Bethan Tuckett-Jones and Hanspeter Tomschi share their experiences of completing an air quality impact assessment on a mine in Botswana
The BCL mine complex, situated near the town of Selebi Phikwe in eastern Botswana, includes both deep mining and smelting operations. It has been producing copper-nickel matte from sulphidic ore since the early 1970s.
Emissions to air occur at all stages in the process and, each year, the complex emits more sulphur dioxide (SO2) than whole of the UK.
Selebi Phikwe area is striving to diversify its economy away from mining, with agriculture and tourism identified as prime sections for development. However, the poor quality of the local environment is a major factor impeding this transition.
A consortium led by Parsons Brinckerhoff was funded through the European Development Fund to provide the BCL mine and Botswana’s government with relevant technical advice, based on environmental impact assessment (EIA) of air quality, to make an informed decision on how to reduce pollution from the smelting operations.
The objective was to:
- review available environmental management data and produce an air emissions inventory for the mine
- assess the current impacts of the mine using dispersion modelling and
- identify technologies for emissions control and markets for the by-products
Prior to smelting, the principal emissions to air from the mining complex are the carcinogenic nickel-rich dusts arising from the crushing of the mined ore. During the smelting process off-gases are discharged to air and consist of 20% SO2, a gas which, in high concentrations, is harmful to both human health and the natural environment.
The EIA process enabled us to demonstrate, beyond reasonable doubt, the link between emissions from the mine and the near complete degradation of the vegetation downwind of the facility and to quantify the emissions reductions required to prevent environmental damage and harm to health both on and off the site.
Methodology
We first prepared an emissions inventory based on a mass balance reconciliation of the sulphur and nickel content of the process raw materials and the final product (nickel matte).
Essentially, with no significant emissions to media other than air, emissions could be estimated by assuming that all sulphur in the mined ore was oxidised and emitted as sulphur dioxide (the final product has no sulphur content), whereas the difference between the nickel content of the input ore and the output matte was assumed to be emitted as nickel.
We then audited the mine’s environmental monitoring programme and commissioned further emissions monitoring to apportion the total emissions between potential sources. Finally, we modelled the air quality impacts of the emissions using the ADMS 4 model.
Previous studies of the BCL complex had been criticised for failing to provide conclusive evidence of the mine’s impact on the environment. While the dispersion model results in our study were consistent with the available ambient air-quality monitoring data, a formal verification of the model was not possible due to the limited range of data.
It was therefore necessary to use alternative approaches to ensure stakeholder engagement. We did this by:
- using all available environmental data, including bio-monitoring, to indirectly verify the modelling
- demonstrating the model was replicating the daily cycle of the real-world behaviour of the emissions, and
- presenting the findings in a stakeholder workshop ahead of the final project report.
Model results
The ability of the dispersion model to reproduce the observed behaviour of the plume and exposure to air pollution was demonstrated through a time-lapse photographic record of the plume, coupled with coincident dispersion model output (see examples below). The model’s predictions accurately replicated the timing of the grounding of the plume and concentrations consistent with the symptoms being experienced by individuals in the area.
| Examples of correlation between time-lapse photographic record and modelled impacts | ||
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| At 0800 hours, the plume remains lofted and coherent for some distance downwind. With only weak turbulence mixing the plume vertically, impacts on the ground do not occur until >5km from the stack and are negligible. | ||
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| By 1400, the average wind speed falls to near calm conditions, fumigation occurs and as the turbulence mixes the plume to the ground rapidly it loses its coherent structure and the point of maximum impact moves to within a few hundred metres of the stack. On site, throat and eye symptoms associated with acute exposure to SO2 are readily apparent. | ||
Impacts on vegetation
Exposure of vegetation to SO2 can result in leaf damage and reduced plant growth. These effects were readily apparent in the variability in the health of vegetation in the vicinity of BCL mine.
Mopane woodland (below picture A) grows only in southern Africa and is important for numerous animal species because of its nutritional value. Degradation of the Mopane vegetation is apparent downwind of the stack (pictures B, C and D), and is increasingly severe as the distance to the stack decreases.
Analysis revealed a consistent exponential decay with distance from the stack which is strongly indicative of a link between the sulphates in the vegetation and the dispersion of emissions from the BCL main stack.
There was also good correlation between the concentration of SO2 in air and reduced growth and foliar damage (pictures B, C and D). Reduced growth is readily apparent at concentrations of SO2 in air of around 100µg/m3 – compare picture B with healthy vegetation in A. Data from locations upwind of the prevailing wind from the stack indicate the EU critical level for SO2 of 20µg/m3 is appropriate.
Figure 1: Examples of Mopane vegetation 40km north of the stack (A) and at distances of 15km (D), 5km (C) and 2km (B) respectively downwind of the stack downwind
Recommendations
To meet World Health Organisation guidelines for the protection of health and ecosystems an immediate 95% reduction of SO2 and nickel emission is required.
Further research concluded it was feasible to capture SO2 and use it to produce sulphuric acid, which could be sold meaning the environmental enhancement could be achieved at a neutral cost.
It was also recommended that a health impact assessment be conducted to consider the health of the Selebi Phikwe population from a holistic view point.
This article was written as a contribution to the EIA Quality Mark’s commitment to improving EIA practice.
Bethan Tuckett-Jones is head of air quality, environment ay Parsons Brinckerhoff. Hanspeter Tomschi formerly worked for Harress Pickel Consult GMBH as a consultant on the project.
