Chemical records of environmental pollution in ombrotrophic peat bogs
Cloy, Joanna Marie
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Human activity has affected metal emissions to the atmosphere on a global scale for several thousand years, resulting in widespread contamination of the environment with toxic heavy metals such as Pb and Hg, thereby threatening both human and environmental health. In recent years ombrotrophic peat bogs have been used to study the changing rates and sources of atmospheric metal deposition, as they receive all their water and nutrients from the atmosphere by dry and wet deposition alone. Cores from such bogs have proved especially useful as archives of atmospheric Pb deposition as Pb is essentially immobile in ombrotrophic peat. The work described in this thesis is primarily concerned with the use of ombrotrophic peat bogs to investigate environmental contamination in Scotland during pre-industrial, industrial and post-industrial (i.e. ca. post-1970 A.D.) times. Cores were collected from ombrotrophic peat bogs at four different geographical locations (Carsegowan Moss, SW; Flanders Moss, W Central; The Red Moss of Balerno, E Central; Turclossie Moss, NE) in Scotland. Air-dried peat samples were dry-ashed and dissolved using microwave-assisted HF IHN03 digestion. Elemental concentrations (e.g. AI, As, Ca, Cd, Co, Cr, Cu, Fe, Mg, Mn, Ni, P, Pb, S, Sb, Sc, Se, Ti, V, Y, Zn and Zr) and Pb isotope ratios (e.g. 206PbP07Pb) were determined using ICP-OES and ICP-MS as appropriate. For Hg determination, samples were digested with HN03/H2S04 and then analysed by CV AAS. Certified reference materials (e.g. Ombrotrophic Peat (NIMT/UOE/FM001), Canadian Peat (1878 P), Bush Branches and Leaves (DC73349), Peach Leaves (GBW 08501) and Coal (BCR CRM No. 40 and NBS SRM 1635) were used for quality control purposes. The distribution and behaviour of the potentially toxic trace elements (As, Cd, Co, Cr, Cu, Hg, Ni, Sb, Se, V and Zn) and of major elements (Ca, Fe, Mg, Mn, P and S) within the four ombrotrophic peat bogs was investigated and there was strong evidence that Ca, Fe, Mg, Mn, P, S, Se and Zn were mobile in ombrotrophic peat, while As, Cd, Co, Cr, Cu, Hg (at least during industrial and post-industrial periods), Ni, Sb and V, like Pb, were essentially immobile in ombrotrophic peat. Deposition records of conservative lithogenic elements (e.g. concentrations of AI, Sc, Ti, Y and Zr) that occur predominantly in soil dust were also investigated and the chosen conservative elements Sc, Ti and Zr were used in calculations to estimate anthropogenic enrichments of As, Cd, Co, Cr, Cu, Ni, Pb, Sb and V in peat bog profiles. 210Pb- and 14C- dated peat cores were used to reconstruct historical records of atmospheric anthropogenic As, Cd, Co, Cr, Cu, Ni, Pb, Sb and V deposition (since the pre-Roman (i.e. to ca. 380 B.C.)/Roman period) and atmospheric Hg deposition (since the onset of the industrial period) across Scotland. For Pb, on the basis of Pb isotopic composition (e.g. 206PbP Pb), clear indications of contamination during the preRoman/Roman and Mediaeval periods were attributed to the mining and smelting of Pb ores (from Britain and elsewhere in Europe). During the industrial and post-industrial periods, variations in the relative importance of contributions of anthropogenic Pb from different sources were apparent. From ca. the early 17th century A.D. at three of the peat bog sites, the mining and smelting of indigenous Scottish Pb ores, until the early 20th century A.D., were found to be the most important sources of anthropogenic Pb deposition. In contrast, at the most southerly site (Carsegowan Moss), influences from the use of both British Ph ores and imported Australian Ph ores (in more southern parts of Britain) since the late 19th century A.D. were evident. At each of the sites, the increasing importance of Australian-Ph-influenced car-exhaust emissions from the 1930s to late 1990s A.D., along with significant contributions from coal combustion (until the late 1960s A.D.) was evident. For Sb, in general, similarities between the major trends in the concentration profiles of anthropogenic Sb and Ph suggested common sources of these two elements. Perturbations in the anthropogenic Sb/Pb ratios since ca. 1800 A.D., however, were attributed to temporal variations in the relative importance of atmospheric emissions from different sources such as Ph ore mining/smelting, coal combustion and, in recent decades, automobile-related use of compounds of Ph (in leaded petrol) and of Sb (in brake linings). For Hg, in general, during the industrial and post-industrial periods, coal combustion and waste incineration, respectively, were likely to be the most important sources of Hg. For As, clear indications of contamination during the Mediaeval period were probably attributable to the mining and smelting of Ph and Cu ores, and for As, Cd, Co, Cr, Cu, Ni and V, during the industrial and post-industrial periods, a variety of sources (e.g. metallurgical activities, coal and oil combustion, use of phosphate fertilisers and waste incineration) were important to varying extents. Also, in recent years, atmospheric Cu emissions from automobile-related use of compounds of Cu (in motor oil, brake linings and tyres) may have been important. Inter-site and inter-elemental comparison of records of atmospheric metal deposition across Scotland indicated that, in general, atmospheric As, Cr, Hg, Ph and Sb deposition was greatest during the industrial period (between the late 1880s and late 1960s A.D.) and atmospheric Cd, Co, Cu, Ni and V deposition was greatest during the industrial and post-industrial periods (between ca. 1900 and the early 2000s A.D.), although increases in As, Co, Cr, Hg, Ph and Sb deposition were earliest (during the late 19th and early 20th century A.D.) at the most southerly site (Carsegowan Moss). During the industrial and post-industrial periods, levels of As, Ph and Cd contamination were generally highest in the south of Scotland, Cu, Co and Sb in south and central Scotland, and Cr, Ni and V in central Scotland. Overall, the existence of a south to north As, Cd, Co, Cr, Cu, Ni, Ph, Sb and V pollution gradient in Scotland was evident.