Retrospective and Historical Monitoring
All biological monitoring is to varying degrees retrospective, but in the present context we use the term for samples grown, exposed or collected many years before the chemical analysis. A number of attempts have been made to trace the changes in pollution levels during the period of man’s increasing influence on the environment. The biological materials used for such studies have included peat, annual growth-rings of trees, bird feathers and human bones (as discussed in Chapter 5, Section 5.3.4) and herbarium specimens, particularly mosses. Generally the methods used have not enabled a quantitative assessment of pollution levels but have provided comparisons with unpolluted sites or periods when pollution was assumed to be minimal. Of the three plant materials mentioned above, probably tree-ring analysis has received the greatest attention but generally has proved to be the most difficult to execute and interpret.
KeywordsZinc Nickel Mercury Chromium Cadmium
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