conducted in order to stimulate algal production and enhance metal scavenging by biogenic particles, while dissolved organic carbon (ethanol) was added to increase oxygen demand and foster the development of sulphate reduction in pit bottom waters. In the Main Zone Pit, the stimulation of algal growth resulted in the pronounced removal of both dissolved and total metals (Zn, Cu and Cd) from surface waters, with higher rates of metal removal being observed at higher nutrient additions. Metal removal could be attributed to the scavenging of dissolved metals by biogenic particles and subsequent particle settling. A similar pattern of removal from surface waters was observed in Waterline Pit (Zn and Cd). The addition of ethanol to deep waters in Waterline Pit was effective in promoting sulphate reduction in the lower layer. The development of reducing conditions resulted in near-quantitative removal of dissolved Zn and Cd, presumably as secondary metal sulphides. Collectively, the results demonstrate that the passive forms of bioremediation tested in these pit lakes may be effective for whole pit-lake remediation.