IDENTIFYING SOURCES OF ELEVATED HEAVY METALS IN MOSS ALONG A MINING HAUL ROAD IN NW ALASKA USING PB ISOTOPES AND CONCENTRATIONS OF IN AND BI

Red Dog Mine is one of the world’s largest producers of zinc (Zn) concentrate. Elevated concentrations of lead (Pb), Zn, and cadmium (Cd) in Hylocomium splendens moss samples have been documented along the mine’s haul road that traverses 32 km of northwestern Alaskan tundra within Cape Krusenstern National Monument. The elevated metal concentrations have been attributed to the transport of ore along the haul road from the mine to the port. This project builds on an earlier study (Neitlich et al., 2017) by analyzing the Pb isotopic compositions in those moss samples to explore how isotopic signatures changed as a function of distance from the road. In addition we examine the elemental ratios Zn/Pb, Zn/Cd, Pb/Cd and concentrations of the ore trace constituents indium (In) and bismuth (Bi) in the moss to provide additional evidence for source.

Elemental ratios in moss samples closest to the haul road exhibited ratios of Zn/Pb, Zn/Cd, Pb/Cd similar to those calculated for Zn and Pb ore concentrates produced by the mine. The trend formed by the isotopic ratios of Pb in moss samples suggests mixing between two endmember Pb compositions. The most radiogenic Pb isotopic compositions were measured in moss collected very close to the haul road and the least radiogenic in moss collected from the most southern sites, 40–60 km from the haul road. The Pb isotopic signatures of moss samples at very close distances from the haul road diverged slightly from the predicted field of ratios based upon previously published Pb isotopic compositions for Zn and Pb ores from the mine. This may be explained by the mixing of Pb and Zn ore concentrate as they are dispersed into the environment. The presence of In and Bi at higher concentrations in samples near the road could be due to the dispersion of concentrate dusts from ore transport. In contrast to the observed local natural In and Bi concentrations of about 1 ppb and 10 ppb, the samples closest to the road showed In and Bi as high as 35 ppb and 80 ppb, respectively. By combining the use of Pb isotopes with elemental analysis of In and Bi this study provides a potential method of tracing ore in the environment.