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 regio ns 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 contaminati on 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 strai ght line is defined by four of the most contaminated tree's. This line ar correlation suggests mixing between two sources of Pb. One of the s ources 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 Mississip pi Valley (i.e., (206)pb/(207)pb = 1.28). This singular pollutant isot ope signature has enabled us to determine the extent of Ph contaminati on in each cypress wood sample. The isotope results indicate that Pb u ptake by the tree is dominated by local-scale root processes and is, t herefore, hydrologically and chemically controlled. In addition, we pr opose that the mobility and bioavailability of Pb in the environment d epends on its chemical speciation.