The concept of ‘dilute and disperse’ was briefly mentioned earlier in this dis-cussion. In principle, it involves the attenuation of pollutants by permitting them to become physically spread out, thereby reducing their effective point concentration. The dispersal and the consequent dilution of a given substance depends on its nature and the characteristics of the specific pathway used to achieve this. It may take place, with varying degrees of effectiveness, in air, water or soil.
In general terms, air movement gives good dispersal and dilution of gaseous emissions. However, heavier particulates tend to fall out near the source and the mapping of pollution effects on the basis of substance weight/distance travelled is widely appreciated.
Typically, there is good dispersal and dilution potential in large bodies of water or rivers, but smaller watercourses clearly have a correspondingly lower capacity. It is also obvious that moving bodies of water disperse pollutants more rapidly than still ones.
Movement through the soil represents another opportunity for the dilute and disperse approach, often with soil water playing a significant part, and typically aided by the activities of resident flora and fauna. The latter generally exerts an influence in this context which is independent of any bioaccumulation potential.
The principle behind this is diametrically opposed to the previous approach, in that instead of relying on the pollutant becoming attenuated and spread over a wide area, it is an attempt to gather together the offending substance and prevent its escape into the surrounding environment.
The inherent contradiction between these two general methods is an enduring feature of environmental biotechnology and, though the fashion changes from time to time, favouring first one and then the other, it is fair to say that there is a place for both, dependent on individual circumstances. As with so much relating to the practical applications of biotechnologies to environmental problems, the idea of a ‘best’ method, at least in absolute terms, is of little value. The whole issue is far more contextually sensitive and hence the specific modalities of the particular, are often more important concerns than the more theoretically applicable general considerations.
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