Application of Zn Isotopes to Resolve Metal Sources In a Sediment Core from An Urban Lake In Seattle, Washington

Lake sediment cores can provide historical records of the input of metals to the environment and are useful for identifying the environmental response to changes in industrialization and land use. In urban lakes, however, it often is difficult to resolve metal contributions from the many different urban sources of metals. We are investigating the use of Zn isotope geochemistry in the Puget Sound area of Washington State where emissions from a metal smelter from 1899 to 1985 are combined with modern urban emissions from a variety of sources. Zn isotopes will be analyzed in a sediment core from Lake Ballinger in suburban Seattle to help identify the sources and temporal variations in Zn inputs. Digested sediment samples have been analyzed for total metal concentrations, and sequential-selective extractions were performed to isolate metals bound in the exchangeable, organic, and Fe-oxide fractions of the sediment. Age dating of the core sediments using Cs-137 and Pb-210 and isotopic analyses of Zn are in progress. Concentrations of most metals, including Pb, As, Cd, Zn, Cu, Cr, Co, and Ni, increased from pre-1900 background (preliminary dates based on a 1998 core from the same location) to a peak around 23 cm depth, which may correspond with increased production at the smelter in Tacoma during WWII. A larger spike in metal concentrations around 12 cm depth may be related to a combination of smelter activity and fluvial inputs from the watershed, which rapidly urbanized during the post-war era. Unlike the other metals, Zn concentrations do not peak in the 1970s but continue to steadily increase, reaching a maximum of 750 ug/g in present-day (2007) sediment. We hypothesize that differences in Zn isotopic ratios in smelter and urban emissions and natural inputs will allow us to resolve contributions from these sources to total Zn in the core. >