Deposited Dust Monitoring
A quarry manager’s guide
By Rachel Cutler, project air quality consultant with SLR Consulting
Dust emissions from quarry operations can cause concern for operators, local residents and regulators. Deposited dust, sometimes referred to as nuisance dust, is potentially a cause of such complaints.
To maintain good local relations and to ensure that residents are not affected by nuisance levels of dust, monitoring schemes are increasingly being written into planning conditions. Not only will monitoring reveal the true extent and nature of nuisance dust, but it can, if necessary, be used in defence by the operator. This article looks into the logistics involved in the planning and implementation of such schemes, and the implications for operators.
Type of monitors and parameters measured
Although numerous techniques exist for monitoring deposited dust, they can be divided into two main categories: automatic and passive monitors. The technique employed will primarily depend on the objective of the scheme and the financial costs involved.
For the majority of deposited dust monitoring schemes at quarry sites, passive monitors are the preferred option. Such monitors are designed to collect deposited particulate matter (precipitated out from the air either by gravity or by contact with water droplets) over a given exposure period, prior to laboratory analyses.
Two parameters often used to measure deposited dust from passive methods are surface soiling and deposition rate. Surface soiling is the measurement of the amount of soiling of a surface and involves the physical coverage, as well as taking into account the appearance of the particles (ie light-scattering properties). Deposited dust only takes into account the mass of the deposit and provides a record of the average deposition rate.
Automatic monitors available for monitoring deposited dust are often very expensive and involve preparation in the field and access to electricity supplies. Continuous samplers, such as the tapered element oscillating microbalance (TEOM), are often used in circumstances where real-time results are required to facilitate a rapid response time.
Case study: Deposition rate and surface soiling combination monitor
A number of simple, cost-effective monitoring schemes that provide robust and reliable results, and which can be operated by on-site personnel, have been designed by SLR Consulting.
At Must Farm, a sand and gravel quarry operated by Hanson Aggregates, SLR implemented such a scheme to meet the requirements of a planning condition. Following correspondence with the local authority, a monitoring scheme of four boundary monitoring locations was agreed, which would assess dust levels at local receptors.
The passive technique of an omni-directional frisbee gauge provided the main structure of the monitor. It collected deposited dust in an up-turned frisbee shape in which dust settled and washed into a collection bottle.
Every four weeks the bottle was replaced and the contents of the removed bottle were analysed by a UKAS-accredited laboratory to produce a dust deposition rate in mg/m2/day for each location.
To increase the effectiveness of this method, an adhesive pad was attached to the shaft of the Frisbee gauge (fig. 1). This pad was able to capture dust flux as well as indicating the primary direction from which dust was originating.
With this monitoring scheme in place, the results provided a monthly average deposition rate, the amount of surface soiling and directional data to help identify the source of the dust. Directionality of dust is of particular relevance when the operator is alleged to be causing the nuisance, as it can, in many cases, demonstrate that a reasonable proportion of coarse dust particles originate from adjacent activities, such as farming or transport.
Deciding on a monitor
Deciding which dust monitor is most suitable for a site depends on many factors. As cost-effectiveness is often a primary operator concern, involving site staff in the monitoring scheme can be a way of significantly reducing the financial outlay involved.
At Must Farm, SLR carried out the preparation for the dust-monitoring scheme, while also training members of site personnel on how to operate the equipment, overcome problems and send off samples to an accredited laboratory. SLR continue to monitor the results, produce summary reports and provide support for site staff undertaking the monitoring.
Monitoring locations
The location of dust monitors is dependant on a number of factors, including the proximity to dust-sensitive receptors, the views of the local authority, the objectives of the scheme, and logistical factors, such as accessibility and the risk of vandalism.
The strategy adopted at Whitehill Quarry, Burford, operated by Smith and Sons (Bletchington) Ltd, located the monitors as close to the two nearby residential receptors as practicably possible. Following sensitive correspondence from SLR, the residents agreed to the installation of the dust monitors within their grounds (fig. 2).
This option reduced the risk of vandalism and also provided representative results of the dust impacts being experienced at each receptor.
Locating dust monitors is very much dependant on site-specific circumstances; monitors may be more appropriate at locations on site, off site or a mixture of both. A mixture of on- and off-site monitoring locations was the approach undertaken at Burford Quarry, also operated by Smith and Sons. As a result of correspondence between SLR, the operators and the local authority, four monitoring points were decided upon, located around the site boundary as well as at surrounding residential properties (fig.3).
Baseline monitoring
Baseline monitoring schemes can be useful to operators in areas where alternative dust sources are present, such as agriculture or transport. If dust monitoring only starts once the quarry site is in operation, elevated levels may be attributed to the quarry – although a high proportion may be background concentrations.
Many planning conditions require the operator to implement a dust-monitoring scheme for a minimum of three months prior to operations commencing. This provides a dataset of baseline dust levels against which a comparison can be made once operations commence.
The monitoring scheme developed and implemented at Whitehill Quarry provides an example of baseline monitoring. The quarry is currently inoperative so the data collated show existing dust levels. In addition to this primary objective, local residents retain peace of mind that the quarry operators are actively seeking to minimize the site’s environmental impacts.
Results and analysis
Often, the two main parameters local authorities request are the rate of dust deposition (mg/m2/day) and surface soiling – percentage of effective area covered (EAC). There are no statutory UK or European Community standards for the control of nuisance dust. In the absence of such standards, the custom and practice guideline for dust deposition used in the UK is 200mg/m2/day. Levels above this should trigger an action by the operator to mitigate the dust source.
It must be kept in mind, however, that the results from a deposition gauge, such as the frisbee gauge, are based upon mass and, therefore, do not take into account the colour or density of the sample, both of which play an important factor in the perception of a dust nuisance.
Similar to dust deposition rates, there are no standard guidelines for what corresponds to an unacceptable level of surface soiling when using such methods as adhesive pads. Studies undertaken provide guideline values to correspond with public perceptions. For example, a surface soiling level of 0.5% EAC per day can correspond to a possible complaint, while 2.0% EAC corresponds to a probable risk of complaint.
Additional analysis options
The results of dust-monitoring schemes can often raise questions over the alleged source of dust. In many cases, the rural location of a quarry site means a sizeable proportion of the dust deposited is from adjacent activities, such as farming.
In addition to directional sampling, such as the use of adhesive pads, if further information is required, additional analysis can be performed on dust samples to assess the colour, size fraction and mineralogy, to trace the source.
Conclusion
Dust monitoring at surface mineral workings is increasingly being requested, either by a planning condition or as a response to local complaints. Monitoring schemes require a high degree of planning to ensure that the results obtained relate to the objective of the scheme. The results can then be used effectively to determine the source of the dust and are in a suitable format to assess against custom and practice guidelines utilized in the UK.
Where appropriate, quarry operators should seek professional advice to ensure that any dust-monitoring scheme that may be needed is designed on a site-specific basis and will help maintain good relations between themselves, local residents and the local authority.
For more information on dust monitoring, contact the author on tel: (01225) 309400; or email: [email protected]
