Characterising and Predicting Tailings Seepage Chemistry

Characterising and Predicting Tailings Seepage Chemistry (North QLD)

There are three approaches to predicting seepage water quality: 1) field monitoring and tracer techniques such as stable isotopes, 2) laboratory column leaching experiments; and 3) geochemical modelling. A column study only shows how a small sample of tailings materials behaves over time. The actual field environmental conditions can be different. A number of factors need to be considered including; water contact - fraction of tailings flushed by unsaturated flow; moisture content effect on oxidation rate; particle size effect on oxidation rate; specific mineral concentration;  surface area of minerals - passivation effects; temperature; oxygen and CO₂ concentrations in tailings; and above all bacterial activity. 

In the geochemical modelling approach, again, in the absence of enough data for parameters like O₂ and CO₂ partial pressure, bacterial activity (oxygen pathway versus iron pathway of oxidation), mineral surface density, secondary mineral precipitation and coatings, assumptions have to be made, which may result in considerable uncertainties.

Through case studies, we use a combination of methods for unravelling sources and pathways of contaminants through the TSF and surrounding surface and groundwater, and predicting water quality at closure. A case study is one the largest active metal mines in Australia, with a tailings storage facility of ~12 km²in area, have accumulated a large data-set of seepage and groundwater chemistry through field monitoring, including those sampled by the authors.