The influence of paste backfill on operational and long-term mine and ground water quality has been identified as one of the priorities of the MEND Program. This report provides a brief summary pertaining to current practices in the geochemical characterization of both cemented and uncemented paste backfill, and methods used to predict environmental impacts to surface and ground water quality associated with the application of paste backfill in underground applications. Data was collected via a literature review, and a survey of mines known to use paste backfill.
The findings indicate that the amount of available information and research on the influence of underground paste backfill on mine water quality is typical of a relatively new field. To date, research by the community at large has focused on the structural characteristics of paste in terms of meeting the required backfill strength using the most economic amount and mix of binder materials. In light of the belief that the chemical reactivity of tailings and the volume of leachate generated are reduced by thickening, and by the addition of alkaline additives such as cement, little information on the influence of paste backfill on mine water quality appears to have been developed.
Exceptions have been where:
• the mineralogy and reactivity are extreme, with potential effects on paste strength;
• a portion of the paste is being deposited on surface (with potential surface water impacts); and,
• concern regarding potential ground water contamination from underground waste disposal in the United States led to initiation of the Underground Injection Control (UIC) Program that incidentally includes placement of mine waste backfill in underground mines under its legislation (Levens et al., 1996).
Recognition of the fact that any backfill has the potential to generate contaminant plumes in the long term, and potentially influence ground and/or surface water appears to have increased the site-specific evaluation of paste characteristics of newly proposed mines in recent years.
Despite the lack of extensive detailed study, the use of paste backfill in underground environments has been generally considered beneficial to reduce overall environmental impacts associated with mining, due to:
1. Reduction in the volume of tailings requiring surface disposal, thereby reducing surface impacts through footprint reduction;
2. Use of the full tailings stream in the backfill, rather than the coarse fraction used in more conventional sand fill, thereby reducing the need to handle and dispose of a separate slimes stream;
3. Reduction in the potential for tailings to oxidize or leach due to the nature of thickened tailings placed as underground backfill because of:
• Less free water, which reduces leachate generation;
• Less available oxygen as a result of the higher degree of saturation;
• Preferential flow of ground water around backfill, rather than through it due to the lower hydraulic conductivity of the paste backfill;
• The addition of cement that provides extra neutralization potential (NP) and decreases effective porosity; and,
• The potential for flooding at closure which reduces sulphide oxidation in long-term.
The general theories associated with paste backfill characteristics and geochemical reactivity appear sound, but there does not appear to be much field validation on the actual influence of key parameters. Lack of controlled conditions in active mine environments appears to significantly limit the ability to separately assess potential scale up issues. The field would benefit from research targeted at the specific components of paste theory (such as the separation of the influence of thickening and binder addition), examination of scale-up issues (preferably in the controlled environment of an isolated well characterized and instrumented backfilled stope), collection of detailed case studies, and additional monitoring of mine waters to assess the influence of paste backfill on mine water quality over time. The lack of detailed information currently available is of concern, and highlights the need to compile detailed site data and monitoring data for future assessment and validation of predictions currently being initiated. And as with any new field, establishing a standard base of terminology would be useful.
In the bigger picture, there may be a need to better define the potential importance of this issue, such that priorities for studying this matter can be assessed. For example, are existing backfilled mines producing significant ground water contaminant plumes? Certainly sidehill mines that continue to drain from portals or other openings are known to be potential closure problems when not suitably mitigated (i.e. Britannia Mine in B.C., Canada; Summitville Mine in Colorado, U.S.). And there appears to be sufficient information to suggest that there might be potential impacts from backfilled mines where the wall rock and backfill are particularly reactive (i.e. Bernier and Li, 2003). However, a general survey of existing underground mines might put the significance of the issue in perspective.
