PRACTICAL DETERMINATION AND LOCATION OF WINDBLOWN DUST SOURCES AND HOW TO ESTABLISH A GOOD MONITORING PROGRAM
This article outlines a practical method of establishing an environmental dust monitoring programme or even an in-depth investigation to establish sources of dust and quantifying the extent of dust fall-out.
Such investigations can be undertaken at landfill sites, mines, slimes or rock dumps, industrial areas, airports and in villages and towns.
Monitoring programs can enable operations to determine unsuspected pollutants, determine bacteriological migration from contaminated landfill sites and composting plants or merely to remain well within the legal requirements. The operations will then be able to design and implement effective suppression measures, once the sources have been located.
The article outlines a systematic approach to the monitoring and sample analysis, offering tips and hints.
With the Environmental Management Bill firmly placing a far greater emphasis on determining and eliminating all sources of airborne or air-carried pollutants and dust, it is vital that a simple, reliable, representative monitoring systems be used at minimal cost.
The airborne sampling of dust using PM10, respirable or total particulate, while being of benefit to the occupational hygienist or health practitioner, offers little help to the environmentalist, as only a small portion of the dust is caught. With these systems only extremely fine particulates are selectively sampled and these could constitute a pollutant level that is only found as a fume or respirable dust. Ideally, the sampling must be continuous with little or no downtime, as the longer the sample, the better the accuracy or representative nature of the sampling.
The following procedure or guide offers a method based on accepted principles and methodology ideally suited to the establishment or quantification of source dust of a variable particulate size, which has proved to be both representative and reliable.
Study the situation
As a departure point there is no substitute for a detailed site examination, a chat with all of the parties involved and time spent in studying the wind patterns and shape of the dump or plant operation.
Residents or workers in the area will be able to provide a remarkably detailed account of occurrences of dust, comment on the weather conditions, or even how much they are affected by the emission. It is noted that while the plant or mine management will tend to minimize the effects of the dust to a neighbouring farmer or village, these neighbours in turn will want to make sure that they maximize their discomfort. Reality usually lies between the two viewpoints.
Should the prevailing winds constitute a straightforward pattern this needs to be considered when locating the monitoring units.
Any ambient dust imported upwind of the mine or plant will also need to be studied in order to establish if the irritant dust actually originates at the mine’s property. It is important that control monitoring of any other possible dust sources be simultaneously undertaken.
Selection of monitors
In the situation presented, we have selected three 4-way fall-out dust monitors that can each monitor the incoming precipitant arriving from four different directions.
In contrast to using a ‘hit or miss’ arrangement of separate open containers and trying to relate the precipitation to the prevailing winds, we note that the most precipitation occurs when the winds slow down or stop rather than while they are blowing at the maximum velocity.
The selection of three units is based on the premise that at least two will be required to actually quantify the dust loads from the various directions while the third will be useful to assist with further determinations of precipitation rates and alternative prevailing wind directions.
Positioning of monitoring and sampling
To assist with the rationalization of the case study, please refer to the sketch (Diagram 1) that outlines the mining operation and dump, together with the prevailing winds and positions for the monitors.
Monitor units A and B are positioned upwind of the mining operation and between this and the farm or village. The third unit C will be mobile to enable the position to be varied.
The combination of units will enable the determination of the movement of dust and particulates around the farm or village and the mine property.
From a study of the diagram, we note that dust reaching the farm or village from the south will potentially contain dust that has been generated by the mine and dump as well as an import from further upwind of the mine property. While the farmer will be tempted to argue that the dust fall-out emanates from the mine, the latter has a case to reduce their export dust value by that being imported from the south on the prevailing wind.
Monitors will assess the incoming dust regardless of the prevailing wind, and it will be of only academic interest to monitor the wind data, obviating the necessity of putting up a weather station. The variable that might be of some interest is the amount of time that there was a wind velocity of less than 0,5 m/s, but this is not usually recorded by the weather station.
Once monitoring starts, samples are retrieved every two weeks from each unit and thus it is possible to correlate the 12 samples against each other. Results are reflected as a mass per area per day unit (mg/m2/day). Over a period of time the captured data can be plotted to indicate trends, which tend to follow an established sine curve pattern between the rainy and dry seasons. Any abnormal peaks that could have resulted from unusual activities on the mine can be immediately noted and attempts can be made to identify the events, ie, veld fires, overburden stripping activities that took place, etc.
Establish the Dust Contents or Fingerprinting
Before this stage, all of the work is mechanical and the next step is to start the detective work, if the exercise is to be of any major value.
By retrieving all of the unit B-South results and making up a composite sample of the filters, (make sure that there is enough filtrate to run a comprehensive multi-element ICP/OES analysis) we can now see exactly what the dust contains and the relevant amount of each of the elements present.
A similar exercise can be done with all of the sample filters constituting the unit-South results. By simple subtraction a good idea of what emanates from the mine can be ascertained:
Cal 1 Total mass (unit B–South) minus total mass (unit A–South) = Dust mass attributable to the mine.
Cal 2 Total silica (unit B–South) minus total silica (unit A–South) = Total silica attributable to the mine.
Calculation 2 can be undertaken for all of the other elements to build up an indicative fingerprint of what the mine dust contains, especially after the winter winds.
To further confirm this fingerprint the exercises can be repeated with samples from unit C-East as well as unit A-East. If four monitor units had been installed, this confirmatory exercise would perhaps be slightly more accurate, covering the prevailing winds during the summer periods in detail as well as in the winter.
Any person undertaking the sampling exercises must also consider other factors as follows:
- The existence of organic substance traces that may give an indication of the type of pollens evident in corresponding samples from the unit A-South and unit B-South. This often offers more definite proof of the dust origin than the chemical element analysis when mine properties are perhaps inundated with invasive vegetation.
- As a similar exercise may have been undertaken nearby, it is worth comparing the results to perhaps gain even more of an insight into the ambient atmospheric dusts. Other concerns may have to be approached in order to access any information that they may have collected.
- Radio analysis of samples can be done to obtain information about the dust’s radioactivity.
- While being far more difficult to assess, it is possible to consider major weather events such as unusual storms in the area or even seasonal events such as the autumn ploughing of the Western Cape wheatlands or the burning of wheat or sugarcane at certain times of the year. These events ‘load’ the atmosphere in these areas and one can then study the movement and occurrence dates at various monitoring stations downwind for considerable distances as the ‘load’ is gradually deposited at differential rates and time periods. Short term monitoring will not normally reveal these events but it is worth considering the influence in a monitoring program over a longer period.
- A natural event such as the occurrence of a locust plague will result in the creation of a central cloud of vegetation fragments that can readily be noted as the product is precipitated further downwind.
- Where toxic substances have been noted or where these have manifested to any large degree, the potentially exposed persons should be biologically sampled for signs of ingestion.
- A final avenue of investigative sampling that can be undertaken with the units involves biological sampling for saprophytic bacteria and microbes, which can become airborne from composting plants, farm yards and abattoir holding pens and sheds. There is much conjecture about the possibly of e-coli (bacteria) becoming airborne and redistributing as a fall-out. This is also currently being researched.
With a common sense approach and time, the use of fall-out monitoring can and does yield more information, allowing a far greater and effective study to be undertaken than any other single sampling method. If used in combination with PM10 or total particulate dust sampling, results can be very conclusive.
About the Authors
The authors, Gerry Kuhn and Chris Loans, are actively involved with the monitoring of most of the 90 stations operating throughout southern Africa.
Gerry Kuhn is an environmental engineer and occupational hygienist who runs the research programme and consults to mines and industrial concerns on a local and international basis. Contact him on 083 308 4764.
Chris Loans is a chemical engineer (UCT) who obtained professional status in 2001. He actively runs the DustWatch program. Contact him on 082 875 0209.
This article was originally published on pages 27-29 of the November 2003 issue of ‘Chemical Technology’, a publication of Crown Publications cc, in Johannesburg, South Africa (Telephone +27 11 622 4770). ‘Chemical Technology’ is endorsed by the South African Institution of Chemical Engineers and is published monthly.