206Pb/207Pb Ratios in Tree Rings as Monitors of Environmental Change

Abstract

Lead concentrations and 206 Pb/ 207 Pb ratios were measured in annual tree rings of sycamore (Acer pseudoplatanus L.) growing at two woodland sites in northwest England. One site, at Prescot, was adjacent to a metal refinery in the center of a large urban conurbation while the site at Croxteth was part of an urban woodland at least 2 km from the nearest major road. Lead concentrations and 206 Pb/ 207 Pb ratios in surface soil were higher at Prescot than at Croxteth. At both sites, Pb concentrations decreased considerably in tree rings formed after 1970. At Croxteth, 206 Pb/ 207 Pb ratios in tree rings formed between 1965 and 1987 were relatively constant, around 1.16, but increased sharply in wood formed after 1987 to over 1.17 in wood formed in 1993. This increase in the 206 Pb/ 207 Pb ratio reflects the recent reduction in Pb derived from vehicle emissions that has a characteristically low 206 Pb/ 207 Pb ratio. There was no corresponding increase in the 206 Pb/ 207 Pb ratio in tree rings at Prescot, although Pb concentrations in tree rings formed after 1986 remained unchanged. In addition, 206 Pb/ 207 Pb ratios at Prescot were generally higher than at Croxteth, and there was considerable annual variation, which is probably due to the multiple sources of Pb at this small woodland site. 206 Pb/ 207 Pb ratios in sycamore tree rings may be used to document historical changes in the sources of bioavailable Pb, which differ between sites, although the majority of Pb in tree rings at these two woodland sites appears to have been derived from industrial and urban sources rather than from motor vehicle emissions.