Long-term migration of metals at historical smelting sites

The vertical migration of metals in soils and rocks is assessed at 3 historical Pb smelters ranging in age from ca. 600 to ca. 1800 a. Core samples were taken through contaminated soils into the underlying rocks and analysed for trace elements by ICP-AES. Surface soils contain high levels of metals, particularly Pb which often exceeds a concentration of 1%. Of the total amount of Pb and Zn present in contaminated soils, only relatively small amounts appear to be in a readily mobile form. Lead concentrations in heavily contaminated horizons were too high to be controlled entirely by exchange reactions. It is suggested that the metal is also present occluded in slag particles, precipitated as insoluble compounds and specifically adsorbed to soil constituents. The low Pb mobility in soils is also related to the elevation of pH in contaminated soils by the release of Ca compounds from the slag wastes during weathering. The rate of vertical migration of Pb varies with rock type with sandstone being greater than limestone which is greater than a clay/sandstone matrix.