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 medieval smelting site in Derbyshire, U.K. has been estimated using the determination of total Pb concentrations and Pb-205/Pb-207 isotope ratio from samples taken down 6 m of drill core. Preliminary studies of total Pb concentrations established that the surface slag derived from the smelting contained up to 16% Pb and that the normal background levels in uncontaminated sandstone were 10+/-2 ppm. Sample analyses beneath the site revealed elevated Pb concentrations in fracture infill clays (270 ppm Pb) and sandstone (76-83 ppm Pb). Both are well above the background Pb concentration. Lead isotope analysis of the slag wastes, the underlying contaminated sandstone and fracture infill has shown that all 3 contain very similar isotope ratios for Pb-206/Pb-207 (1.1802-1.1820). However, matched control sandstone samples show that the background Pb-206/Pb-207 isotope ratio (1.1670+/-0.003) is distinctly different. This would indicate that both the sandstone and fracture infill underlying the historical smelting site contain a substantial proportion of Pb that has been derived from the overlying contamination. The application of total Pb concentrations along the core and isotope analysis suggest that anthropogenically derived Pb from the smelting site (that 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 been estimated from small variations in the Pb-206/Pb-207 isotope ratio. If the slag is considered to contain 100% anthropogenic Pb and the uncontaminated sandstone considered to contain 100% natural Pb, the linear interpolation can be applied between the 2 end members of the isotope ratio. The use of this approach to the Pb-207/Pb-207 ratio measurements 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 derived from the anthropogenic Pb at the surface. For the fracture infill sample 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 approximately 98% (equivalent to 265 ppm Pb). The remaining Pb in both these samples (9.4 and 5 ppm, respectively) is deduced to have originated from the natural background concentration of Pb in the sandstone. The closeness of these estimates to the measured background concentration, suggests that a simple two-source model of Pb contamination is valid for this site. (C) 1997 Elsevier Science Ltd.