Lauri Solismaa, Geological Survey of Finland, PO Box 1237, 70211 Kuopio, Finland; firstname.lastname@example.org
The term particulate matter (PM) is used to describe the amount of particles that exists in the air we breathe. Total Suspended Particle Matter (TSP) refers to the total of all particles suspended in the air. PM10 describes particles smaller than 10μm in diameter and PM2.5 represents fine particle, smaller than 2,5 μm. Particles in the size range 2.5μm to 10μm in diameter are referred to as coarse particles (PM2.5-10).
The vast majority of dust from mining activities consists of PM10 and particles larger than PM10, generated from natural activities such as mechanical disturbance of rock and soil materials by dragline or shovel, bulldozing, blasting, and vehicles on dirt roads. Dust is also generated when wind blows over bare ground and different types of stockpiles. These larger particles can have amenity impacts as well as health impacts. (NSWMC, 2011)
Description of the methodology
Environmental risk assessment methods concerning dust emissions are metal level monitoring in environmental samples (soil, moss, fungi, berries, etc.) and estimation of the volume and spreading of dust using emission and deposit measurements and spreading modeling.
Covering of tailings may be required if the area is susceptible to rising of dust by wind ablation, or if the establishment of desired vegetation demands landscaping and improvement of soil quality. The nature of tailings, waste rocks and mine host rocks varies considerably, depending on whether the mine has been exploiting industrial minerals, sulphide ores or various other metals. Conditions and requirements for covering also vary further within these groups. Therefore, it is important to characterize the properties of the material to be covered in order to assess both the potential environmental impacts and risks related to the material, prior to deciding implementation strategy. There are essentially two options for isolating tailings and waste rock – burial beneath a layer of overburden, or covering with water (Heikkinen et al 2008).
- High-Volume air sampler systems
- Atmospheric fallout collectors
- Personal dust collectors used in occupational hygiene measurements
- optical particle counters (example nephelometry)
Spreading modeling methods:
- US EPA Fugitive Dust Model (FDM)
- System for Integrated modeling of Atmospheric composition (SILAM)
Picture 1: Dust collector near tailings area at Pyhäsalmi mine, Finland. Photo: Lauri Solismaa GTK
Generation of dust from tailings areas can be minimized by watering or spreading lime slurry on top of the tailing area. Common method to prevent dusting is covering the tailings with dry cover or water cover.
Dry cover layers
The thickness, structure and material properties of the cover layer will be determined by the nature of the tailings and other site characteristics, including the local water table, and the availability of and access to source material, all of which need to be considered thoroughly. If the material to be covered is reactive or susceptible to acid production, and the intention is to isolate the tailings from oxygen diffusion and water infiltration, the following features should also be examined carefully (Naturvardsverket 2002).
The potential for interaction between the atmosphere and tailings or waste rock, for example via plant root systems or disruption of surface integrity by subsidence or some other mechanism.
The possibility that seasonal temperature and rainfall fluctuations induce frost heave or desiccation cracking and thereby affect the effectiveness of the covering layer.
Measures to mitigate the effect of erosion should also take into account extreme and unlikely events, such as flooding, or freezing and blockage of drainage networks.
The risk of excessive acid production can be addressed by moderating the pH of the material to be covered by addition of lime, crushed limestone or fly ash, prior to covering.
The most straightforward method is to spread the cover material without any attempt at compa- ction or consolidation. This is appropriate in situations where there is no need to prevent the water infiltration into the covered material. If the covering material is inherently relatively impermeable, then it is possible to make an effectively impervious barrier by spreading and compacting two or more layers. Oxygen diffusion can be dealt with by designing the system such that the water table lies within the cover layer, but in this case it is necessary to ensure that there is no possibility of accidental release of acidic waters or solid waste material.
A more effective approach to prevention of oxygen and water infiltration is by spreading two separate layers, a sealing layer (for example compacted impervious clay) and the surface layer. The upper layer thus protects the lower layer against the effects of erosion and structural degradation due to seasonal dehydration or freezing, as well as minimizing direct interaction with humans and other animals, plants and microbial activity. Moreover, this layered structure both disrupts upwards capillary flow and favours retention of any metal complexes transported by pore waters.
The EC’s BAT reference document (EC 2004) also describes a further option in which the compacted layer is overlain by another layer intended to prevent the water infiltration and to promote drying of the substrate. However, excessive drying and potential desiccation of the surface layer may lead to oxygen infiltration to deeper levels. Moreover, in the long-term, the geotextile between the layers may deteriorate, leading to mixing of the layers and reduction of the effectiveness of the desiccation layer.
In oxygen consuming cover layer a water-saturated, relatively impermeable structure also acts as an effective barrier to oxygen diffusion, since breakdown of organic matter in the surficial layer consumes oxygen. However, if organic matter promotes biologic reduction of iron, there is an increased risk of metal leaching.
Conditions might also be favourable for bacterial sulphate reduction, although so far, neither of these processes has been documented during site monitoring. Dry cover methods have been widely used in mine site rehabilitation in the Nordic countries, for example at Enonkoski and Keretti (Outokumpu) in Finland and at Viscaria in Sweden.
The use of water covers
Covering by a layer of water requires that the following conditions are met:
- Water must be available, even if the area is subject to seasonal dryness, in sufficient amounts to ensure that both the water table and water chemistry remain stable
- Dams and impoundments are structurally stable over the long-term
- Outflow channels are stable and with a capacity designed for coping with isolated extreme flood events, and also other eventualities, such as ice dams and log jams
- Water depth above the tailings is deep enough such that wave action does not cause erosion or hydraulic separation and concentration of tailings.
The impoundment will function best if it is integrated with the surrounding watershed and is fed by inflow from a natural stream. This accelerates the restoration of a natural ecosystem by supplying organic matter, nutrients, and aquatic biota. Moreover, influx and deposition of sediment enhances the existing barrier to oxygen diffusion within the underlying tailings. The greatest uncertainties with respect to this approach lie in ensuring the long-term stability of the impoundment dams and that an adequate supply of water is maintained over the longer term, even after monitoring of the site has ceased.
Heikkinen, P. (ed.). 2008. Mine closure handbook—environmental techniques for the extractive industries. http://arkisto.gtk.fi/ej/ej74.pdf
New south Wales Minerals Council Ltd. 2011. Fact sheet: Mine Dust and You. http://www.nswmining.com.au/NSWMining/media/NSW-Mining/Publications/Fact%20Sheets/ Fact-Sheet-Mine-Dust-and-You.pdf
Naturvardsverket 2002. Uppfoljning av efterhandlingsprojekt inom gruvsektorn – Atgarder, kostnader och resultat. Rapport 5190 Maj 2002.
EC 2004. Reference Document on Best Available Techniques for Management of Tailings and Waste Rock in Mining Activities. July 2004. European Commission, Directorate-General JRC Joint Research Centre, Institute for Prospective Technological Studies, Technologies for Sustainable
Development, European IPPC Bureau. 511 pp.