Air Quality & Noise
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 stronger 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 (45 µg/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.
Noise – An overview for students
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 as well as a primary school. The planners had to look at ways that noise from the mine and its activities could affect the neighbouring areas.
Noise levels were monitored around Waihi for seven years before mining started so that information could be collated about pre-mining noise levels. This information was used to set the standards for noise levels once mining began.
Noise monitoring continues throughout the operation. Technicians take regular weekly and monthly readings and other readings are taken when they are needed. The location and number of sites are reviewed from time to time to make sure the monitoring clearly reflects the activities happening on site.
How is sound monitored?
Noise monitoring is carried out with a sound level meter connected to a chart recorder that graphs the sounds as they happen. While noise monitoring the technician has to note down any significant sounds such as traffic, birds, cicadas or barking dogs. The microphones are very sensitive and a lot of care must be taken to see that the sound samples are recorded in the right way.
A fifteen minute sample is recorded at three separate times over one day for each monitoring site. The technician must also use an anemometer to check the wind speed and wind direction for each sample. This is because wind speed and direction can affect noise levels.
After samples are recorded the information is loaded into a computer and later downloaded and the results sent to the Hauraki District Council.
How is sound measured? What are the permitted levels?
The basic unit of sound is the decibel (dB). The A-weighted sound level (dBA) is a good measure of sound that is detectable by the human ear.
L10 denotes the sound level which is equalled or exceeded for 10% of the measurement time. This level may be considered as the average maximum sound level. L95 denotes the sound level which is equalled or exceeded for 95% of the measurement time. This level may be considered as the average minimum sound level. A sound reading of L10 75 dBA means that the average human ear is hearing up to 75 decibels for at least one tenth of the measurement time.
Noise monitoring is carried out at houses close to mining activities. The permitted noise levels vary, depending on the activity and the time of day. At night time the limits for noise are lower (L10 40 dBA) than for the day time. The day time noise limit is either L10 55 dBA or L10 50 dBA, depending on when and where the activity is taking place.
How is noise managed?
The mining company and mining contractor take all practical steps to keep down noise levels by:
- choosing and maintaining good equipment so the engines are running quietly and efficiently
- building noise bunds (earth barriers) and planting them with trees
- building close-boarded fences and using acoustic materials where needed around noisy machinery or on perimeter fencing
- keeping stockpiles low so that any machinery working on the stockpiles stays below the level of the noise bunds
- reviewing excavation methods and using different methods when mining machinery is near the crest of the open pit.