The use of fallout radionuclides as tracers for studying heavy metal records in peat bogs

 

P.G.Appleby* (Department of Mathematical Sciences, University of Liverpool, Liverpool L69 3BX, UK), W.S.Shotyk (Geological Institute, University of Berne, Baltzerstrasse 1, CH-3012 Berne, Switzerland) & A Grünig (Swiss Federal Institute of Forestry, CH-8903 Birmensdorf, Switzerland).

 

ABSTRACT

Although peat bog cores have in many instances been shown to contain high quality records of heavy metal pollution, their precise relationship to levels of atmospheric pollution is not well understood.  Major uncertainties include the extent to which heavy metals deposited on the bog are retained, redistributed spatially over the its surface, or transported downwards into the older layers by advection or diffusion.  The fallout radionuclides 210Pb, 137Cs and 241Am are ideal tracers for studying these processes.  Their atmospheric fluxes are relatively well known, they have contrasting chemical properties, and there are now a number of sites where records have been studied over many years.  Although there are instances where 210Pb inventories vary spatially over the bog, at many sites the inventory is close to the value supported by the direct atmospheric flux.  Since these sites have also yielded reliable 210Pb chronologies it is reasonable to conclude that they contain good records of 210Pb and presumably stable Pb.  At sites where the 210Pb inventory deviates significantly from the fallout value reliable 210Pb dates can still be determined using chronostratigraphic markers such as 241Am and pollen as reference points.  In these cases estimates of atmospheric fluxes of heavy metal from peat bog records can be improved by normalising against the 210Pb.  Reliable models for more soluble species must take account of differences in speciation.  137Cs records from the 1986 Chernobyl accident have shown dramatic changes through time reflecting the impact of the high degree of mobility of this radionuclide in peat.

 

INTRODUCTION

Peat bogs are increasingly seen as important natural archives for records of atmospheric pollution by trace metals (c.f. Shotyk et al. 1998).  If these records are however to be used to make accurate reconstructions of historical levels of atmospheric pollution, it is essential to have a clear notion of the parameters being used to measure atmospheric pollution, and of the relationship between those parameters and the data determined from the bog.  Depositional fluxes across the peat bog surface (measured as a mean annual average) are mainly controlled by the atmospheric concentration and total rainfall.  Concentrations at particular points in the bog will depend on the peat growth rate, the impact of any lateral and vertical transport processes at the time of deposition or following incorporation in the peat matrix, and on the rate of vegetative decay.  Since pollutants for the most part have unknown depositional histories, models of the impact of these processes can not be assessed from the bog records themselves.  There are however a number of natural (210Pb) and artificial (137Cs, 241Am) radiouclides with well defined fallout records that are ideal for this purpose.  In particular cases where the pollutant and radionuclide are likely to have similar geochemical behaviour (e.g. stable Pb and 210Pb), the pollution history can be measured simply by normalising the data against that for 210Pb.

 

METHODS

Peat cores collected from 8 sites in Switzerland during 1993-8 (Weiss et al. 1999; Shotyk et al. 2000) were analysed for fallout radionuclides in the University of Liverpool Environmental Radioactivity Research Centre by direct g assay using Ortec HPGe low-background intrinsic germanium detectors (Appleby et al. 1986). At two of the sites (Tourbière de Genevez and Gola di Lago) cores had also been collected in 1986 shortly after the Chernobyl reactor accident and analysed following the same procedures. Stable Pb records in each core were determined using an EMMA miniprobe XRF (Cheburkin & Shotyk 1996).  All the sites are in rural locations relatively remote from obvious pollution sources.

 

RESULTS AND DISCUSSION

210Pb profiles

Figure 1 shows unsupported 210Pb concentrations versus depth in each core, including two cores from Etang de la Gruère (collected in 1991 and 1993), and two cores from Gola di Lago (collected in 1986 and 1995).  To compensate for differences in dry bulk density the depth has been measured in terms of cumulative dry mass.  Although the profiles differ in detail, the depths at which 210Pb reaches equilibrium with the supporting 226Ra are very similar, apart from Schopfenwaldmoor (in central Switzerland) where the 99% 210Pb equilibrium depth (corresponding to c.150 years) occurs at a depth of 1.3 g cm-2. The results suggest that, apart from SWM, the bogs all have had similar mean net peat accumulation rates during the past 150 years.  Differences between individual profiles are most likely due to fluctuations in growth rates, changes in the surface topography, and variations in the atmospheric 210Pb flux.        

 

 

       (a)                                     (b)                                           (c)

 

Figure 1:  210Pb activity versus depth profiles in peat cores from (a) Etang de la Gruère (EGR), La Tourbiére des Genevez (TGE), (b) Pré Rodet (PRD), Schopfenwaldmoor (SWM), Hagenmoos (HGM) and (c) Gola di Lago (GDL), Suossa (SUO), Mauntschas (MAU).

 

 

Atmospheric fluxes

Figure 2 plots the mean 210Pb flux to each core site required to support the measured 210Pb inventories.  Also shown are the stable Pb inventories in each core accumulated since the industrial revolution. Atmospheric deposition is largely controlled by wet deposition, and the differences between sites reflect differences in rainfall as well as differences in levels of atmospheric pollution.  Mean annual precipitation at the study sites varies from c.915 mm at Hagenmoos to 2070 mm at Suossa.  Since concentrations of 210Pb in rainfall are relatively uniform on this spatial scale, the ratio of stable Pb to 210Pb may be a better index of atmospheric pollution.  Figure 3 plots values of the stable Pb/210Pb ratios.  The results suggest that atmospheric Pb pollution was relatively uniform throughout much of central Switzerland (PRD, SWM, SUO).  At these sites the Pb/210Pb ratios all lie between 0.013-0.015.  Levels were c.50% higher in northern Switzerland (EGR, TGE, HGM) where the ratios ranged from 0.019-0.023, and nearly 300% higher at GDL in southern Switzerland.  The latter site almost certainly records pollution from a relatively localized source south of the alps (Weiss et al. 1999).  The lowest levels of Pb pollution (as measured by the Pb/210Pb ratio) are near St Moritz (MAU).

 

 

 

            Figure 2:  210Pb fluxes and stable Pb inventories in Swiss peat cores.

 

 

 

 

            Figure 3:  Stable Pb/210Pb ratio in Swiss peat cores.

 

 

Post-depositional mobility

The relative stability of Pb records in these sites is evidenced by the good agreement between peat dates determined from the 210Pb record and those determined independently from the 241Am stratigraphy (Appleby et al. 1997).  Migration of either radionuclide would almost certainly result in discrepancies between the two dating methods.  This is not however the case for relatively more soluble metals.  Figure 4 compares 137Cs activity versus depth profiles in cores from two sites (TGE and GDL) collected immediately after the 1986 Chernobyl accident, and again in the 1990s.  The surficial 137Cs concentration in the 1986 GDL core exceeded 40,000 Bq kg-1, reflecting the very high fallout of Chernobyl radionuclides at this site.  By 1995, however, the maximum 137Cs concentration had fallen by over an order of magnitude, to less than 3,000 Bq kg-1.  Similar reductions were observed at TGE in the space of just 5 years, between 1986 and 1991, though concentrations were lower due to the lower fallout at this site.

 

(a)                                                                    (b)

Figure 4:  137Cs activity versus depth profiles in cores from (a) La Tourbiére des Genevez (TGE) and (b) Gola di Lago (GDL), collected in 1986 immediately after the Chernobyl accident, and again in the 1990s.  The dramatic reductions in surficial 137Cs activity during the years following 1986 illustrate the high degree of mobility of this radionuclide in peat bogs.   

 

These results illustrate the importance of considering the speciation of heavy metals when studying their records in peat bogs.

 

ACKNOWLEDGMENTS

We gratefully acknowledge financial support from the Swiss National Science Foundation (Grant 21-55669-98 to W.S.).  Dr. Andriy Cheburkin of EMMA Analytical Inc., Elmvale, Ontario, designed and built the miniprobe XRF analyzer, and carried out the measurements of stable Pb in the peat samples.  Philipp Steinmann, Martin Otz, and Dominik Weiss helped with peat sample preparation and pore water analyses.

 

 

REFERENCES

Appleby PG, Shotyk W, Fankhauser A (1997), Water Air Soil Poll., 100:223-231.

Appleby PG, Nolan PJ, Gifford DW, Godfrey MJ, Oldfield F, Anderson NJ, Battarbee RW, (1986), Hydrobiologia, 143:21-27.

Cheburkin AK, Shotyk W (1996), Fresenius J. Anal. Chem. 354:688-691 

Shotyk W, Weiss D, Appleby PG, Cheburkin AK, Frei R, Gloor M, Kramers JD, Reese S, van der Knaap WO (1998), Science 281:1635-1640.

Shotyk W, Blaser P, Grünig A, Cheburkin AK (2000), Sci. of the Tot. Environ. 249:257-280.

Weiss D, Shotyk W, Appleby PG, Cheburkin AK, Kramers JD (1999), Environ. Sci. Technol. 33:1340-1352.