Accumulation Rates of the Heavy Metals Lead, Mercury and Cadmium in Ombrotrophic Peatlands in the West of Ireland

Abstract

The vertical distributions of three heavy metals: Hg, Pb and Cd were determined in 3 cores sampled from two ombrotrophic bogs in the west of Ireland, one at Knockroe Co. Mayo, and the second at Letterfrack National Park, Co. Galway. Core chronologies were established using 210Pb dating techniques and were checked with fallout radionuclides from weapons testing. Variations were found in metal concentrations and cumulative inventories of each of the metals within each site and between the two sites. Maximum accumulation rates of the anthropogenically derived elements Hg, Pb, and Cd, were found in peat sediments dated between 1950 and 1970s at both sites. Pb and Hg accumulation rates are slightly lower than those found in similar studies from remote sites in Europe. Hg accumulation rates are fairly similar to those found in peatlands in America. Unlike the Pb and Hg concentration profiles, the Cd concentration profiles at the Letterfrack site were dominated by a surface enrichment, thought to be due to biological cycling of Cd in the peat. However Cd accumulation rates calculated at the Knockroe site are lower than those observed in Eastern Europe. Local meteorological conditions at the sites chosen for this study may account for the lower concentrations profiles observed when compared with some of the European studies. The similarity between the timing of the increase in metal accumulation rates in peat bogs in Northern America and this study could indicate that long range transportation of trace metals from Northern America may be occurring. Lead accumulations in the surface peat sediments (1993–1996) were between 1.5–3.0 mg m−2 yr−1 and 4–5 mg m−2 yr−1 at Knockroe and Letterfrack, respectively. Mercury accumulation rates for the same period at Knockroe were found to be between 6–11 μg m−2 yr−1, and between 19–24 μg m−2 yr−1 at Letterfrack. A greater variation in surface Cd accumulation rates was observed at both sites, with surface layer accumulation rates varying from 25 to 50 μg m−2 yr−1 at Knockroe and between 166 and 405 μg m−2 yr−1 at Letterfrack. Higher metal concentrations were found at the Letterfrack site, which are most likely due to local sources and the history of the site.