Use of 87Sr/86Sr and δ11B to identify slag-affected sediment in Southern Lake Michigan

Slag is a ubiquitous byproduct of the iron-smelting industry and influences geochemistry and water quality in adjacent geologic units, ground and surface water. Despite extensive slag deposition along the Indiana shoreline of Lake Michigan, definitive evidence that slag has affected lakebed sediments has not been established. Concerns for the protection of water and ecosystem resources in the Great Lakes motivated this study to determine if strontium and boron isotopes could be used to identify and delineate slag-affected bed sediment in Lake Michigan. Sixty-five samples of bed sediment were acquired from the southern lobe of Lake Michigan and analyzed for Sr-87/Sr-86 and delta(11)B. Samples immediately offshore from Indiana steel mills and slag-disposal sites contained higher median Sr-87/Sr-86 values (0.70881) than shoreline sediments collected elsewhere in the basin (0.70847) and uniquely decreased with increasing distance from the shoreline. The highest delta(11)B values occurred in sediments from the Indiana shoreline (+12.9 to 16.4parts per thousand) but were also elevated in sediments collected offshore from three Lake Michigan cities (+11.7 to 12.71parts per thousand). Contoured isotope data indicated that 82-154 km(2) of bed sediment along the Indiana shoreline had elevated Sr-87/Sr-86 and delta(11)B values relative to shoreline sediments elsewhere in southern Lake Michigan.