DURATION OF SOIL CADMIUM AND LEAD POLLUTION FROM SHALE-BASED ALUM PRODUCTION

The scope of this study is to assess the present impact from an histor ical emission source, in order to provide information on the duration of this metal pollution problem. The present distribution of cadmium a nd lead, emitted from a Swedish alumworks between 1726-1840, was studi ed in soils, sediments and ground water. The soil data imply that cadm ium has disappeared from the surface soils during the 150 to 250 years that have passed since the emissions ceased. Lead concentrations of t he soil are elevated only within 200 m from the site of the works. Fur ther away, the lead concentrations are almost at background level. A p ossible explanation for this pattern, is that lead dispersed in large particles has not yet fully disintegrated and leached down the soil pr ofile, in contrast to the smaller particles deposited further away. Th e metal concentrations in the ground water wells are slightly higher t han the Swedish average but probably governed by the pH of the water, rather than the occurrence of large amounts of metals. Elevated lead c oncentrations were found in a few sediment samples in the river mouth downstream from the works. These elevated concentrations coincided wit h the abundant occurrence of small particles of used shale, indicating their origin. Cadmium is not found, at present, in the soil around th e alumworks in concentrations substantially above background levels. C onsequently, it is likely that the cadmium deposited in the area, has leached through the upper layers of the soil profile on its way to the sea sediments. Likewise, the lead concentrations in the soil beyond 2 00 m from the alumworks are not above background levels which indicate that losses through leaching have occurred at a rate of 0.5 to 1.5% y r(-1). Finally, comparisons with other long-term perspective studies g ive an opportunity to discuss the rate for the disappearance of an env ironmental problem.