Osmium isotopes demonstrate distal transport of contaminated sediments in Chesapeake Bay

Because the isotopic composition of anthropogenic Os is normally distinctiv e in comparison to continental crust and is precisely measurable, this plat inum-group element is attractive as a tracer of transport pathways for cont aminated sediments in estuaries. Evidence herein and elsewhere suggest that biomedical research institutions are the chief source of anthropogenic Os. in the Chesapeake Bay region, uncontaminated sediments bear a crustal Os-1 87/ Os-188 signature of 0.73 +/- 0.10. Slightly higher Os-187/Os-188 ratios occur in Re-rich Coastal Plain deposits due to post-Miocene Re-187 decay. The upper Susquehanna Basin yields sediments also with higher Os-187/Os-188 . Beginning in the late 1970s, this signal was overprinted by a low Os-187/ Os-188 (anthropogenic) source in the lower Susquehanna Basin. In the vicin ity of Baltimore, which is a major center of heavy industry as well as biom edical research, anthropogenic Os has been found only in sediments impacted by the principal wastewater treatment plant. Surprisingly, a mid-Bay site distant from anthropogenic sources contains the strongest anthropogenic Os signal in the data set, having received anthropogenic Os sporadically since the mid-20th Century. Transport of particles to this site overrode the nor thward flowing bottom currents. Finding anthropogenic Os at this site cauti ons that other particle-borne substances, including hazardous ones, could b e dispersed broadly in this estuary.