PCDD/F and PCB history in dated sediments of a rural lake

Concentrations of polychlorinated dibenzo-p-dioxins (PCDD), dibenzofurans (PCDF) and biphenyls (PCB) were analysed from a sediment core, which had been sampled from a remote lake in Finland. The deepest subsamples were analysed from a depth of 4.0 m, and were more than 8000 years old. At the surface of the sediment core, low levels of industrial background contamination were detected. The sum of 2,3,7,8-substituted PCDD/Fs was 35 ng/kg d.w. (dry weight), or 1.2 ng/kg d.w. in WHO's toxicity equivalent, WHO-TEQ. The annual accumulation rate of 2,3,7,8-substituted PCDD/Fs was 1.26 ng m(-2) a(-1). Deeper in the core, PCDD/F concentrations ranged between 2.11 and 11.1 ng/kg d.w. (0.24 and 1.70 ng/kg d.w. WHO-TEQ) and the accumulation rate was 0.11 ng m(-2) a(-1). A characteristic PCDD/F congener profile was found in all pre-industrial sediment layers. The order of concentrations was OCDD > 1,2,3,4,6,7,8-HpCDD > 1,2,3,7,8,9-HxCDD > 1,2,3,4,7,8-HxCDD > 2,3,7,8-TCDD, and concentrations of 2,3,7,8-chlorinated dibenzofurans were below the detection limit. Similar congener profiles have previously been described in samples for which natural PCDD/F formation has been proposed. PCBs were present at low levels in all of the sediment samples. The sum of the PCB concentrations of the sediment subsamples ranged between 50 and 2540 ng/kg d.w., and the three predominant congeners in the core were PCBs 18, 52, and 110.