DETERMINATION OF THE EXTENT OF ANTHROPOGENIC PB MIGRATION THROUGH FRACTURED SANDSTONE USING PB ISOTOPE TRACING

The extent of vertical migration of anthropogenic Pb beneath a medieva l smelting site in Derbyshire, U.K. has been estimated using the deter mination of total Pb concentrations and Pb-205/Pb-207 isotope ratio fr om samples taken down 6 m of drill core. Preliminary studies of total Pb concentrations established that the surface slag derived from the s melting contained up to 16% Pb and that the normal background levels i n uncontaminated sandstone were 10+/-2 ppm. Sample analyses beneath th e site revealed elevated Pb concentrations in fracture infill clays (2 70 ppm Pb) and sandstone (76-83 ppm Pb). Both are well above the backg round Pb concentration. Lead isotope analysis of the slag wastes, the underlying contaminated sandstone and fracture infill has shown that a ll 3 contain very similar isotope ratios for Pb-206/Pb-207 (1.1802-1.1 820). However, matched control sandstone samples show that the backgro und Pb-206/Pb-207 isotope ratio (1.1670+/-0.003) is distinctly differe nt. This would indicate that both the sandstone and fracture infill un derlying the historical smelting site contain a substantial proportion of Pb that has been derived from the overlying contamination. The app lication of total Pb concentrations along the core and isotope analysi s suggest that anthropogenically derived Pb from the smelting site (th at was operated between 665 and 445 a BP) has migrated to a depth of 4 .50 m. Assuming a uniform migration rate and a mean time of migration of 555 a, then the mean migration rate is estimated to be 8+/-2 mm/a. The proportion of natural versus anthropogenic Pb in the samples has b een estimated from small variations in the Pb-206/Pb-207 isotope ratio . If the slag is considered to contain 100% anthropogenic Pb and the u ncontaminated sandstone considered to contain 100% natural Pb, the lin ear interpolation can be applied between the 2 end members of the isot ope ratio. The use of this approach to the Pb-207/Pb-207 ratio measure ments has shown that 88% of the Pb in the contaminated sandstone (i.e. 69 ppm from a mean total Pb concentration of 78.5 ppm) has been deriv ed from the anthropogenic Pb at the surface. For the fracture infill s ample taken at a depth of 4.50 m, and with a total Pb concentration of 270 ppm, the % of Pb that has been derived from the slag wastes is ap proximately 98% (equivalent to 265 ppm Pb). The remaining Pb in both t hese samples (9.4 and 5 ppm, respectively) is deduced to have originat ed from the natural background concentration of Pb in the sandstone. T he closeness of these estimates to the measured background concentrati on, suggests that a simple two-source model of Pb contamination is val id for this site. (C) 1997 Elsevier Science Ltd.