Isotopic evidence of the relative retention and mobility of lead and radiocaesium in Scottish ombrotrophic peats

The reconstruction of historical trends in atmospheric deposition of contaminant metals by use of their concentration profiles in accumulating, ombrotrophic peat deposits, in conjunction with radionuclide-derived chronologies, is subject to uncertainty because of potential post-depositional mobility both of the radionuclides and of the contaminant metals. Results are presented here for a study of the distribution of 210Pb, Pb, stable Pb isotopes and radiocaesium in the unsaturated, surface zone of ombrotrophic peat deposits in three contrasting locations in Scotland. 210Pb chronologies are derived and the implied historical variations in atmospheric depositional fluxes and atom ratios of contaminant Pb are considered, along with total inventories, relative to known historical trends in input and proximity to heavily populated, industrial areas. Similarly, radiocaesium concentration profiles, activity ratios and inventories are considered in the context of the 210Pb chronologies and known inputs. It is concluded that the historical trends in the depositional flux and stable Pb atom ratio of contaminant Pb derived on the basis of the 210Pb chronologies are in good agreement with known temporal variations in these parameters, consistent with post-depositional immobility of Pb under the conditions investigated here. The study confirms that the high degree of mobility of radiocaesium in organic soils renders it of little value in providing chronological information, but suggests that the presence of small amounts of mineral matter may restrict this mobility.