A previous review of MEND studies on subaqueous disposal of tailings presented several recommendations (RAAS 1992). Proposed work in Anderson and Buttle lakes included a mass balance of selected chemicals, measurements of geochemical fluxes across the sediment interface and the application of comprehensive QA/QC procedures to ensure reliable data. A literature study of biological effects of metals was also proposed. This report reviews progress since 1992.
The literature study on biological effects demonstrated that sorption reactions on Fe, Mn oxyhydroxides and organic matter, and reactions with acid volatile sulphides allow reasonable prediction of geochemical behaviours under oxic and anoxic conditions. Biological availability of dissolved metals is reasonably well described by the free ion activity model.
Attempts to combine field data and historical records (water quality/quantity and effluent loadings) provided valuable background information but were not sufficient to detail the effects of sulphide-rich tailings on either lake, or to support an acid-base mass balance for Anderson Lake.
QA/QC protocols provided much high quality data about the diffusive flux at the sediment/water interface in both lakes. Weight of evidence indicates there can be a small loss of metals (mostly Zn) from sediment to the overlying water, and some recycling (including metals) is probably related to the natural degradation of organic matter. Tailings exposed to subaqueous oxidation react very slowly but actual rates in the lake environment are uncertain.
The diffusive flux is one of several factors that control the effect of tailings in subaqueous environments and in man-made containments it may be the dominant factor. Man-made containments and lakes with naturally anoxic bottom water are likely to be most suitable for subaqueous tailings disposal.
It is concluded that a comprehensive assessment of the advantages and disadvantages of subaqueous and subaerial disposal would be useful. A model study (with examples) could address costs, risks and potential benefits, and focus on long term stability. The need for a better understanding of the "cradle to grave" effects of tailings remains but study costs are high. New studies on such effects in complex natural environments may be non-generic and made only on a case-specific basis.