Lead isotopes in tree rings: Chronology of pollution in Bayou Trepagnier, Louisiana

We have measured the Pb isotopic composition of tree rings from seven trees in both highly contaminated and relatively noncontaminated regions of Bayou Trepagnier, a bayou in southern Louisiana that has had oil refinery; effluent discharged into it over the past 70 years. To our knowledge, this is the first time that Pb isotope treeing records have been used to assess the sources and extent of heavy-metal contamination of the environment through time. When tree ring (206)pb/(208)ph and Pb-206/Pb-207 isotope ratios are plotted against one another, a straight line is defined by four of the most contaminated tree's. This linear correlation suggests mixing between two sources of Pb. One of the sources is derived from the highly polluted dredge spoils an the banks of the bayou and the other from the natural environment. The nature of the contaminant Pb is unique in that it is, isotopically, relatively homogeneous and extremely radiogenic, similar to ores of the Mississippi Valley (i.e., (206)pb/(207)pb = 1.28). This singular pollutant isotope signature has enabled us to determine the extent of Ph contamination in each cypress wood sample. The isotope results indicate that Pb uptake by the tree is dominated by local-scale root processes and is, therefore, hydrologically and chemically controlled. In addition, we propose that the mobility and bioavailability of Pb in the environment depends on its chemical speciation.