Buffering from secondary minerals as a migration limiting factor in lead polluted soils at historical smelting sites

Examination of calcareous slags from several historical smelting sites has indicated that the specific soil environment, in particular the soil pH, ma y have a very significant effect on the rate of weathering and metal releas e. A series of acid titration experiments were used to investigate whether the weathering of the calcareous slags could be increasing the buffering ef fect of the soils through accumulating CaCO3 in the slag-rich horizons. Suc h a buffering mechanism would maintain high pH levels and so limit the migr ation of Pb through the soil profile. Three sites were chosen; one with hig h levels of Ca in the soil, one with relatively low Ca levels and one with intermediate Ca levels. Analysis of metal concentrations was determined usi ng ICP-AES. The results support the hypothesis that, while the soil pH rema ins between 8 and 5, the CaCO3 provides an effective buffer against the mob ilisation of Pb, Between soil pH 5 and 4 it is suggested that both CaCO3 an d PbCO3 participate in the buffering reaction, which slows down with a furt her drop in pH. However, this reaction ultimately releases Pb into the soil solution, although at a much slower rate than would be the case in an unbu ffered soil. An important implication of these findings is that migration r ates of metals in soils cannot be assumed to be constant over time, if such buffering mechanisms are operational. (C) 2001 Published by Elsevier Scien ce Ltd.