Observations on historical, contemporary, and natural PCDD/Fs.

PCDD/Fs were determined in samples of archived surface soils collected from different locations around the world in the early 1880s, in contemporary surface soils from around the world, in archived subsurface soils collected at Rothamsted Experimental Station in the 1870/1880s, and in sections of peat core deposited between 5000 BP and the present. PCDD/Fs were detected in most of the samples. The potential sources and implications of the levels and mixtures of PCDD/Fs present in the samples are discussed. The homologue and isomer patterns observed in most of the contemporary European surface soils are commonly observed for European air samples and soil samples. The homologue pattern in the Rothamsted surface soils collected in the 1800s was similar, suggesting that similar sources of atmospheric emissions of PCDD/ Fs were operating in the UK in the 1800s as currently. Very different patterns, dominated by OCDD and with contributions of HpCDD and HxCDD, were found in some other samples. It is proposed that the PCDD/Fs present in the subsurface Rothamsted soils, archived (1880s) surface soils from Illinois and the Congo, clay beneath the peat bog (deposited approximately 5000 BP), and possibly surface soil samples from Thailand and Australia are of a natural origin. The most abundant TeCDD/F congeners measured in the peat samples here were also those observed by previous workers who studied a Canadian peat bog and are consistent with the microbially mediated oxidative coupling of chlorophenols. The study provides evidence for the widespread occurrence of PCDD/Fs in the environment prior to 1900 and for a complex array of sources (including natural) and environmental transformation processes.