Environmental geochemical risk assessments have become a routine practice in the environmental evaluation of most greenfields and brownfields mining and mineral processing related projects. Brownfields projects, e.g. extension of an existing tailings facility, usually have timeseries monitoring data which can be used in the risk assessment. The greenfields scenario is more complex in that very little, if any, existing data is available, thus often requiring follow-up studies.

In terms of the assessment of environmental geochemical risks from leachate from mineral waste facilities, there is, at least in some circles, a prevailing misconception that acidic conditions cause environmental impacts due to leaching of contaminants from mineral waste facilities, while neutral to alkaline conditions does not.

This paper is aimed at in identifying a decision strategy to guide project managers in determining when follow-up studies specifically for greenfields mineral waste generating projects is warranted and also addresses the notion of pH being an indicator of risk. These two aspects will be addressed by applying numeric geochemical modelling, which takes geochemical processes over time into account to determine likely impacts from mineral waste Tailings from two types of ore will be considered. The first is an acid producing tailings material from the Witwatersrand gold mining basin, while the second is tailings material from a Fluorspar mining project (non-acid producing Both types of tailings contain iron sulphides which have the potential to produce acid mine drainage. The difference is the host geology of the two deposit types, with the Fluorspar ore associated with and located in dolomitic lithologies. Even though the Fluorspar tailings is likely to generate a neutral leachate from the tailings, it also contains SO42- and As in concentrations exceeding regulatory guideline values. This demonstrates that even though a mineral waste facility may have circum neutral pH values, it may still contained potentially hazardous dissolved components in excess of regulatory guideline values. The Fluorspar tailings material also contains F- below, but within 20%, of the regulatory guideline value, implying that follow-up studies may be warranted. This can be done relatively inexpensively with potential significant financial implications of the project net present value. Both of these types of tailings material are associated with greenfields projects.

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