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Thermal infra-red scanners are also useful in monitoring algae, since definite 
temperature changes are associated with their growth. Oswald (1967) suggested 
the possible use of thermal infra-red sensing in defining areas of potential 
algal blooms, which are frequently related to thermal discharges into streams. 
3.3 Synthetic Chemicals 
( i) Detergents - unfortunately there are substances in domestic sewage which, 
at present, pass through the treatment plant and pollute the effluent. The most 
notorious are the synthetic detergents. The 'hard! detergents, which were in 
general use a few years ago, caused foaming in the rivers, at concentrations of 
as little as 1.0 ppm and reduced the uptake of oxygen considerably. They did 
not, however, appear to be particularly toxic. This problem has been markedly 
reduced, as detergents with a slightly different chemical composition, which are 
more easily decomposed by bacteria in the sewage works are substituted. Today, 
the main complaint regarding detergents is that they release large amounts of 
phosphates, which cause an increase in algal growth, as discussed in the section 
on eutrophication. 
( ii) Pesticides - the pollutants so far considered have been substances produced 
by man or his iadustries, which have caused unintentional damage to the environment. 
Their control has been achieved by taking greater care over their disposal and their 
dispersal. Pesticides, on the other hand, are poisonous substances deliberately 
disseminated in order to exploit their toxic properties. They become pollutants 
when they reach the wrong targets. Although there are many chemicals with a 
remarkable degree of selectivity, so that the chosen pest is destroyed by a lower 
exposure than that required to damage other plants or animals, nevertheless, 
complete selectivity is virtually impossible. This means that there is always 
the risk that pesticides will cause damage to man or other, non-target, organisms. 
The presence of abnormal amounts of pesticides can only be detected using remote 
sensors if the toxic properties of the pollutant have removed the natural strean- 
edge vegetation or in extreme cases fish-kills have occurred. Understandably vite 
detection of pesticide pollution is a difficult task. 
(iii) Chemical and Mineral Substances - these substances result from mining, 
manufacturing and agricultural processes. A major pollutant is acid mine drainage 
from coal seams. It can be a natural polluting agent, but most damage comes from 
the tailings of coal seams that have been worked by man. The acid is formed when 
pyrite, marcasite and sulphur compounds come into contact with water and oxygen. 
These substances turn the stream bed a red or rusty yellow colour and are extremely 
toxic to flora and fauna, which can easily be detected on colour photography. 
3.4 Marine Pollution 
The oceans cover 56 x 109 sq.kms, and contain 1420 x 109 cubic metres of water. 
Often it is assumed that the sea can absorb, unharmed, all the wastes discharged 
into i*. However, pollution of at least some of the seas does occur, largely 
because noxious substances do not become equally mixed but remain concentrated 
in limited areas. 
( i) Sewage and Poisonous Wastes - a large amount of raw sewage is discharged 
into the sea. Where long outfall pipes are used, so tbere is little risk of fouling 
the beaches, this system is reasonably unobjectionable. The organic mavter breaks 
down and the nutrients are re-cycled. However, the discharge is often too near 
to the land. Other ovzgenic wastes are dumped, such as vegetable wastes, into 
the sea which may promote the growth of mussels and some seaweeds. 
Today, the main cencern is that the ocean may be polluted by poisons, which do 
not break down as rapidly as the brganic materials. There are reports of dis- 
charges of heavy metals, including mercury and lead of industrial chemicals such 
as polychlor-biphenyls and other organo-chlorine insecticides. There is little 
control of dumping of poisonous whste materials outside territorial wates and no