TRACING ANTHROPOGENIC CONTAMINATION IN A LAKE SEDIMENT CORE USING HG, PB, AND ZN ISOTOPIC COMPOSITIONS

The isotopic compositions for mercury (Hg), lead (Pb), and zinc (Zn) were measured in a lake sediment core collected from Lake Ballinger, in Seattle, Washington. The lake was affected by a nearby smelter in Tacoma, which operated from 1887 to 1986. The smelter is about 50 km upwind from Lake Ballinger and known to have emitted metal-bearing particulates that were deposited in the lake. The sediment core dates from present-day to about 1450. Three periods in the core with distinct isotopic Hg compositions (relative to NIST SRM 3133) were identified and consist of (1) a pre-smelting period (~1450 to 1886, δ²⁰²Hg -3.02 to -2.50‰, mean -2.77‰), (2) a smelting period (1887 to 1986, δ²⁰²Hg -2.22 to -0.79‰, mean -1.41‰), and (3) a post-smelter period coincident with extensive urbanization (1986 to present day, δ²⁰²Hg -0.89 to -0.48 ‰, mean -0.65‰). Similarly, variations were observed for ²⁰⁸Pb/²⁰⁴Pb ratios for the three periods, with means of (1) 38.204, which varied from 38.123 to 38.300, (2) 38.100, which varied from 37.677 to 38.349, and (3) 38.305, which varied from 38.294 to 38.316. The mean isotopic compositions of Zn (δ⁶⁶Zn) for the three periods were (1) 0.39‰, ranging from 0.45 to 0.35‰, (2) 0.15‰, ranging from 0.23 to 0.01‰, and (3) 0.00‰, ranging from -0.04 to -0.04‰. The isotopic compositions of Hg, Pb, and Zn all change significantly in Lake Ballinger sediment coincident with operation of the Tacoma smelter, indicating smelter emissions were a primary source of these metals to the lake. All three metal isotope ratios change again following closure of the smelter, with Hg and Pb toward pre-industrial ratios, but distinctly different than Zn, which was increasingly lighter indicating urban sources. Isotope ratios for Pb and Zn have been shown to be reliable tracers of historical sources of industrial and anthropogenic contamination. The correlation of Hg isotopes with those for Pb and Zn suggest that the Hg isotopic signature in Lake Ballinger was primarily influenced by the source of metal pollution and not by subsequent biogeochemical cycling of the element. Thus, the stable isotopes of Hg were useful in tracing the sources of Hg in this environment.