MINING WASTE MANAGEMENT / 5 Feb 2019
Nordic winter conditions – special challenges in mining waste management
Cold climate sets a lot of special requirements for the mining industry. Also management of mining waste and seepage waters in cold climate differs from warmer environments.
Seasonal variation in mine water concentration
In the Nordic Region, the most visible parts of annual variation are - quite obviously - the winter time snow cover and the enormous water released during the spring melt. Spring melt brings a lot of “clean” water to a mining area. Only a part of the snow melt infiltrates and is in such contact with the mineral surfaces that water chemistry changes. Therefore, mine site water has usually relatively low concentrations of its typical substances during the spring.
During the early winter, there is also ground frost forming. Partial freezing works as nature's own freeze-distillation process: the frozen portion is poorer in the dissolved substances than the remaining liquid. Remaining water (especially in individual sampling points) may suddenly include surprisingly high metal concentrations in the winter. This is one of the many reasons why trend interpretations should never be based on short term observations.
In extractive waste facilities containing sulphide minerals, metal and sulphate concentrations often peak already during the autumn. Sulphide oxidation in mineral surfaces is faster during the warm summer period. In summer time, also microbiological processes contributing to the oxidation are more active. Sulphate, metals and acidity produced by these processes are typically flushed out in the autumn, when rainfall increases.
Extractive waste behavior changes in frozen ground
There may be even permafrost in parts of a mining area. A lot of snow and ice gets buried into overburden and waste rock storage facilities, as new material is dumped on the top. Often the ice melts in the spring, but not necessarily. Biogeochemical processes are almost non-existing in the frozen material and ice also functions as a liner, stopping gas and fluid movement into the waste. Sulphide containing material may seem to behave like inert waste in this kind of situation – at least for a while, until the ice melts. There may be natural exothermic reactions warming the facility from inside, but also the climate is warming.
In general, trying to understand extractive waste behavior only by sampling groundwater or surface water outside the waste facility can be tricky, sometimes even misleading. Groundwater flow paths in frozen ground are not identical to summer conditions. Sampling results may seem rather random. Unfortunately, downstream monitoring is sometimes the only monitoring applied. Using also in-facility instrumentation can give valuable information and help to understand the waste facility geochemical behavior.
Variation of conditions has also an impact on behavior of nitrogen. Nitrogen in mine sites comes typically from explosives. Nitrification-denitrification circle is impacted by both temperature variation and freezing related variation of aerobic and anaerobic conditions. If overland flow fields are being used, also possibility to bind nitrogen to vegetation and organic matter varies by season.
Holistic approach for predicting mine site water qualities and quantities
Understanding extractive waste behavior in cold climate is complicated due to the large quantity of contributing factors. Anyhow, there are measuring, testing and modelling methods to help understanding and (what is most important) to predict the long-term behavior of the waste. Potential risks related to geochemical behavior of the material must be understood in site specific conditions in cold climate and even changing climate should be taken into consideration. Predictions must cover the different construction stages of the extractive waste facility and the time after mine closure. Uncertainties cannot be completely removed, but they can be significantly decreased.
Planning the waste management facilities must be based on understanding the extractive waste long-term behavior in interaction with its environment. Case specific performance criteria must not be replaced by standard designs based on waste categories. Considering our seasonal variation, a holistic approach is needed. Holistic approach enables understanding mine site water quantities and qualities better and planning the best suitable water management. In the end of the day, mine site water management must be able to cope with the whole range of challenges a Nordic year provides: from small water quantities with high concentrations to high water quantities with small concentrations.
Päivi Picken, PhD
Environmental services for Mining and Exploration