ISOTOPIC EVIDENCE OF THE RELATIVE RETENTION AND MOBILITY OF LEAD AND RADIOCESIUM IN SCOTTISH OMBROTROPHIC PEATS

The reconstruction of historical trends in atmospheric deposition of c ontaminant metals by use of their concentration profiles in accumulati ng, ombrotrophic peat deposits, in conjunction with radionuclide-deriv ed chronologies, is subject to uncertainty because of potential post-d epositional mobility both of the radionuclides and of the contaminant metals. Results are presented here for a study of the distribution of Pb-210, Pb, stable Pb isotopes and radiocaesium in the unsaturated, su rface zone of ombrotrophic peat deposits in three contrasting location s in Scotland. Pb-210 chronologies are derived and the implied histori cal variations in atmospheric depositional fluxes and atom ratios of c ontaminant Pb are considered, along with total inventories, relative t o known historical trends in input and proximity to heavily populated, industrial areas. Similarly, radiocaesium concentration profiles, act ivity ratios and inventories are considered in the context of the Pb-2 10 chronologies and known inputs. It is concluded that the historical trends in the depositional flux and stable Pb atom ratio of contaminan t Pb derived on the basis of the Pb-210 chronologies are in good agree ment with known temporal variations in these parameters, consistent wi th post-depositional immobility of Pb under the conditions investigate d here. The study confirms that the high degree of mobility of radioca esium in organic soils renders it of little value in providing chronol ogical information, but suggests that the presence of small amounts of mineral matter may restrict this mobility. (C) 1997 Elsevier Science B.V.