Environ. Sci. Technol. 2010, 44, 8497–8502
Storage and Behavior of As, Sb, Pb, and Cu in Ombrotrophic Peat Bogs under Contrasting Water Table Conditions J A M E S J . R O T H W E L L , * ,† KEVIN G. TAYLOR,‡ SIMON R. N. CHENERY,§ ANDREW B. CUNDY,| MARTIN G. EVANS,† AND TIMOTHY E. H. ALLOTT† Upland Environments Research Unit, School of Environment and Development, The University of Manchester, Manchester M13 9PL, U.K., Department of Environmental and Geographical Sciences, Manchester Metropolitan University, Manchester M1 5GD, U.K., British Geological Survey, Keyworth, Nottingham NG12 5GG, U.K., and School of Environment and Technology, University of Brighton, Brighton BN2 4GJ, U.K.
Received April 28, 2010. Revised manuscript received September 9, 2010. Accepted October 8, 2010.
Concentration depth profiles and inventories of solid-phase As, Sb, Pb, and Cu were determined in 210Pb-dated cores from an ombrotrophic peat bog in northwest England. Cores were collected from the peat dome and adjacent to an eroding gully. Down-core distributions of As, Sb, Pb, and Cu in the dome core are almost identical. The water table is close to the dome surface with only short-term draw-down. Under these conditions, As, Sb, Pb, and Cu are immobile, allowing the reconstruction of trends in historical contaminant deposition. The peak in atmospheric deposition of As, Sb, Pb, and Cu (4.59, 2.78, 147, and 26.7 mg m-2 y-1, respectively) occurred during the late 19th century. Stable Pb isotope ratios reveal that Pb deposition during this period was from indigenous and foreign sources. The mean water table is much lower at the gully edge, and there are pronounced interannual fluctuations. These conditions have not affected the integrity of the Pb and Cu records but have caused postdepositional mobilization and redistribution of As and Sb. Cumulative inventories show significant loss of As and Sb at the gully edge site. Longterm water table draw-down in ombrotrophic peat bogs has the potential to alter the geochemistry and fate of previously deposited As and Sb.
Introduction Arsenic (As) and antimony (Sb) are potentially toxic trace metalloids (1-3). Fossil fuel combustion, mining and smelting activities, and industrial applications have released substantial quantities of As and Sb into the atmosphere in the last few decades (2) and throughout history (4). Due to their toxicity, there is a need to understand the distribution, cycling, * Corresponding author e-mail: james.rothwell@manchester. ac.uk. † The University of Manchester. ‡ Manchester Metropolitan University. § British Geological Survey. | University of Brighton. 10.1021/es101150w
2010 American Chemical Society
Published on Web 10/26/2010
and fate of atmospherically deposited As and Sb in soils and sediments (5-8). Ombrotrophic peat bogs receive inputs solely from the atmosphere (9) and as a result, these organic-rich soils have been shown to record an archive of past atmospheric deposition of As (e.g., refs 10, 11) and Sb (e.g., refs 12, 13). It has been suggested that the chronology of historical As and Sb deposition onto peat bogs is similar to that of Pb, with maxima occurring between the late 19th century industrial period and around the middle of the 20th century (9-13). Peat bogs play a major role in the storage, transformation, and mobilization of trace metals and metalloids (14). It has been suggested that As and Sb are immobile in ombrotrophic peat bogs (13). However, some research has contradicted this view, suggesting that short-term drying and rewetting of peat soils, and the associated change in redox conditions, can lead to a transfer of As from soils to pore waters (15) and to peatland streams (7). It has been suggested that As and Sb display similar geochemical behavior in soils and sediments (5). Therefore, there may also be the potential for mobilization of Sb in peat bogs under fluctuating water table and changing redox conditions. A significant proportion of the ombrotrophic peat in the UK is eroded with extensive gullying. Erosion has increased in intensity in the last 200 years and has been attributed to climatic and anthropogenic pressures (16). In these degraded peat bogs, water tables are lower than those in intact peats, due to a local draw-down effect immediately adjacent to erosional gullies and a reduction in the hydrological contributing area caused by drainage diversion into gully channels (17). In these gullied peatlands, there has been longterm modification of soil hydrology. Predicted anthropogenic climate forcing, through changes in the seasonality of rainfall and temperature (18), could lead to similar long-term change in water table dynamics in other ombrotrophic peat bogs. This could have significant impacts on the mobility of redoxsensitive trace metals and metalloids. There is a paucity of data on As and Sb distributions and inventories in ombrotrophic peat bogs where there has been long-term modification of water table conditions. The aim of the study was to assess the storage and geochemical behavior of As, Sb, Pb, and Cu in hydrologically contrasting peat bog environments in the Peak District uplands (NW England). The solid-phase distributions of As and Sb in 210Pbdated cores were compared to those of immobile trace metals, Pb and Cu (19-21), and redox-sensitve elements, Fe and Mn (14). Cumulative element inventories were calculated for each site and inferences about element mobility under contrasting hydrological conditions aided by detailed measurement of water table variability over 6 months. Stable Pb isotopes (206Pb, 207 Pb, 208Pb) were used to identify the major sources of Pb to the peat bog. Furthermore, this study provides the first quantified information on the chronology and intensity of atmospheric As and Sb deposition to an English ombrotrophic peat bog located in the industrial heartland of the 18th and 19th century.
Materials and Methods Site Description and Sampling. Peat cores were collected from Alport Moor (AM) in the Peak District, NW England (OS grid reference SK1192). This upland area is situated between the industrial cities of Manchester and Sheffield (Figure S1, Supporting Information). AM is dominated by grasses and is characterized by a large peat dome and incising peat gullies (ref 22; Figure S1, Supporting Information). It has been shown VOL. 44, NO. 22, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Profiles of element concentration (mg kg-1) versus depth (cm) in cores collected from (A) the dome at AM and (B) the gully edge at AM. 210Pb dates are shown on the right-hand axis. Dotted lines indicate the As and Pb concentration maxima. that intact and eroding peat areas have marked differences in hydrology (17). In order to investigate the storage and geochemical behavior of metals and metalloids in these contrasting settings, a peat core was collected from within 2 m of an eroding gully (2007) and from the dome (2008). Cores were extruded using a stainless steel Russian corer (77 mm internal diameter). Some of the vegetation was removed at the peat surface to facilitate core extraction. Water table depths at the dome and gully edge sites were measured between August 9, 2008 and January 21, 2009 using capacitance probes and associated data loggers (TruTrack WR HR1000) installed in dipwells (see ref 17 for details). Geochemical Analysis. The upper 30 cm of each core was sliced at contiguous 1 cm intervals (using a stainless steel blade) and homogenized (using a mortar and pestle). Following concentrated HNO3 digestion at 175 °C in closed vessels (USEPA Method 3051a) using a microwave apparatus (MARS Xpress, CEM), an ICP-OES (Perkin-Elmer Optima 3300RL/2100DV) was used to determine Pb, Cu, Fe, and Mn. An ICP-MS (Agilent 7500CX) was used to determine As and Sb concentrations. Metal/metalloid concentrations represent pseudototal values (all but the residual fraction). Blanks, check standards, replicates, and a certified reference material (CCRMP LKSD4, organic-rich sediment) were included in the analysis. Experimental and certified values were within 10%. Replicates yielded an RSD of e5% for Pb, Cu, Fe, and Mn, and e3% for As and Sb. Isotope ratio determinations were made using a quadrupole ICP-MS (Thermo PQ ExCell). Prior to analysis, peat digests were diluted using 1% HNO3. Raw isotope intensity count rates were collected for 206Pb+, 207Pb+, and 208Pb+ ions. Processing consisted of blank correction and application of mass bias factors derived from SRM981 (23). Quality control was provided by replicate analysis of an in-house solution of Glendenning galena (Table S1, Supporting Information). Uncertainties for individual samples were calculated using expanded precisions to include the mass bias correction and a coverage of k ) 2 (2σ), equating to approximately 95% confidence intervals. The overall analytical precision of the measurements (1σ), including mass bias correction, was
