HEAVY-METAL MIGRATION IN SOILS AND ROCKS AT HISTORICAL SMELTING SITES

The vertical migration of metals through soils and rocks was investiga ted at five historical lead smelting sites ranging in age between 220 and similar to 1900 years. Core samples were taken through metal-conta minated soils and the underlying strata. Concentration profiles of lea d and zinc are presented from which values for the distances and rates of migration have been derived. Slag-rich soil horizons contain highl y elevated metal concentrations and some contamination of underlying s trata has occurred at all sites. However, the amounts of lead acid zin c that have migrated from soils and been retained at greater depths ar e comparatively low. This low metal mobility in contaminated soils is partly attributed to the elevation of soil pH by the presence of calci um and carbonate originating from slag wastes and perhaps gangue miner als. Distances and rates of vertical migration were higher at those si tes with soils underlain by sandstone than at those with soils underla in by clay. For sites with the same parent material, metal mobility ap pears to be increased at lower soil pH. The mean migration rates for l ead and zinc reach maxima of 0.75 and 0.46 cm yr(-1) respectively in s andstone at Dole A where the elements have moved mean distances of 4.3 and 2.6 m respectively. There is some evidence that metal transport i n the sandstone underlying Bole A and Cupola B occurs preferentially a long rock fractures. The migration of lead and zinc is attenuated by s ubsurface days leading to relatively low mean migration rates which ra nge from 0.03 to 0.31 cm yr(-1) with many values typical of migration solely by diffusion. However, enhanced metal migration in clays at Cup ola A suggest a preferential transport mechanism possibly in cracks or biopores.