The long-term effects of waste disposal at sea must obviously depend upon the length of time taken for material to be broken down to a harmless form. The extent to which continuous addition of substances may be cumulative in effect will depend partly on their persistence in the water. Pollutants can be tentatively classified as transient, moderately persistent, very persistent and virtually permanent. Examples of the length of time that various litter items may persist are given in Table 10.2. These are maximum times which may be shortened by wave action and abrasion.
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