Lake Baikal is a unique freshwater ecosystem that has been declared a UNESCO World Heritage Site. It contains high levels of PCBs, and Baikal seal were recently found to have PCDD/F concentrations comparable to those in the Baltic Sea. In this work fish and soil were analyzed to trace the sources of these compounds to the lake. The fish samples indicated that the PCDD/F and PCB contamination of Lake Baikal does not originate from background inputs and that the contamination increases from north to south. The soil inventory (quantity of chemical per m2 ground) was determined at 34 sites around Lake Baikal and in the Angara River valley. For the PCDD/Fs and most PCBs, the soil inventory is a good approximation of the cumulative atmospheric deposition. It varied over a factor of 1000, with the highest levels in Usol'ye Sibirskoe, a city 110 km north of the southwestern tip of the lake in the highly industrialized Angara River valley, and the lowest values in the pristine areas to the northeast of the lake. A continuous decrease in the soil inventory was observed moving from Usol'ye S. up the Angara River valley to Lake Baikal and from there northeastward along the lake. This indicates that there was a major atmospheric source of these compounds in the Usol'ye area. The cumulative deposition to the lake was estimated to be 1.2 kg of TEQ (PCDD/F + PCB). The cumulative deposition of ΣPCB to Lake Baikal was comparable to the ΣPCB inventory in Lake Superior in 1986, indicating that the atmospheric emissions in the Usol'ye S. area have been a major source to Lake Baikal. The soil inventories of the PCDD/Fs and PCBs were highly correlated, and the PCDD/F pattern in the soils was similar to the PCDD/F pattern in technical PCB mixtures. There is a large chloralkali chemical complex in Usol'ye Sibirskoe, and the chloralkali industry has caused environmental contamination with PCBs elsewhere. This chemical complex is suspected to have been the source of the PCB and PCDD/F contamination, but due to the paucity of information about this facility it has not yet been possible to confirm this hypothesis. This study illustrates the utility of soil contaminant inventories to trace sources of persistent hydrophobic organic contaminants.

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