Air quality – an overview for students
What is dust?
There’s dust in the air all the time, even if you can’t see it. The amount and type of dust varies a lot and depends on many factors, including source, climate, wind direction and activity.
Dust is generated from a range of human activities and natural sources. It may be made up of soil, pollen, volcanic emissions, vehicle exhaust, smoke, or any other particles small enough to be suspended or carried by wind. The stonger the wind, the larger the particles lifted, and the more dust carried.
Planning for the new Martha Mine had to take into account that the mine would be operating very near to people’s houses and properties. The planners had to look at ways that dust from the mine and its activities could affect the neighbouring areas.
Dust monitoring equipment was set up around Waihi in 1982, five years before mining started, so that information could be collated about pre-mining dust levels. This information was used to set the standards for dust levels once mining began.
Dust monitoring continues throughout the operation. Technicians take regular weekly and monthly samples.
Dust particles that are bigger than about 10-20 microns are collected in dust deposition gauges. This is the dust that settles on clean surfaces such as cars and window ledges. It is called ‘nuisance dust’ because it is visible and can be annoying but is not a health hazard.
A dust deposition gauge consists of a frame with a funnel at the top that leads into a bucket below. The funnel collects rainfall, along with heavier particles of dust in the air.
Once a month the bucket is collected and the water is poured through a filter. The filter is dried and weighed. The weight results for each gauge are entered into a database and regularly reported to the regional council. There are several dust deposition gauges that must be checked every month.
Smaller dust particles are collected by air monitors. Tiny particles of dust, less than 10 microns in size, stay airborne for a longer time – from several minutes to several days.
Air monitors are fixed in raised positions, such as on power poles. A motor drives a pump that sucks in air, just like a vacuum cleaner. The air passes through a filter, drawing airborne dust with it.
Once a week the filter is changed. The volume of air that has passed through the pump is noted. The used filter is taken to the laboratory and weighed. The weight results for each gauge are entered into a database and regularly reported to the regional council. There are several air monitors that must be checked every week.
In addition continuous monitoring for PM10/silica has been undertaken by Waikato Regional Council since 2007. Before that, the Council employed a mobile monitoring unit to record data for a few weeks once every two years. The equipment currently used is called a Partisol air sampler. An electric pump sucks in air, just like a vacuum cleaner. The air is first drawn through a special inlet that only accepts particles up to 10 microns and then filters it. The sampler automatically changes filters daily.
How is dust measured? … and what are the permitted levels?
Dust filters collected by environmental technicians are weighed on a set of sensitive scales. The weight of the clean, dry filter is subtracted and the calculations are made.
The dust collected in dust deposition gauges is measured in grams per square metre per month. The Air Quality Management Plan guidelines allow up to 4 grams per square metre per month (4g/m2/month).
The dust collected in air monitors is measured in micrograms per cubic metre per week. The Air Quality Management Plan guidelines allow up to 45 micrograms per cubic metre averaged over seven days (45µg/m3/7days). One microgram is equal to one-millionth of one gram – those scales must really be sensitive!
How is air quality managed?
The mining company and mining contractor take all practical steps to keep down dust levels by:
- applying dust suppression product
- watering haul roads and using sprinkler systems
- dust collectors and filters on drill rigs and crushers
- using a windbreak fence adjacent to the crushers
- keeping stockpiles low so wind is less likely to spread dust
- planting grass on long term stockpiles
- planting pasture, shrubs and trees as soon as rehabilitation areas are available
- washing wheels of all vehicles before they leave the site to travel on public roads.