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8 Tier 3 Full Assessment Process

The aim of a tier 3 assessment is to provide reasonably accurate estimates and a detailed assessment of the likely air quality impacts associated with a proposal. This is usually done through the use of detailed emission information, a topographical profile, dispersion modelling and background air quality and meteorological data. For any aspect of the assessment where detailed information is not available, or is not required, it is appropriate to adopt the conservative assumptions discussed for tier 2 assessments.

An overview of the tier 3 assessment is shown in Figure 8-1.

Figure 8-1: Tier 3 assessment process

Thumbnail of image. See figure at its full size (including text description).

8.1 Characterising the discharges to air

Characterising the discharges to air includes:

  • identifying all air discharges associated with the development (they may be point sources such as stacks and vents, area sources such as ponds or disturbed soils, or volume sources such as fugitive emissions from buildings)
  • describing the sources of the discharge (including all process factors influencing the quantity and nature of the discharges)
  • detailing the nature of the emissions (pollutant types, concentrations and mass emissions, emission temperatures, velocities, particle sizes, etc)
  • detailing the measures employed to prevent and minimise the discharges.

The following discussion provides guidance on the type of information that should be provided to describe the proposed development and the potential sources of air emissions.

8.1.1 Description of the proposal

The description of the proposal should be sufficient to enable a full understanding of the application from an air discharge consent viewpoint, and should also provide sufficient information to ascertain whether any other consents are required.

The depth of information required will vary depending on the type of activity, although for industrial activities the following should generally be provided:

  • a detailed process flow diagram or, if more appropriate, a process and instrumentation (P&D) diagram showing all existing and/or proposed plant and equipment included in the proposal, including emissions control equipment and emission points
  • a written summary of the process flow from raw materials to final product, focusing on aspects that give rise to emissions to air
  • details of alternatives considered and reasons for their rejection
  • details of any relevant historical information, including past changes to the activity
  • details of any proposed changes to an existing activity
  • any relevant timeframes or constraints for undertaking the activity
  • details of all emissions control equipment, including design-criteria calculations and design drawings
  • details of potential upset/emergency conditions, including their influence on emissions
  • any mitigation and/or preventive measures undertaken on site for both ordinary and accidental emissions to air, including management techniques (eg, management plans), alarms, interlocks, monitors and control equipment
  • maximum and normal processing capacities
  • maximum and normal ratings, capacities and throughput of all major plant equipment, including boilers, air discharge control equipment, driers, mixing tanks and crushing plant
  • details of any materials-handling procedures and mitigation measures in place for raw, intermediate, by-product and finished materials.

Because the application is defined by the process description, consent can only be granted for what is applied for. Therefore, all details possible relating to current and proposed operations should be included to enable all matters to be properly considered. Any proposed changes to an existing process should be highlighted.

Consent applicants should recognise that data provided in an assessment will often form the basis for setting the consent conditions. For example, fuels in a combustion process may be limited to those identified in the application, or dispersion-modelled emission rates may be adopted as stack emission limits. It is therefore important that any application reasonably reflects all anticipated operational scenarios for a development in order to provide for the desired flexibility.

8.1.2 Defining the discharge for dispersion modelling

Section 4.1 of the Good Practice Guide for Atmospheric Dispersion Modelling (Ministry for the Environment, 2004a) provides a description of the information required to define the variety of discharge points for the purpose of dispersion modelling.

An assessment of emissions from industry to air should also address abnormal or uncommon emission scenarios, including start-up, shut-down, upset conditions and emergency release. These scenarios often result in elevated emissions, or emissions of chemical intermediates that would not normally be released. It is often appropriate to assess these using a risk-based (probabilistic) approach; that is, considering not only the consequences of the release but also the likelihood of it occurring. Consideration of these occurrences is often tailored to a specific situation; for example, considering only short-term average assessment criteria commensurate with the duration of exposure.

Where a project goes through different stages of development, such as in a mine or an industrial process with anticipated production growth, these project-life operating and emission changes should also be addressed in the assessment.

Compliance Monitoring and Emissions Testing of Discharges to Air (Ministry for the Environment, 1998) provides guidance for obtaining pollutant emission rate and concentration data by measuring an existing source. If the assessment will be based on previous monitoring results, the data collection method should be audited against the methods specified in the guide.

Proprietary process simulation software can also provide useful emissions data (eg, GT PRO for gas turbine emissions). A number of air quality professionals also make use of combustion calculation software. [For instance, Flue 2, for the generation emission data from combustion processes. The freeware is available from Terry Brady Consulting Limited (terry@ebg.pl.net).]

As noted above, the Ministry for the Environment provides guidance on both characterising release points for the purpose of dispersion modelling and on emissions monitoring (Ministry for the Environment 2004a and 1998, respectively).

8.2 Characterising the receiving environment

8.2.1 Existing air quality

Information on existing air quality is essential for assessing the effects of new industrial developments. It is not just the incremental increase in air quality that is used in assessing discharges, but the cumulative impact of any increase to existing pollution levels and how that compares with the appropriate air quality criteria.

When existing air quality data are required

Existing air quality should be considered in all assessments of discharge to air. The degree to which it should be addressed is influenced by the following considerations.

  • The nature of the discharge - large discharges or discharges of pollutants of high toxicity, which may have the potential to adversely impact the environment, would be expected to require a thorough assessment of the existing air quality.
  • The anticipated air quality - areas that are anticipated to have poor air quality due to a combination of existing emission sources and/or adverse terrain or meteorology would be expected to require a more robust definition of the existing air quality.
  • Sensitivity of the receiving environment - where discharges have the potential to affect highly sensitive receiving environments (see Table 6-1), existing air quality would be expected to be well-defined.

It is the combination of these considerations that determines the extent to which existing air quality should be addressed. A small emission of a low-toxicity pollutant within a commercial/ light industrial area, for example, might only require a qualitative statement on existing air quality identifying the reasons existing air quality is anticipated to be good. Conversely, a large-scale industrial source with the potential to impact on residential suburbs might be expected to provide good quality, representative and quantitative air quality data.

As well as the generic considerations identified above, where a proposal might result in increased PM10 emissions within a gazetted airshed (as defined under the Standards), existing air quality is likely to be given greater attention.

Identifying existing air quality

The range of options for generating air quality monitoring data, ranked in order by the increasing effort required to obtain the data, are:

  • use surrogate data from locations with air quality characteristics representative of the area of interest
  • use existing monitoring data sets for the area of interest
  • use atmospheric dispersion modelling to predict air pollutant concentrations due to existing sources (as well as the concentrations due to the proposed development)
  • commission a monitoring programme specifically for the purposes of the consent.

Pre-existing air quality data can be obtained from a range of sources. The consenting authority will usually have the best knowledge of the full range of data available within its region, and will also be able to provide an opinion as to whether the pre-existing data are sufficient for the assessment proposed.

The range of air quality data sources is identified in section 6.4.1. of the Good Practice Guide for Atmospheric Dispersion Modelling (Ministry for the Environment 2004a). Atmospheric dispersion modelling may be the preferred approach for estimating existing air quality where:

  • there are a small number of existing emission sources in the area for which reliable emission data are available
  • any contribution to ambient levels from other hard-to-characterise sources (such as vehicle emissions, domestic fires or dust from wind erosion) is negligible.

This situation would be unlikely to occur in urban areas of New Zealand. Again, the Good Practice Guide for Atmospheric Dispersion Modelling provides advice on the appropriate application of dispersion models.

Overall, the pollutants released from the proposal and the assessment criteria that are available for those pollutants determine the type of existing air quality data required for any assessment.

Reviewing existing air quality data

Guidance relevant to assessing the quality of currently available ambient air quality data as well as quality assurance and control procedures for the collection of new air quality data are provided in the Ministry for the Environment's (2000a) Good Practice Guide for Air Quality Monitoring and Data Management.

The assessment criteria often include requirements for the air quality monitoring technique to be used. For example, the Standards contain specific requirements for the monitoring of pollutants within gazetted airsheds. The Ministry for the Environment's Ambient Air Quality Guidelines also contain recommended monitoring methods.

The use of such methods reduces uncertainty and minimises inaccuracy. Before using existing air quality data in an assessment, it is important that the monitoring technique and protocols be audited against the requirements of the Standards and the Good Practice Guide for Air Quality Monitoring and Data Management to demonstrate that the existing air quality data are of appropriate quality.

The location of a monitoring site and the time of the monitoring also affect how representative existing air quality data might be. The site should be representative in terms of location (ideally being within the affected airshed), but also representative in terms of land use, physical setting, etc. The specific location of the monitoring site (eg, its proximity to major sources such as roads) will also be important.

The time of the monitoring is also relevant, in that data collected at the site in previous years may not be representative if the character of the area has changed markedly since monitoring was last undertaken. For example, historical data from a roadside monitoring site in an area that has experienced significant traffic growth would no longer be representative of current levels.

Trends in air quality should be considered, and it is preferable for several years of data to be analysed so that any improvement or deterioration of the air quality of an area can be ascertained. As a minimum, one year of data could be used if other longer-term monitoring sites in a similar location can be used to provide an indication of long-term trends. Ideally, 10 years of data are required to determine trends.

As noted above, monitoring data should be reviewed with reference to techniques identified in the Standards and other Ministry for the Environment guidance (Ministry for the Environment, 2000a , 2002 and 2004a).

8.2.2 The built environment

Sensitivity to air quality impacts will vary with land-use type. For example, residential land use (including schools) will typically have greater sensitivity than an industrial setting.

The land use surrounding a proposed development should be reviewed and described in any assessment of air quality impacts. The review will provide both an indication to any developer of the likely acceptability of, or objection to, a proposal and a guide to the depth of consultation required. The sensitivity will also be one factor that influences the level of assessment of environmental effects required for a proposed development.

Land-use zoning must also be reviewed in the relevant district plan to ascertain whether the proposal (in its proposed location) is permitted. District plans provide maps that generally zone the district by land-use types. District plans also provide rules that identify the limitations on land use, either within those zones or in the wider district.

Authors of district plans and developers should also be aware of the issue of 'reverse sensitivity'. Reverse sensitivity relates to sensitive land uses encroaching on, for example, industrial facilities. Allowing such encroachment is seen as having potentially adverse effects on the health, safety or amenity values of people, as well as potentially adversely affecting the economic and safe operations of industries.

Case precedents in the Environment Court (see ARC vs ACC, RMA10/97) mean that new designations of sensitive land uses within the vicinity of industry may be turned down on the basis of reverse sensitivity. The use of buffers minimises the effects of reverse sensitivity, and it is recommended that buffers be owned by the industry creating the discharge.

8.3 Exposure estimates

8.3.1 Dispersion modelling

Atmospheric dispersion models are used to estimate contaminant concentrations downwind of a discharge source. The prediction of pollutant concentrations using atmospheric dispersion models has been covered in detail in the Good Practice Guide for Atmospheric Dispersion Modelling (Ministry for the Environment, 2004a). It contains guidance on the application of models for a range of emission types, and meteorological and terrain scenarios. Of particular note is Recommendation 59, which deals with the issue of model accuracy and how it should be dealt with in a regulatory context.

8.4 Assessing the effects

8.4.1 Reconciling predicted and background concentrations against assessment criteria

Background air quality data and predicted pollutant concentrations are considered together against the selected assessment criteria. Adding the background data and predicted results to provide an estimate of the cumulative impact for comparison with the selected assessment criteria, is reasonable for annual average concentrations. For short-term concentrations, this simplistic approach is appropriate where the criteria are not breached, although it is a very conservative approach and a more accurate assessment may be necessary where compliance is an issue.

The above approach can lead to an overly conservative assessment due to issues relating to the spatial and temporal coincidence of background and predicted concentrations, as follows.

  • Spatial co-incidence problems − it is often difficult to know whether the background data are representative of the point at which the modelled peak occurs. In general, they will not be located in the same place, so adding the two will overestimate actual future concentrations.
  • Time co-incidence problems − both modelled and background concentrations vary with the time of day due to factors such as meteorological patterns, operational variations and changes in background emission sources (eg, traffic peak hours). In most cases, the peak caused by a point-source emission does not occur at the same time as the background peak. Adding the two together may again overestimate the future concentration.

For the highest percentiles (ie, concentration values close to the peak short-term concentration of a year's worth of such concentration predictions), simple addition can overestimate the source contribution, and, in general, the overestimate is more severe for the higher percentiles.

The best predictive assessment technique is to use hourly, sequential ambient air quality monitoring data that is recorded in the airshed of interest, and add the hour-by-hour predicted concentrations. These predicted concentrations should be made using meteorological data recorded at the same time as the recorded air quality data. Where data are available, such an approach is recommended.

It is rare for all of this data to be available, however, and the UK Environment Agency study investigated some alternative approaches. A simpler approach, which gave better accuracy than some, and equal accuracy to the best alternative statistical approach, was to add the predicted short-term average concentration to twice the annual average background concentration. This may not be generally applicable in New Zealand, and more locally specific techniques could be used. For example, if the air quality is known through monitoring in some nearby location that has obvious geographical and emissions similarities, then this can be used as a proxy for the background concentration.

8.4.2 Atmospheric chemistry

The chemical transformation of emissions during transport in the atmosphere can be another important consideration. For example, the perception of odour can change between source and the receiving environment due to chemical transformation, although there is no practical way to assess this effect.

Perhaps the most commonly encountered issues with regard to atmospheric chemistry are:

  • the oxidation of NO to NO2 when NOx (oxides of nitrogen) are released from industrial emissions
  • the formation of the secondary pollutant ozone (O3) following release of NOx and volatile organic compounds (VOCs) from industrial and other anthropogenic sources
  • the formation of secondary particulates from sulphur and nitrogen discharges.

Methods for assessing these issues were discussed and identified in section 4.3.6 and Appendix C of the Good Practice Guide for Atmospheric Dispersion Modelling (Ministry for the Environment, 2004a).

To estimate NO2 concentrations from modelled NOx concentrations, the methodology proposed in Appendix C of that guide is recommended. This is a simply applied, conservative approach based on the US Environmental Protection Agency O3-limiting method, together with knowledge of the available background O3 concentrations within air masses moving off the oceans and across New Zealand.

The formation of O3 following the release of NOx and VOCs from anthropogenic sources is a large-scale regional effect, affecting rural areas surrounding major urban centres (where the precursor chemicals - NOx and VOCs - are released). The estimation of O3 creation is technically complex due to the wide range of chemical reactions involved. Individual industrial emissions would not normally be expected to have a significant impact on O3 creation in isolation of other urban sources, and only large industrial emissions sources of NOx and/or VOCs might be expected to have a discernable additional effect. Typically, only major industrial emission sources in, or near, large urban areas might be expected to assess such effects.

Complex models are available to assess the photochemical production of O3. Consenting authorities might find such tools useful for airshed management of O3. These tools are also available for developers for assessing potential effects of large-scale industrial emission sources of NOx and VOCs in major urban areas.

Section 4.3.6 of the Good Practice Guide for Atmospheric Dispersion Modelling (Ministry for the Environment, 2004a) recommends using models such as CALGRID and UAM-V. These are identified as having sufficiently complex chemistry schemes to enable examination of small changes in urban emissions generally associated with an individual industrial source.

A further model to consider is CSIRO's IER-Reactive Plume Model. The model is identified in the NSW Department of Environment and Conservation's Approved Methods for the Modelling and Assessment of Air Pollutants (2001 and 2005). The model is identified as being suitable for predicting the effects of industrial source emission on ground-level O3 concentrations.

8.4.3 Non-human health considerations

Air pollution effects on ecosystems

Section 4 of the Ambient Air Quality Guidelines (Ministry for the Environment, 2002) provides critical levels for protecting ecosystems from sulphur dioxide, sulphate particulate, nitrogen dioxide, ammonia, ozone and fluoride. The Effects of Air Contaminants on Ecosystems and Recommended Critical Levels and Critical Loads (Ministry for the Environment, 2000b), provides some guidance on methods for calculating pollutant deposition rates from predicted or ambient monitoring results, and guidance for assessing whether a discharge is likely to cause adverse effects on ecosystems.

In Europe and North America, the effects on sensitive ecosystems of acid deposition and elevated pollutant concentrations from industrial and other anthropogenic sources have been subject to legislative controls for some decades. Similar effects in New Zealand are not so evident, although they have been reviewed in a number of Ministry for the Environment technical reports (eg, Ministry for the Environment, 1999b and 2000b).

Much of the work has drawn on knowledge of the effects on non-New Zealand (North American and northern European) plant species. The recommended critical levels and loads and the provisional guidance on assessing deposition and its effects given in The Effects of Air Contaminants on Ecosystems is based on this knowledge. There is scant information on either the effects of air pollutants on native New Zealand species or the current level of pollutant deposition or concentration in New Zealand's natural environments. The robustness of any assessment of air pollution effects on ecosystems in New Zealand is therefore very vulnerable to these knowledge gaps. Despite these limitations, it is good practice to assess potential effects on ecosystems for any significant source that may have an impact on sensitive ecosystems.

Global climate change

Sections 104E and 104F of the RMA place climate change outside of the remit of consenting authorities in their consideration of discharge consents. The assessment of effects of greenhouse gas emissions from industry on global climate change is therefore outside of the scope of this document. Policy measures to control the emission of greenhouse gases are developed and led by the Ministry for the Environment.

Ozone depletion

The 1987 Montreal Protocol is an international agreement under which substances that deplete the ozone layer are being phased out.

The ozone layer, which sits about 15−30 kilometres above the Earth, reduces the amount of dangerous ultraviolet light which reaches the Earth from the sun. Too much ultraviolet light can cause skin cancer and cataracts in people. It also distorts plant growth, damages the marine environment and leads to the breakdown of materials such as plastics.

New Zealand has ratified the Montreal Protocol and implemented its objectives through the Ozone Layer Protection Act 1996 and the Ozone Layer Protection Regulations 1996. This legislation controls ozone-depleting substances to prevent their release to the atmosphere through bans on their import and use, etc. Site-specific assessments of the effects of releasing such substances to the atmosphere should therefore not be necessary. The full list of ozone-depleting substances can be found in the Regulations, but these include several chlorine compounds, and methyl bromide, which is still in wide use in New Zealand as a fumigant.

8.5 When a health risk assessment is required

In some situations it may be necessary to undertake a more comprehensive air pollution health risk assessment as part of a detailed study. This would include determining exposure and dose via a number of different pathways (inhalation, dermal, ingestion, etc), assessment of dose-response data, and characterisation of the health risks from the exposure and dose assessments.

The ambient air quality standards are health-based standards, which are intended to provide a guaranteed level of protection for the health of all New Zealanders. In most circumstances, it is appropriate to assess the potential health effects of discharges to air from industry by comparing model predictions with the ambient air quality standards and, where appropriate, the Ambient Air Quality Guidelines. An air pollution health risk assessment is typically only required if the Standards or guidelines are breached.

Health risk assessment should not be confused with health impact assessment, which may be required for significant projects or strategies. Health impact assessment is a formal approach used to predict the potential health effects of a policy or project, with particular attention paid to impacts on health inequalities. Guidance on undertaking health impact assessments for policy development is available from the Public Health Advisory Committee (www.nhc.govt.nz/PHAC/phac_pubs.html).

Circumstances in which a more comprehensive air pollution health risk assessment is recommended include when:

  • there is a significant discharge of contaminants with no clear threshold for adverse effects
  • community consultation outcomes and/or plan provisions require this
  • there is a significant discharge and/or background concentration of contaminants that are toxic, carcinogenic, teratogenic, mutagenic or bioaccumulative
  • background ambient air concentrations already approach or exceed guideline or target levels.

The circumstance identified in the last bullet point needs special consideration, particularly if the guideline levels are the same as the ambient air quality standards. In this case, it may be necessary to ensure that air quality is not further degraded as a result of the proposal (by implementing offsets, for example).

Health risks assessments are specialised tasks and are typically only undertaken for large, or particularly toxic, discharges. This subsection describes some of the issues that might be encountered, but it is recommended that additional expert assistance be sought for any full health risk assessment.

8.5.1 Introduction

A risk assessment is a scientific process for evaluating exposure to an agent, and the likely effects of this exposure. Consideration is given to the population potentially exposed, the method of exposure, the dose received, and the toxicological effects of such an exposure. Through the risk assessment process, information may be provided to policy makers and managers in order to help make decisions on the management of human health and environmental risks. Quantitative health risk assessments are highly technical and complex studies, and should be undertaken by qualified professionals with experience in this specialised field.

For air exposures, health risk assessment is the backbone of the process used to derive ambient air quality standards and guidelines, and for plant design limits used throughout the world. Ambient air quality standards and guidelines are usually based on conservative assumptions (eg, lifetime exposure) and sometimes take into consideration sensitive receptors such as children and the elderly. For particular chemicals, ecological considerations may be more stringent than human health, such as in the case of fluoride emissions or some organochlorine compounds like DDT. Consideration is also given to the scale of exposure, because ambient air quality standards and guidelines are set to protect large populations (eg, entire cities).

It should be possible for most assessments to be adequately carried out using air dispersion modelling and comparison with appropriate ambient air criteria, or by comparing in-stack air concentrations with design limits. There are, however, a number of situations where a site-specific risk assessment may be required. These are discussed below.

8.5.2 Guideline criteria exceeded or nearly exceeded

Comparing ground-level concentrations with ambient air quality criteria would usually precede any formal site-specific risk assessment, and usually becomes the basis for triggering this course of action. If the maximum cumulative (including background), predicted ground-level pollutant concentration exceeds or nearly exceeds (say 80 percent of) a guideline value, then consideration should be given to conducting a site-specific health risk assessment.

There are a number of reasons and considerations for conducting a site-specific risk assessment.

Exposure averaging time

It is important when modelling the dispersion of air pollutants that the appropriate averaging time is used to match the averaging time for toxicity and people's likely exposure. If toxicity is associated with long-term chronic exposure, then an annual, average concentration should be used to estimate health risk. Similarly, if the toxic effects occur only at times of day when people are not present, then this should be taken into account in the total risk exposure assessment.

Sensitivity analysis

There is uncertainty in any risk assessment and dispersion modelling. Evaluating this uncertainty by examining the sensitivity of key parameters is important in any risk assessment. By varying modelling and risk assessment parameters, additional information may be obtained on the sensitivity of the risk assessment results to these factors.

Multiple chemical exposure

Most standards and guidelines are based on the assumption that exposure to a chemical is independent of any other chemical exposure, but this is not always the case. Cumulative effects may occur between chemicals that exhibit similar toxicological effects on target human organs. In addition, incremental cancer risk is usually added for all non-threshold (cancer-causing) chemicals, for all exposure pathways.

In order to determine whether multiple chemical exposure is a potential issue, the individual percentage exceedances may be totalled to determine a hazard index. For example, if the ground-level concentration of one chemical is 60 percent of the guideline value and another chemical is 70 percent of its guideline value, then individually both chemicals satisfy the requirements for compliance, but combined they exceed with a total of 130 percent. A risk assessment may therefore be required to evaluate the health endpoints of these chemicals to determine if cumulative effects are likely to be significant.

Unlisted chemicals

Ambient air quality standards and guidelines usually cover comprehensive lists of chemicals, but compared to the hundreds of thousands of known chemicals in common use the list is quite small. Ambient air criteria have been derived for the most well-known air pollutants associated with industrial processes, but in the event that an air quality assessment is required to assess a chemical for which there are no criteria, a risk assessment approach may be used either to estimate risks from a particular exposure scenario, or to derive an acceptable ambient air concentration for that chemical.

Change of default exposure assumptions

Design limits for industrial plant are usually based on the most conservative assumptions to protect the majority of the general population, including the sick and young children. Exposure is usually assumed to occur for 24 hours per day, 365 days per year for 70 years (ie, a continuous lifetime exposure). This is usually the requirement where emissions impact on a residential neighbourhood.

In commercial and industrial areas, however, long-term exposure usually does not occur to children. Likewise, exposure to an individual usually does not occur for 24 hours per day, every day. Hence, through the use of risk assessment it can be shown that the risk posed by emissions is generally less to commercial/industrial workers than to residents and children. As a result, in some situations it may be possible to allow ground-level concentrations greater than the ambient air criteria in these regions. Risk assessment may be used to derive these alternative criteria.

Care should be taken that the toxicity endpoint is appropriately selected. Air quality criteria for chemicals based on acute (short-term) exposure impacts cannot be modified in this way.

Exposure duration and non-threshold chemicals

When assessing exposure to emissions it is important to consider the time basis of the standard or guideline values. If the dominating consideration is toxicity, then exposure duration becomes an important consideration, particularly for non-threshold chemicals. Non-threshold (also known as carcinogenic) chemicals are potential cancer-causing agents that have been assumed to have no safe threshold for a potential health risk.

The non-threshold model of toxicity assumes there is a calculable probability of an adverse health risk, no matter how small the dose. The toxicity criteria for such chemicals are represented by slope factors (SF) or the inhalation unit risk (UR). For a particular dose, the incremental lifetime cancer risk may be calculated and compared with predefined goals for acceptable risk.

In New Zealand, an acceptable environmental risk for exposure to environmental pollution of 1 in 100,000 has been adopted by the Ministry for the Environment in a range of guidelines for the management of contaminated land, most recently in the Guidelines for Assessing and Managing Petroleum Hydrocarbon Contaminated Sites in New Zealand (Ministry for the Environment, 1999a).

The significance of incremental cancer risk is that it is averaged over a lifetime. Conservative, ambient air quality goals assume a lifetime of exposure (70 years by convention). However, if exposure is expected to be of shorter duration, then the incremental cancer risk will be less than the default 70-year exposure scenario.

Generally, industrial works must be assumed to be operating for 70 years to be conservative, as there is no way to know the real operational duration of an industry. However, sometimes a shorter duration is known. An example is a mobile thermal desorber unit operating at a contaminated site. These units may be used to treat soil contaminated with heavy organics (eg, dioxins). The unit will only be operational for the duration of the remediation project (say five years for a large site) and not the default 70 years continuous exposure inherent in guideline values. While a guideline value may be exceeded, the incremental lifetime cancer risk may be within acceptable levels since exposure will only occur over five rather than 70 years.

Particulate deposition

Particulate deposition by itself is generally not a major concern, except where it has an impact on amenity. However, non-volatile toxic chemicals can be emitted in the form of particulate matter, or adsorbed on the surface of particulate matter. Particles smaller than 10μm in diameter may be inhaled and trapped within the respiratory system. Guideline values for ground-level pollutant concentrations are usually based on inhalation exposure only. However, if the emission is primarily in particulate form, then accumulation of contaminants in soil occurs over time, and exposure from other pathways is then possible.

A site-specific risk assessment may be required if it is determined that accumulation of a toxicant in soil could potentially become high enough to pose a health risk to the occupants of the premises. In such situations, the exposure needs to consider not only the inhalation pathway but also ingestion, and even dermal contact with contaminated soil.

Where it is known that home-grown produce is grown, contaminant uptake and produce ingestion also need to be considered. In addition, special consideration needs to be given if the location is a primary production site for food, such as milking cows or farm crops, as there are strict guidelines with respect to residual contamination in food products (Food Standards Australia New Zealand, 2001).

To determine if there is a potential issue from particulate deposition, soil chemical criteria may be used as an indicator. Using dispersion modelling techniques it is possible to estimate the mass of the chemical deposited in a particular location over a lifetime (70 years). By assuming a soil-mixing depth (say 5cm), a soil concentration can be estimated. This can then be compared to national or international soil criteria. If the resulting soil concentration is within 10 percent of the soil criteria for residential land use, this is considered significant enough to warrant a site-specific risk assessment. A site-specific risk assessment should also be conducted for special circumstances, such as deposition on crop production.

Things to consider in the risk assessment are the particulate size distribution and the locations of greatest deposition. If the particulate is primarily fine fractions, then inhalation would be potentially of greatest concern. If particulates are coarse, then inhalation risks would be less significant as only small particles are likely to be inhaled and the potential accumulation of chemicals in the soil might be more significant. Both degradation and bioaccumulation of chemicals also need consideration in the risk assessment.

Health risk assessments are very specialised tasks, and are often designed to account for very specific local concerns. Generic approaches, or recommended methodologies, are beyond the scope of this good practice guide.

8.6 Accuracy

Air quality assessment is a highly technical process. By way of illustration it relies on:

  • monitoring ambient air quality
  • monitoring meteorology
  • numeric prediction of meteorology by prognostic and diagnostic models
  • monitoring stack gas and other source emissions
  • estimating emissions (combustion calculations, simulations, etc)
  • numeric prediction of plume dispersion in the atmosphere.

Accuracy is improved by adopting good practice techniques and equipment. The areas identified above are largely covered in the Ministry for the Environment good practice guides on ambient monitoring, emissions monitoring (Ministry for the Environment, 1998) and dispersion modelling (Ministry for the Environment, 2004a) (see Figure 1-2 in section 1).

Standard quality control techniques, such as the use of calculation checking and aspects as basic as checking that model data are correctly input, are extremely important and should be used. It is also recommended that within an assessment report, each aspect of an assessment is auditable and repeatable so that the approach, assumptions and calculations can be independently reviewed.

In summary, an assessment report should include, or be supported with, supplementary reports or data including:

  • emissions testing reports
  • ambient monitoring reports
  • equipment specifications
  • model output files
  • all input data
  • copies of spreadsheets and calculations
  • electronic model data input files, etc.

When using dispersion models in situations where there is a reasonable degree of uncertainty about any input parameters, sensitivity analysis should be undertaken. Model runs should be carried out simulating the higher and lower boundaries of expected input parameters (such as emissions estimates or stack temperature), as well as the best estimate. Such sensitivity analysis can improve the confidence in an assessment, particularly where it is used to guide the management of uncertainties. This is particularly important where there is vulnerability to adverse effects at the upper boundary of input estimates.

For example, a process might be new and emission levels based on simulation. Sensitivity analysis might be undertaken by modelling both the expected and the more unlikely upper estimate emissions. Where such analysis predicts ambient concentrations to be above the assessment criteria for the upper case estimate only, a management response would be expected. The management response might require continuous emissions monitoring under a consent condition to verify the best estimate of emissions, with an associated emission management response agreed in the event that the upper emission estimate is subsequently found to be true.

8.7 Reporting a tier 3 assessment

The results of a tier 3 assessment should be included in any AEE. The report should summarise the findings of the assessment, including the basis for traffic information, air quality information, any assumptions and their justification. Section 4.4 describes the recommended content of an assessment report. The size and nature of the report depend on the project, but for any tier 3-type assessments each of the sections described will be included.

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