A TALE OF TWO PIT LAKES: VARIED BIOGEOCHEMICAL CONDITIONS, PROBLEMS AND REMEDIAL SOLUTIONS

Abandoned or discontinued open-pit, mining operations have resulted in the creation of numerous pit lakes throughout the world. Depending on the mineral composition of the rocks surrounding the pit, particularly the presence or absence of carbonates and/or sulfides, water in the resulting pit may be acidic, near neutral or even alkaline. In addition, these water bodies can become highly stratified with respect to temperature, dissolved oxygen, redox, and TDS, with stratification being very stable or subject to disruption through physical mixing. The dynamics of these biogeochemical and physical processes often determines the environmental problems associated with a particular pit lake and frames the potential remedial solutions. Here, two pit lakes that present highly contrasting physical and biogeochemical environments are examined. One is a hydrological sink, with a near neutral pH, relatively low TDS that undergoes seasonal mixing. In contrast, the other is a flow-through system, with an acidic pH, and is stably stratified having low TDS surface water and high TDS bottom water. Each of these systems is associated with a unique set of environmental problems and intern presents a unique set of challenges in terms of environmental restoration.