Organic substances constitute the major freshwater pollutants, coming from domestic sewage discharges (even after treatment) and from certain industries such as food processing. This section will deal with the biodegradable forms, but there are also inert (non-biodegradable) toxic forms. Organic substances can be natural (in which case they are normally biodegradable) or synthetic (in which case they can often be degraded by microorganisms that have adapted to utilising them).
The major polluting effect of biodegradable organic materials is the reduction in oxygen concentration in the water. Bacteria and other organisms (decomposers) break these materials down into simpler organic or inorganic substances. They use up oxygen in the process, and as their population increases there is an extra demand for dissolved oxygen.
When a potentially polluting effluent is released into a stream, there follows a sequence of events in time and distance. This sequence leads to different environmental consequences and different aquatic communities compared with those immediately upstream and the successive reaches downstream. After a certain distance, natural biodegradative processes will break down the pollutants, often returning the river to something like its original condition.
Three stages of organic pollution can be defined.
The sequence of events following significant pollution of a waterway by organic material is shown in Figure 4.
Inorganic materials can also cause deoxygenation, e.g. when ferrous iron from mine drainage water enters a river (Stumm and Lee, 1961). In the reduced ferrous (Fe(II)) state the iron is in solution, but on meeting the oxygen in the river it is oxidised to red insoluble ferric (Fe(III)) iron, a process that reduces the concentration of dissolved oxygen in the river water. The oxidised iron is now in suspension so that, as well as reducing the oxygen content, it reduces light penetration. It finally settles out slowly downstream of the discharge point, giving rise to all the problems associated with suspended solids. This type of problem is usually associated with coal-mining effluents.
Identify which of the following statements concerning the effects of both organic pollution and eutrophication are true. If a statement is false, give the reason why.
Statements (d) and (e) are true.
Statement (a) is false: depletion of oxygen occurs in both cases due to the activities of decomposers. An increase in primary producers (plants) would increase oxygen levels.
Statement (b) is false: only in eutrophication are plant nutrients present, and these will usually encourage plant growth.
Statement (c) is false: in organically polluted waters the producers are only a small proportion of the ecology, making up less than 25% of the population, whilst in eutrophic waters they constitute more than 75%. The proportion of consumers, on the other hand, is similar in both situations (though slightly greater in organically polluted waters).
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