Zn and Cu Isotopes as Tracers of Anthropogenic Contamination in a Sediment Core from an Urban Lake

In this work, we use stable Zn and Cu isotopes to identify the sources and timing of the deposition of these metals in a sediment core from Lake Ballinger near Seattle, Washington, USA. The base of the Lake Ballinger core predates settlement in the region, while the upper sections record the effects of atmospheric emissions from a nearby smelter and rapid urbanization of the watershed. delta Zn-66 and delta Cu-65 varied by 0.50 parts per thousand and 0.29 parts per thousand, respectively, over the 500 year core record. Isotopic changes were correlated with the presmelter period (similar to 1450 to 1900 with delta Zn-66 = +0.39 parts per thousand +/- 0.09 parts per thousand and delta Cu-65 = +0.77 parts per thousand +/- 0.06 parts per thousand), period of smelter operation (1900 to 1985 with delta Zn-66 = +014 +/- 0.06 parts per thousand and delta Cu-65 = +0.94 +/- 0.10 parts per thousand), and postsmelting/stable urban land use period (post 1985 with delta Zn-66 = 0.00 +/- 0.10 and delta Cu-65 = +0.82 parts per thousand +/- 0.12 parts per thousand). Rapid early urbanization during the post World War 11 era increased metal loading to the lake but did not significantly alter the delta Zn-66 and delta Cu-65, suggesting that increased metal loads during this time were derived mainly from mobilization of historically contaminated soils. Urban sources of Cu and Zn were dominant since the smelter closed in the 1980s, and the delta Zn-66 measured in tire samples suggests tire wear is a likely source of Zn.