Contents
| AIR HYGIENE REPORT no. 10 | |
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Contents |
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2.1 Method selection
A well designed monitoring programme will aid the determination of cause and effect relationships. There is often a need for pilot studies to determine usefulness of different biological materials and approaches in detecting environmental pollution. As demonstrated in the preceding chapters several important factors therefore need to be considered.
When devising an air quality biomonitoring programme using plants a major choice lies between using passive or active biomonitoring. Passive monitoring is generally quicker, simpler and may allow the assessment of long-term pollution exposure. In contrast, active methods such as transplantation exercises generally represent a pollution regime over a short period of time but may allow quantitative assessments such as deposition rates to be made. In addition the genetic state and physiology of the plant is known and results can be more reliably related to air pollution. However transplantation monitoring requires the use of control plants for comparison. References
As illustrated in the above chapters several air quality biomonitoring methods are available. Monitoring can be qualitative or quantitative and can employ single indicator species or use community changes. Other methods include physiological/biochemical plant responses or visible injury as indicators of air pollution. By analysing element content from plant tissue samples at different distances from a pollution source, the type of pollution and the size of the fallout zone can be determined.
The choice of method depends among other factors, on the purpose of the survey, the size of study area, the resources available and the desired detail of the output.
Careful selection of plant bioindicator species during the design of a monitoring programme enables not only the identification of the pollutants but can supply approximate estimates of the pollutant dose, the strength and location of the polluting source and assist in the demarcation of the spatial and temporal distribution of the pollutant. References
The sensitivity and tolerance of plant species is fundamental to their selection. Bioaccumulative indicators tend to be tolerant to the pollutants under investigation, whereas sensitive species indicate air pollutants by showing recognisable responses. The selection of a species will depend on the techniques employed and the objectives of the monitoring programme. For example, if the sole reason for a study is to determine whether air pollution is having an impact or not, then it may be useful to observe all sensitive species. Alternatively, the most sensitive species with the widest distribution are the preferred bioindicators in mapping sources of pollution. Species selection will also depend on whether an array of air pollutants are the suspected contaminants or a single pollutant is responsible. In metal deposition biomonitoring the species utilised and its effectiveness will depend to an extent on the elements to be monitored, certain species being better bioaccumulators of a particular element than others.
In general, bioindicators should show a distinct, easily measured response to a pollutant and the response should be measured with an acceptable accuracy and precision. Other factors which affect plant species selection include the availability of the species and ease of sampling.
The density and location of sampling sites will depend very much on the type of survey required by the monitoring programme. Larger scale surveys covering larger areas will obviously require more sites than studies investigating point emission sources. In the latter, sites are frequently spaced along transects or gradients in relation to the pollution sourceIn general intensity of sampling sites should be adequate to detect gradual changes along the study area. If indigenous species are to be utilised, the number and location of sites will depend on the natural distribution of the species whereasif transplantation techniques are used, choice of sites are at the discretion of the investigator.
The collection of additional environmental data at a sampling location is often necessary and will often aid the interpretation of results.
Biomonitoring surveys can generate large quantities of data. Data analyses and interpretation methods should be addressed at the earliest stages of sampling design. Semi-quantitative and quantitative indices are a useful summary tool and allow comparisons between datasets. Indices often involve less fieldwork and provide good baseline data (Miller, 1989). A good biological index system is ideally simple, rapid, robust and user friendly but based on sound mathematical reasoning. According to Muir and McCune (1987) the ideal index uses quantitative information which is equally weighted or carefully weighted.
The application of statistical analytical methods should be considered. For example multivariate analysis techniques have successfully been used to assess the importance of air pollution in causing plant responses in relation to other environmental factors. For example, multivariate analyses techniques such as factor analysis have been used in the accurate interpretation of results from large-scale moss surveys by including an assessment of the contribution from other sources. References
Chemical analysis of plant tissue is generally applicable to bioaccumulative monitoring studies. Analysis of tissues for sulphur, nitrogen, heavy metals or organic compounds has been used in air quality biomonitoring.
Over the years a number of lessons have been learned. These include:
The choice of analytical method will depend on the purpose of survey. Some analytical methods are non-destructive and are useful for repetitive surveys such as baseline studies. Such samples can also be archived and used at a later date for additional analysis. Alternatively although destructive techniques result in the loss of the plant they may be more effective in achieving the desired results.
When estimating elemental content in plant tissue, the reliability of procedures can be assured by including measures such as: use of standard solutions; use of blanks; reanalysis of selected samples and the standard solutions used in calibrating the instrument after analysis of every five samples for atomic absorption spectrophotometry (Rao and Dubey, 1992).
Effective quality assurance during a biomonitoring programme is essential to the production of high quality data. Quality assurance procedures are required for two reasons:
Quality assurance is of particular importance in situations where data may be exposed to legal challenge, or when regional, national or international comparisons of monitoring results are required.
The application of general quality assurance and analytical quality control is more widespread and comprehensive for chemical methods than for biological ones, although it is of equal importance for both disciplines. References
At its most basic, quality assurance procedures should take the form of a predetermined level of random repeat surveys, analyses, measurements or identifications (as appropriate to the individual method), with a pre-set action level for disparity. Training provision, inter-laboratory calibration exercises and accreditation schemes for individual methods should all be considered in the overall planning of quality assurance. Minimisation of the bias introduced by different surveyors employing the same method needs to be minimised. This may be partly achieved by training and the application of statistical measures of assurance. If standard methods of biomonitoring are to be developed, formal training procedures are necessary. The United Nations Economic Commission for Europe (UN ECE) monitoring programme mentioned regards training of surveyors an important element of international co-operation.
Throughout the published literature, quality assurance procedures were seldom addressed, although some standard national and international (e.g. UN ECE) protocols ensure that a consistent approach is taken during surveys. The spects of quality assurance addressed by certain authors during compilation of this report are provided here. In their study of O3 distribution using tobacco plants in northern Italy, Mignanego et al. (1992) described how measurements at sites were always undertaken by the same recorder. In addition, inter-calibration exercises were stressed and samples were sent off to external auditors on a monthly basis. More recently Chappelka et al. (1997) detailed specific quality assurance procedures employed during a survey of O3 plant injury in Great Smoky Mountains National Park. Sampling teams were trained in species recognition, O3 symptom recognition and quantification before embarking on fieldwork. Survey teams were also evaluated against an 'expert system' for testing the ability to rate injured foliage using herbarium samples. An acceptable score was defined as 75% of the leaves correctly estimated to within one class of the actual amount of foliar injury.
In biomonitoring studies, the standardisation of sample collection, preparation, analytical techniques and data analysis is recommended. This will ensure consistency and comparability between different sampling sites, surveys and between different regions. References
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