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Analysis of the river sufficiently downstream from the discharge point to allow for dispersal will give a direct measurement of the concentration in the river, but there will be some uncertainty as to the source of the pollution.
Analysis of the pollutant in living organisms found in the river will give a direct indication of the environmental problem, but unless the organisms are sampled close to the discharge point, the analytical results would be difficult to relate to individual discharges.
The discharge composition was included to encourage you to extend your ideas to consider what extra analytical information would be useful. Quality control of the starting materials would reduce the concentration of the contaminants in the discharge, as would process control to minimize the manufacture of side products.
National legislation may in fact specify the requirements of monitoring such as detailed in this response.
The first prerequisite is that the compound must be in a form which allows it to become widely dispersed. Dispersal may be via the atmosphere or the hydrosphere. The properties which would affect this dispersal include volatility, solubility in water, and if the compound is a solid, its particle size.
The compound must have a high resistance to degradation within the atmosphere and hydrosphere and to metabolism (chemical reaction) within organisms. If the compound rapidly degraded, there would be little possibility of toxic concentrations building up. Beware, however, of the possibility that the degradation products themselves might pose similar or worse environmental problems.
If the pollution problem is related directly to the effect of the material on living organisms, rather than on physical structures, then the compound must have an ability to reconcentrate within organisms. This will be described in detail in the following sections.
Finally, if the compound is seen as a pollution problem, it must have some deleterious effect. However, this need not always be the case.
Responses to Self-Assessment Questions
DDT is a high-molecular-mass neutral organic compound. As with all similar compounds, it would be expected to have a significant vapour pressure. The following illustration indicates some possible dispersal routes - drift of the spray from the field of application, run-off into watercourses, and also volatilization.
Figure SAQ 2.2 Illustration of the dispersal of a pesticide from its field of application.
Both types of pollutant may be dispersed through the atmosphere, as well as in watercourses. Atmospheric dispersal of metals is usually as particulates (in the form of metal salts), whereas organics are found both in the particulate form and in the vapour state.
Deposition may occur on to land or into water courses.
The solubility of the organic compounds in water is often low, but this can lead to a large bioconcentration in organisms living in the water. The solubility of metals is very dependent on the chemical compounds involved, but for most species will increase with a decrease in pH. There is normally no pH effect on the solubility of neutral organic compounds. The bioconcentration of metals is very dependent on the element being considered; however, for some metals, such as cadmium, this can be extremely high (see Table 2.1).
Introduction to Environmental Analysis
Both types of pollutant will concentrate in sediments. The detailed mechanism for this is quite different in the two cases. Reducing conditions would, for instance, increase the deposition of lead, whereas these would have no effect on neutral organic compounds.
Entry into the food chain in an aquatic environment is in both cases by bottom-dwelling fish and filter-feeders. Once again, the detailed mechanisms are different in the two cases and this may lead to concentration in different organs in the body.
If you had said that there were some similarities in the dispersal and reconcentration you would have been correct, but there are many differences in the detailed mechanisms.
The metal ions would be likely to concentrate in the sediment on the sea bed and be ingested by filter-feeders. A likely critical group would be the local people who consume large quantities of the sea-food, perhaps the families of the fishermen themselves.
A second likely path could exist. Some of the sediment may be washed up on the shore. This may dry out and be blown into the atmosphere, or it may be propelled into the atmosphere via sea spray.
The critical group could then be people who spend a large proportion of their time on the sea-shore, with the metal ions entering their bodies through inhalation.
In a highly polluted area, there is the possibility that contamination may occur even at the sampling stage. This is particularly the case if the same apparatus is being used for a number of samples without thorough cleaning. A suitable sample container must be used to prevent contamination or loss during storage and transport. We have already discussed (see Section 2.6) that this should be glass rather than plastic for organic compounds. When working in the laboratory, there is the possibility of contamination by solvents in the laboratory atmosphere. At the instrumental analysis stage, there is the possibility of cross-contamination by consecutive samples. A suitable quality control procedure would be to analyse blank samples which have been introduced at the stages when contamination is likely - at the sampling stage, before transportation, before storage in the laboratory, and immediately prior to the instrumental analysis (see Table 2.3). Not only would any contamination be identified but also the point at which it was introduced.