Sources Dispersal Reconcentration and Degradation

Virtually every form of human activity is a potential source of pollution. The popular concept of industrial discharge being the primary source of all pollution is misguided. It is just one example of a point source, i.e. a discharge which can be readily identified and located. Discharges from sewage works provide a second example. In some areas these are the major source of aquatic pollution. Sometimes, however, it is not possible to identify the precise discharge point. This can occur where the pollution originates from land masses. Examples include the run-off of nitrate salts into watercourses after fertilizer application and the emission of methane from land-fill sites into the atmosphere. These are examples of diffuse sources.

Both water and the atmosphere are major routes for the dispersal of compounds. What comes as a surprise are the pathways by which some of the compounds disperse. It is very easy, for instance, for solid particulate material to be dispersed long distances via the atmosphere. There has been, for example, an approximately equal quantity of lead entering the North Sea off the coast of Britain from atmospheric particulates as from rivers or the dumping of solid waste. To illustrate this, a typical transport scheme for a metal (lead) is shown in Figure 2.1.

Equally surprising are the dispersal routes of 'water-insoluble' solid organic compounds. No material is completely insoluble in water. For instance, the solubility in water of the petroleum component, isooctane (2,2,4-trimethylpentane), is as high as 2.4 mg l-1. Watercourses provide a significant dispersal route for such compounds.

The significant vapour pressure of organic solids is also often forgotten. Consider how readily a solid organic compound such as naphthalene, as used in mothballs, volatilizes. In these cases, transportation through the atmosphere is partly in the solid phase and partly in the vapour phase. If you wish to monitor the concentration of these materials in the atmosphere, you not only have to analyse the suspended particulate material but also the gaseous fraction.

The atmosphere also provides a dispersal route for volatile organic compounds. Hydrocarbons will be quickly degraded but will contribute to localized pollution

Figure 2.1 Transport of lead in the environment; concentrations are given in parentheses. Reproduced with the permission of Nelson Thornes Ltd from Environmental Chemistry 3rd Edition ISBN No 0 7514 04837 first published in 1998.

in the form of photochemical smog. If the compound is stable, or is only slowly degraded, in the lower atmosphere, as is the case with many chlorine- or bromine-containing compounds, some may eventually reach the stratosphere (the portion of atmosphere at an altitude of 10-50 km). Decomposition, promoted by the intensity of low-wavelength radiation at this altitude, initiates a series of chemical reactions which deplete the protective layer of ozone.

Distances which are travelled by pollutants in the atmosphere may be as long as hundreds or thousands of kilometres. The movement of sulfur oxides has been studied over distances covering the whole of Europe, and when Mount St. Helens volcano erupted in the USA, the particulate material which was discharged resulted in the production of vivid sunsets several thousand kilometres away.

Dispersal of a pollutant in water or in the atmosphere will inevitably lead to a dilution of the pollutant. As we have seen that the effect of a chemical compound in the environment can be related directly to its concentration, you may think that the dispersal process will simply spread out the pollutant such that it could have little effect away from the source. This would especially be the case when we consider that most forms of pollution are eventually broken down by microbial attack, photochemical or other degradation, and so there would be little chance of the concentration building up to toxic levels. Indeed the phrase 'Dilution is the solution to pollution' was often heard in the early days of environmental concern.

What factors do you think this statement does not take into account?

Answer

(a) The possibility that some pollutants can reconcentrate at particular locations or within organisms remote from the original source.

(b) The non-degradation or slow degradation of some pollutants so that there is a gradual concentration buildup in the environment at large.

(c) Contamination of large areas before sufficient dilution has taken place.

Examples may be given for all these cases, as follows:

(a) Toxic metals, such as cadmium, may be found in the organs of shellfish in concentrations up to 2 million times greater than in the surrounding water (Table 2.1).

(b) The major constituent of the pesticide DDT (^,_p'-dichlorodiphenyltrichloro-ethane) is now a universal contaminant due to its widespread use over several decades and its slow degradation. There is little organic material on the earth which does not contain traces of this at the ng 1-1 level or greater concentration.

(c) Dilution does not take into account localized pollution effects which may occur around discharge pipes or chimneys before dispersion occurs. One of the observed effects of pollution by endocrine disruptors is the 'feminization' of male fish. This particularly occurs close to sewage outfalls where several of the compounds first enter the environment.

The effects of pollution have also been often underestimated in the past. The discharge of sulfur dioxide in gases from tall chimneys was, until recently, seen as an adequate method for its dispersal. The potential problem of 'acid rain' was not considered.

Table 2.1 Examples of metal enrichment in shellfish relative to the surrounding water

Metal Relative concentration in shellfish3

Metal a

Cadmium Chromium Iron Lead

Manganese

Molybdenum

Nickel

2 260 000 200 000 291 500 291 500 55 500

90 12 000

The following sections will discuss two major categories of pollutants which have caused environmental concern due to their ability to reconcentrate (accumulate) in specific areas and within living organisms. These provide good examples of how a knowledge of the transport of pollutants can be used to determine suitable sampling locations where high concentrations may be expected.

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