PIT LAKES AS MINE-WASTE REMEDIATION CELLS: PLUSES AND PITFALLS

Dissolved metals released to surface and ground waters as a consequence of ARD generation can theoretically be captured by sulphide precipitation at depth in meromictic pit lakes and be "permanently" sequestered providing that the bottom waters remain oxygen-free. Use of such lakes requires establishment of significant vertical density gradients, and sufficient SO₄^2- supply and oxidant demand to guarantee persistent sulphate reduction in the monimolimnion. Experiments underway in pit lakes at the Equity Mine site in north-central B.C. confirm that such conditions can be induced with anticipated "permanent" removal of dissolved metals to the sediments on the lake floor. Sulphate depletion at depth in pit lakes can also have adverse consequences, however. Sulphate reduction in submerged tailings deposits at uranium mines, for example, can lead to (Ba,Ra)SO₄ dissolution and Ra release.

Like meromictic water bodies, well-mixed pit lakes can also be used effectively for "permanent" storage of certain metal-rich phases. For example, deposited Fe-oxyhydroxides, produced by plants that treat ARD with lime, will remain chemically stable provided that bottom waters remain perpetually oxygenated and benthic oxidant demand remains low. Should anoxia develop above such deposits, major release of metals to bottom waters can be anticipated.

Examples of these phenomena gleaned from several recent field studies in Canada will be presented. These demonstrate that the use of pit lakes as environmentally beneficial reactors requires detailed understanding of their long-term physical characteristics as well as an appreciation of biogeochemical responses that are specific to the types of materials deposited on the lake floor.