Environ. Sci. Technol. 2009, 43, 1756–1762
Retention of As and Sb in Ombrotrophic Peat Bogs: Records of As, Sb, and Pb Deposition at Four Scottish Sites J O A N N A M . C L O Y , * ,† J O H N G . F A R M E R , † MARGARET C. GRAHAM,† AND ANGUS B. MACKENZIE‡ School of GeoSciences, University of Edinburgh, West Mains Road, Edinburgh, U.K., and Scottish Universities Environmental Research Centre, East Kilbride, U.K.
Received September 26, 2008. Revised manuscript received December 16, 2008. Accepted December 23, 2008.
Possible postdepositional As migration in ombrotrophic peat bogs was investigated by comparing depth profiles of As with those of Sb and Pb, two elements considered to be essentially immobile in peat, and those of redox-sensitive, potentially mobile nutrient elements such as Mn, Fe, P, and S in 210Pb-dated cores from four Scottish bogs. Concentration profiles of As were similar to those of Sb and Pb rather than these other elements, indicating that As is bound strongly to organic matter and is relatively immobile in ombrotrophic peat. Historical records of atmospheric anthropogenic As, Sb, and Pb deposition during the industrial and postindustrial periods were derived, sitespecific maxima (up to 1.55, 1.33, and 45 mg m-2 y-1, respectively) occurring between the late 1890s and 1960s, reflecting emissions from diverse sources such as mining and smelting, coal combustion, and also, in the case of Pb, exhaust emissions from the use of leaded gasoline. Since the mid1980s, fluxes of Pb decreased (4-7 fold) more rapidly than those of As and Sb (2-3 fold), attributable to both the gradual elimination of leaded gasoline and recent new sources of the latter elements. Relative trends in derived anthropogenic As, Sb, and Pb deposition largely agreed with other Scottish peatandmossarchiverecords,directmeasurementsofdeposition, and UK emissions, i.e., four different types of data source.
Introduction Ombrotrophic peat bogs, which receive all of their nutrient, pollutant, and water inputs solely by deposition from the atmosphere, have proved especially useful as archives of atmospheric Pb deposition (e.g., 1-5). The chalcophilic elements As and Sb frequently occur together with Pb in sulfidic minerals and coals, and all three elements are largely emitted simultaneously to the atmosphere during mining/ smelting and coal combustion (6). There have been comparatively few peat core studies of historical atmospheric deposition records, however, in which all three elements were featured (7). Furthermore, although Pb is considered essentially immobile (1-5) and there is growing evidence for the immobility of Sb (8, 9), much less is known about the geochemical behavior of As in peat bogs. In freshwater lake * Corresponding author e-mail:
[email protected]. † University of Edinburgh. ‡ Scottish Universities Environmental Research Centre. 1756
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sediments, As is known to undergo postdepositional diagenetic remobilisation, in association with the redox cycle of Fe, resulting in significant natural enrichment of As in nearsurface layers and perturbation of the historical pattern of As deposition (e.g., (10)). In ombrotrophic peat bogs, where there may be fluctuating water table depths and varying redox conditions, there is thus potential for vertical mobility of As, although the predominantly organic bulk peat matrix is markedly different from that of typical lake sediments. The aim of this study, therefore, was first to assess the geochemical behavior of As in peat cores from four Scottish ombrotrophic bogs via the comparison of concentration profiles of As with those of Sb and Pb, and also with those of redox-sensitive and biologically essential elements Mn, Fe, P, and S, which may be subject to postdepositional redistribution. Second, a subsequent historical interpretation of the As, Sb, and Pb profiles was aided by 210Pb dating, temporal trends in the stable isotopic composition of Pb (e.g., 206Pb/207Pb), and profiles of Ti, a conservative lithogenic element indicative of soil dust input.
Materials and Methods Sampling. Cores were collected from four sites in Scotland: Turclossie Moss (TM), northeast; Flanders Moss (FM), west central; Red Moss of Balerno (RM), east central; Carsegowan Moss (CM), southwest (Figure 1). Methods of core collection from moss hummocks and preparation for analysis of sections (ranging from 1 to 3 cm in thickness) cut from TM04M-1/ TM04CM-4, FM01CM-1, FM04-1-M, RM03CM-2, CM04CM1, and their sister cores are detailed elsewhere (9, 11). Analysis. Following microwave-assisted HNO3/HF digestion of duplicate peat sections (12), a Thermo Jarrell Ash IRIS ICP-OES (Thermo Electron, Cambridge, UK), and/or a PerkinElmer Optima 5300 DV ICP-OES (Perkin-Elmer, Beaconsfield, UK), was used to determine total Fe, Mn, P, Pb, S, and Ti concentrations in 2% HNO3 (v/v) analyte solutions, with an overall analytical precision ((1 RSD) of e(8%. A PlasmaQuad (PQ) 3 ICP-MS (Thermo Electron, Winsford, UK) was used to determine As and Sb concentrations and Pb isotopic ratios in single sectional sample digests, with analytical precision of e(10% and
