Human Activities as Geochemical Processes

Despite the difficulties, there have been many efforts in recent years to evaluate trace metal concentrations in natural systems and to compare trace metal release and transport rates from natural and anthropogenic sources. There is no single parameter that can summarize such comparisons. Frequently, a comparison is made between the composition of atmospheric particles and that of average crustal material to indicate whether certain elements are enriched in the atmospheric particulates. If so, some explanation is sought for the enrichment. Usually, the contribution of sea spray to the enrichment is estimated, and any enrichment unaccounted for is attributed to other natural inputs (volcanoes, low-temperature volatilization processes, etc.) or anthropogenic sources. 

A second approach is to compare total mining production of a metal to an estimate of its total natural flux, making the implicit assumption that all mined materials will be released to the environment in the near future (a reasonable assumption when comparing with geologic processes). 

Finally, some authors have computed metal loading to the environment from specific human activities, such as discharges of wastewater, and compared these with natural release rates. While the details of the computations and conclusions vary, the general observation for many metals is that anthropogenic contributions to metal ion transport rates and environmental burdens are approaching and in many cases have already exceeded natural contributions. A few such comparisons are provided in Tables 15-1 to 15-4. 

The amount of metals released as the byproduct of a single activity, the burning of coal, illustrates the potential importance of anthropogenic sources. Inorganic, non-combustible materials are present in coal, and these materials constitute the ash that remains after combustion. Fly ash (the ash that leaves the furnace and is collected by flue-scrubbing 

Element Natural rate (Gg/year) Anthropogenic rate (Gg/year) Anthropogenic/ natural ratio 

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