Siskiwit Lake Revisited:  Time Trends of Polychlorinated Dibenzo-p-dioxin and Dibenzofuran Deposition at Isle Royale, Michigan

We have investigated the time trends of polychlorinated dibenzo- p -dioxins and polychlorinated dibenzofurans (PCDD/F) in two dated sediment cores from Siskiwit Lake, a remote lake on Isle Royale in Lake Superior. Both cores indicated that atmospheric deposition of PCDD/F to Siskiwit Lake increased sharply in the 1930s and peaked in the late 1970s at around 9.5 pg cm - 2 yr - 1 . Since then, atmospheric deposition to Siskiwit Lake has continued to decline. Analysis of the top 1.0 cm of these cores showed that in 1996−1998, the PCDD/F flux to Siskiwit Lake was about 4.5 pg cm - 2 yr - 1 . We compared this sediment-derived flux to a soil-derived flux from Ryan Island, the largest island in Siskiwit Lake, and found a similar value. Thus, the atmospheric deposition of PCDD/F has decreased about 50% since the late 1970s. This decline is slower than might be expected as a result of combustion source regulations alone. Examination of the PCDD/F homologue distributions throughout the sediment cores showed that the relative percent of the hepta- and octachlorinated dibenzo- p -dioxins to total PCDD/F increased with time. We propose that de novo synthesis of these compounds from pentachlorophenol in the atmosphere explains these data.