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. δ⁶⁶Zn and δ⁶⁵Cu varied by 0.50‰ and 0.29‰, respectively, over the 500 year core record. Isotopic changes were correlated with the presmelter period (∼1450 to 1900 with δ⁶⁶Zn = +0.39‰ ± 0.09‰ and δ⁶⁵Cu = +0.77‰ ± 0.06‰), period of smelter operation (1900 to 1985 with δ⁶⁶Zn = +0.14 ± 0.06‰ and δ⁶⁵Cu = +0.94 ± 0.10‰), and postsmelting/stable urban land use period (post 1985 with δ⁶⁶Zn = 0.00 ± 0.10‰ and δ⁶⁵Cu = +0.82‰ ± 0.12‰). Rapid early urbanization during the post World War II era increased metal loading to the lake but did not significantly alter the δ⁶⁶Zn and δ⁶⁵Cu, 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 δ⁶⁶Zn measured in tire samples suggests tire wear is a likely source of Zn.