The emerging immission landscape
In order to describe the immission landscape at different times, a simple flow model was used, where emissions to land and water, flows from soil to water, and, finally, accumulation in sediments and soils were considered. Detailed maps of chromium and lead loads in Sweden from 1880 to 1980 have been published elsewhere (Bergbäck et al., 1989, 1992). In figures 2 and 3 a comparison is made for accumulated amounts of chromium and lead in soils and sediments between 1950 and 1980.
These calculated amounts of metals in soils in Sweden correspond to measurements taken during the 1980s, particularly in areas where high production emissions had taken place. Exact comparisons, however, are difficult to make, as our calculations represent mean loads in administratively defined areas. Comparisons with monitoring data reveal similarities but also discrepancies in the patterns, and the latter could in some cases be explained by the composition of the bedrock. (Uncertainties in the calculations are further discussed in Bergbäck et al., 1989, 1992.)
The present rate of lead consumption in Sweden is approximately 25,000 tonnes per year, excluding ammunition and gasoline . If this rate were to remain constant between 1980 and 2080, another 2.5 million tonnes would be added to the 2 million tonnes already accumulated in the last 100 years, giving a total of 4.5 million tonnes of lead in the anthroposphere.
It would also be relevant to compare the anthropogenic release of lead into the environment with the mobilization of lead from the bedrock during the weathering cycle. Weathering mobilization may be calculated by using average trace metal concentrations in soils and the suspended sediment flux in rivers. According to Nriagu, the global weathering rate for lead is approximately 180,000 tonnes per year (Nriagu, 1990). In Sweden's case, this would mean about 500 tonnes per year. Thus, 50,000 tonnes might have been released by natural weathering processes within the last 100 years. This amount should be compared with our calculations of total emissions, which are approximately four times higher.
The rate of emissions (see figure 4) and accumulation of chromium is higher than for lead. The use of chromium products in Sweden (e.g. stainless steel) has increased dramatically since the Second World War. As long as consumption emissions remain at a high level, chromium will have a strong impact on the environment in the future.
In figure 5 the emission rates of lead, chromium and cadmium are compared, with a roughly calculated weathering rate for these metals. Obviously, the anthropogenic contribution is significant, especially for lead.
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