Lead contamination in a wetland watershed: isotopes as fingerprints of pollution

The Pb-isotope composition of soils and sediments has been measured from both highly contaminated and non-contaminated regions of Bayou Trepagnier, a bayou in southern Louisiana that has had oil refinery effluent discharged into it over the past 66 years. Spoil banks created by the dredging of the bayou bottom approximately 50 years ago are the main source of contamination within the ecosystem. The ²⁰⁶Pb/²⁰⁷Pb isotope composition of the contaminant is relatively constant averaging 1.275 ±0.008. A literature search reveals that such radiogenic values are typical of ores from southeastern Missouri. When surficial soil ²⁰⁶Pb/²⁰⁸Pb and ²⁰⁶Pb/²⁰⁷Pb isotope ratios are plotted against each other, a straight line is defined (r²=0.99). The linear correlation suggests mixing between Pb from the spoil banks and Pb from a natural source. The latter source may consist of Pb in soil that has been leached of its natural radiogenic component during weathering processes. Mixing calculations indicate that transport of contaminant Pb is widespread and occurs several hundred meters from the spoil banks. Despite the low Pb concentrations of some of the soils, the isotope data demonstrate that a significant amount of the Pb is derived from the pollutant source. Received: 12 July 1999 · Accepted: 14 September 1999