Environ. Sci. Technol. 2005, 39, 8207-8213
Introduction
spheric metal deposition, especially for Pb (1-3) and Hg (4-7). In contrast, there are far fewer studies on the distribution and fate of Cu (8-14), Ni (13, 15), Co (12), Cd (8, 10), or Zn (8, 10-12, 14, 16-18). In a previous study (19) we compared high-resolution reconstructions of atmospheric metal deposition with the known metal emission histories at three sites in Finland. Nickel, Zn, and Cd accumulation rates did not reflect the chronology of their atmospheric inputs. In contrast, Cu and Co accumulation rates reflect the known metal deposition histories very well, except for the site of greatest metal loading. As an independent check on the interpretation of the previous study, trace metals were also measured in the porewaters, and these data are the subject of the present paper. To date, porewater analyses from ombrotrophic peat bogs have been restricted to gaseous compounds, anions, and major element cations, as well as Fe, Mn, and Zn (20-22). A few studies have been published which report concentrations of trace elements in bog surface waters (23-25). The porewaters in ombrotrophic bogs represent a unique natural water system. Because metals are supplied only by the atmosphere (precipitation and dust), the concentrations of major elements such as Ca are comparable to rainwater values. Although Ca can be added to the porewaters by dissolution of calcite or apatite supplied by atmospheric dusts to the bog surface (26), or removed in deeper layers by ion exchange (27), in general these waters are dilute. Below the living vegetation layer, the partial decomposition of plant matter results in the formation of peat and consumes dissolved oxygen. Conceptually, the bog surface can be viewed as consisting of an aerated “acrotelm”, where decay is comparatively rapid, overlying a “catotelm”, which is permanently anoxic. Despite the popular use of these terms, there have been few quantitative studies of the redox chemistry of bog porewaters (20, 21). Peat in the acrotelm tends generally to be poorly decomposed and therefore has a relatively high hydraulic conductivity; during seasonal water table draw down, oxygen may penetrate these surface layers. All of the porewaters are rich in dissolved organic matter and contain elevated partial pressures of CO2. In this study, we used a purpose-built sampler to study the release of Cu, Ni, Co, Zn, and Cd to the porewaters of ombrotrophic peat bogs. Samples were collected from peat bogs in Finland which had already been subjected to preliminary studies, including the measurement of these metals in age-dated peat cores from each of the sites (19, 28, 29): Hietaja¨rvi, a “low-background” control site which receives atmospheric trace metals predominantly from longrange transport, Outokumpu, near the famous Cu-Ni mine, and Harjavalta, nearby a Cu-Ni smelter. In selected peat layers, metal-containing particles were identified using SEM/ EDX analysis. The main goal of the study is to determine the extent of Cu, Ni, Co, Zn, and Cd mobility in ombrotrophic peat bogs of varying metal loads and to discuss possible mechanisms of metal release from the solid phase, with special emphasis on Cu.
Peat cores from ombrotrophic bogs have recently proved to be meaningful archives of recent as well as ancient atmo-
Materials and Methods
* Corresponding author phone: +49-6221-544801; fax: +49-6221545228; e-mail:
[email protected]. † University of Heidelberg. ‡ Finnish Forest Research Institute. # Current address: European Commission, Joint Research Centre, Institute for Transuranium Elements, Box 2340, 76125 Karlsruhe, Germany.
Study Sites and Hydrological Conditions. Porewater samples were collected in undisturbed, Sphagnum-dominated ombrotrophic peat bogs at Hietaja¨rvi (HIJ), Outokumpu (OUT), and Harjavalta (HAR) (Figure 1). A detailed description of the sampling sites is given elsewhere (28, 29) but summarized here. The HIJ site is located in the Patvinsuo National Park, eastern Finland, where the peatland area remains in a natural
Porewater Evidence of Metal (Cu, Ni, Co, Zn, Cd) Mobilization in an Acidic, Ombrotrophic Bog Impacted by a Smelter, Harjavalta, Finland and Comparison with Reference Sites N I C O L E R A U S C H , †,# LIISA UKONMAANAHO,‡ TIINA M. NIEMINEN,‡ MICHAEL KRACHLER,† AND W I L L I A M S H O T Y K * ,† Institute of Environmental Geochemistry, University of Heidelberg, Im Neuenheimer Feld 236, 69120 Heidelberg, Germany, and Finnish Forest Research Institute, Box 18, 01301 Vantaa, Finland
Porewaters were collected from three Finnish peat bogs subjected to varying inputs of atmospheric trace metals: Hietaja¨ rvi (HIJ), a low-background site, Outokumpu (OUT), near a Cu-Ni mine, and Harjavalta (HAR), near a Cu-Ni smelter. Samples for metal analyses were collected at depths ranging from 10 to 70 cm using a purpose-built syringetype sampler. Metal concentrations were determined using inductively coupled plasma-sector field-mass spectrometry (ICP-SF-MS). Porewater concentrations at HIJ and OUT (Cd
