A study of PCB concentrations and fluxes in lake sediments was conducted to test the “global fractionation” hypothesis that deposition of semivolatile organics will decline while more volatile congeners will be enriched in polar regions. Sediment cores were collected from 11 remote lakes in Canada ranging from 49° N to 82° N and were dated using excess 210Pb and 137Cs. Sediment extracts were analyzed for up to 90 PCB congeners by capillary GC-ECD with confirmation by GC/high-resolution MS. Total PCB (∑PCB) concentrations in surface slices ranged from 2.4 to 39 ng g-1 (dry wt) and showed no latitudinal trend. Fluxes (ng m-2 yr-1) and inventories of ∑PCB as well as total tetra- to octachlorobiphenyls declined with increasing north latitude while those for di/trichlorobiphenyls showed no latitudinal trend. The proportion of di/trichloro congeners of ∑PCB also increased significantly with latitude, while total octachlorobiphenyls declined. Maximum ∑PCB concentrations were observed in subsurface slices dating to the 1960−1970s in most lakes except those in the high Arctic, where maxima were generally in surface slices. The onset of elevated ∑PCB deposition was delayed in the high Arctic (1950−1960s) relative to the midlatitude and sub-Arctic lakes (1930−1940s). The high proportions of lower chlorinated congeners and the delayed appearance of PCBs are consistent with predictions of the global fractionation model.
The geographical variation in atmospheric input fluxes of pollutants is of paramount importance for understanding the implications of atmospheric pollution. In regions with high levels of industrial and agricultural activity, the natural distribution of some pollutants could be affected by existing point sources of chemicals. Due to its geographic location and size, Canada offers opportunities for studying geographic variations in fluxes of airborne materials independently of local point sources. The sub-Arctic covers 53 percent of Canada and is very sparsely populated, hence most of Canada is not affected by point-source anthropogenic impacts. Most changes in loadings of contaminants reflect changes in regional atmospheric inputs. Some of the most convincing examples of this include synthetic organic compounds never used in the Arctic. To reconstruct histories of chemical contamination over the past 150 years, sedimentary records of 20 lakes have been examined. Fifty-four sediment cores collected from 1987 to 1993 were analyzed using Pb-210 (t 1/2 =22.26 yr) and Cs-137 (t 1/2 =30 yr) dating methods. A wide range of depositional characteristics including sedimentation rates, mixing depths, geochronologies of sedimentation, radionuclide inventories and fluxes were obtained. It was assumed that sediments from basins with no local industrial or domestic sources served as natural collecting systems recording the history of net inputs from the atmosphere. Since radionuclide inventories in the sediments are affected by physical, chemical and geological conditions, estimates of sediment focusing factors were made. Focusing factors were determined by dividing the measured excess Pb-210 flux in the core by the predicted atmospheric flux of excess Pb-210. The predicted flux values were determined experimentally from terrestrial soil profiles.
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