INVESTIGATIONS OF THE POTENTIAL MOBILITY OF HEAVY METALS IN AN OMBROTROPHIC PEAT BOG

Alex J. Freeman, Margaret C. Graham (Margaret.Graham@ed.ac.uk), John G. Farmer (Department of Chemistry, University of Edinburgh, Edinburgh EH9 3JJ, Scotland) and David G. Lumsdon (Macaulay Land Use Research Institute, Aberdeen AB15 8QH, Scotland).

ABSTRACT

Analysis of the solid phase and associated pore waters of individual sections of a core from the ombrotrophic Flanders Moss peat bog confirmed the diagenetic cycling and surficial enrichment of redox-active Mn and Fe and suggested the downward diffusion of Zn. In contrast, both Pb and Cu appeared to be immobile. The absence of vertical mobility of Pb was supported by the good agreement between the ²⁰⁶Pb/²⁰⁷Pb ratios of solid phase peat and corresponding pore waters over the top 25 cm. A significant decrease in the ²⁰⁶Pb/²⁰⁷Pb ratio over the top 5 cm is attributable to the atmospheric deposition of ²⁰⁶Pb-depleted Pb from car-exhaust emissions.

INTRODUCTION

Ombrotrophic peat bogs are widely regarded as excellent archives of atmospheric Pb deposition (Farmer et al, 1997; MacKenzie et al, 1997; Shotyk et al, 1997, 1998; Weiss et al, 1999). It has been suggested, however, that Pb could be mobile in peat bogs, perhaps in association with dissolved organic matter or as a result of release from slightly soluble PbSO₄, itself formed from the oxidation of PbS as the water table drops (Damman, 1978; Urban et al, 1990; Jordan et al, 1997). Pb mobility would have implications not only for archives of stable Pb (²⁰⁴Pb, ²⁰⁶Pb, ²⁰⁷Pb, ²⁰⁸Pb) but also for radioactive ²¹⁰Pb, a popular tool in reconstructing the timescales over which atmospheric deposition of much anthropogenic stable Pb has occurred (Appleby et al, 1997). This study investigates the potential mobility of Pb, other trace elements Cu and Zn, and the known redox-active elements Mn, Fe and S in an ombrotrophic peat bog.

METHODS

Several 1-m cores (5 cm x 5 cm) were collected in July 1999 from Flanders Moss peat bog, near Stirling, Scotland. These cores were cut on-site into 2-cm depth sections and then sections from the same depth in each of five cores were combined. On return to the laboratory, pore water was extracted (by centrifugation and subsequent filtration through a 0.2 mm cellulose nitrate filter) from a portion of each of the combined 2-cm depth sections. The residual peat material was dried and used for determination of ‘pseudo-total’ metal concentrations (8 M HNO₃; microwave digestion). A certified reference material (LGC CMI7004) was included with each batch of acid digestion samples. Determination of elemental concentrations (Al, Mn, Fe, S, Pb, Cu and Zn) in both the pore waters and the solid peat was carried out by AAS, ICP-OES and/or ICP-MS. Pb isotopic data were obtained using ICP-MS (VG-PQ3). Some of the remaining peat material from each depth was used to obtain water, ash and organic matter contents (through drying at 110°C and ashing at 450°C). Pore water DOC was also determined.

RESULTS AND DISCUSSION

Figure 1 shows the vertical variations in the ash, Al and organic matter contents, and in the concentrations of redox-active elements (Mn, Fe, S) and heavy metals (Pb, Cu and Zn) in the solid phase peat. The ash content decreases from a peak at 2-4 cm to ~80 cm, where there is a slight increase. The Al content also declines similarly from a peak at 4-6 cm and has a small peak at ~80 cm. In contrast, the organic matter content increases with depth. The main feature of the vertical distributions of Mn and Fe is a near-surface enrichment (74 mg/kg at 0-2 cm and 3000 mg/kg at 0-4 cm, respectively) whilst the concentration of S is much more uniform at 810-1270 mg/kg over the length of the core. The near-surface distributions of Pb and Cu have some similar features with peaks of 328 mg/kg at 4-6 cm and 165 mg/kg at 20-22 cm for Pb and peaks of 18 mg/kg at 2-4 cm and 5 mg/kg at 18-20 cm for Cu, with concentrations decreasing markedly to < 5 mg/kg and < 3 mg/kg, respectively, below 38 cm. In comparison, the decrease in Zn with depth is more gradual from concentrations of 202-325 mg/kg in the uppermost 38 cm to 121-275 mg/kg below.

The vertical variations in moisture content of the peat and pore water concentrations of Al, dissolved organic carbon (DOC), Mn, Fe, S, Pb, Cu and Zn are shown in Figure 2. There is an increase in moisture content from ~ 85% at the surface to 94% at 36 cm, below which it is constant. The concentration of Al is highest in the 0-2 cm section and decreases with depth whilst DOC has peaks of 173 mg/l at 0-2 cm and at 20-22 cm and then decreases with depth. The profiles for Mn and Fe again have near-surface peaks (0-4 cm and 0-6 cm, respectively) but there is an additional peak in the Fe profile at 22-24 cm which almost directly co-incides with a pore water S peak. The S profile has an additional larger peak at 8-10 cm. The depths of the main peaks in the Pb profile, 0.043 mg/l at 2-6 cm and 0.024 mg/l at 20-24 cm, respectively, differ from those for Cu (0-4 cm and 8-12 cm). Zn, as in the solid phase, is more uniformly distributed throughout the core but concentrations do decrease with increasing depth.

From the solid and pore water data, there is evidence of redox cycling of Mn and Fe in the near surface sections of the core. The presence of DOC, Fe and S peaks in the pore water may indicate that further redox reactions are occurring at ~ 20-26 cm. Of the trace metals, the vertical profile of solid phase Cu is similar to that of Pb which, assuming immobility within the peat, might suggest a similar pattern of past anthropogenic emissions. For Zn, which exhibits a more uniform solid phase concentration profile, the pore water trends perhaps suggest downwards diffusion of Zn. In this case, the solid phase profile for Zn may thus be indicative of significant post-depositional alteration.

Further work has considered the isotopic signatures of Pb in the solid phase peat and in the pore water. Figure 3 compares the vertical trends in ²⁰⁶Pb/²⁰⁷Pb for solid phase and pore water samples over the 0-25 cm portion of the core. The conventional interpretation of the former is the increasing influence of ²⁰⁶Pb-depleted Pb towards the surface, primarily as a consequence of the atmospheric deposition of car-exhaust emissions of Pb arising from the use of petrol alkyllead additives (²⁰⁶Pb/²⁰⁷Pb ~ 1.07) manufactured largely from Australian lead (²⁰⁶Pb/²⁰⁷Pb ~ 1.04) (Farmer et al, 1997, 2000). There is good agreement at all depths between the ²⁰⁶Pb/²⁰⁷Pb ratio in the solid phase peat and in the pore water, the slight difference in the top section perhaps being attributable to the influence of recent rainwater on Pb isotopic composition of the pore water. It would therefore appear that there is a simple equilibrium between Pb in the pore water and in the solid phase and that there is no evidence to support downwards movement of significant quantities of Pb with isotopic signature typical of more recent deposition, i.e. ²⁰⁶Pb/²⁰⁷Pb~1.14 (Farmer et al, 2000). The apparent absence of Pb mobility is in accord with the recent experimental results of Vile et al (1999). Further work is now under way to investigate the associations of Pb in the peat and to model the distribution of Pb between the solid phase and pore water. ²¹⁰Pb dating is also being carried out on the core to assist the historical interpretation of the data.

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Figure 1: Concentration profiles for ash, Al, organic matter (OM), redox-active elements (Mn, Fe, S) and heavy metals (Pb, Cu and Zn) in the solid phase of the Flanders Moss peat core.

Figure 2: Vertical variations in moisture content of Flanders Moss peat and associated pore water concentrations of Al, dissolved organic carbon (DOC), Mn, Fe, S, Pb, Cu and Zn.

Figure 3: Vertical trends in ²⁰⁶Pb/²⁰⁷Pb for solid phase (o) and pore water (�) samples over the 0-25 cm portion of the Flanders Moss peat core.