1. Review of metal behaviours
According to Skjelkvåle et al. (2006), the main (non-point) sources of metals influencing surface water concentrations in Norway are:
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Geological. Metals are derived directly from the weathering of bedrock and surficial deposits
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Atmospheric. Metals may be transported from elsewhere in Europe as “long-range, transboundary air pollution” (LRTAP), or from local sources within or near Norwegian borders. LRTAP is particularly important in Southern Norway
The relative importance of these sources is different for different metals. Historically, As, Cd, Hg, Pb and Zn have been primarily determined by LRTAP, whereas Cr, Cu and Ni have been mostly influenced by geology (except close to the Russian border, where Cu and Ni are strongly affected by local industry). Furthermore, the mobility of metals derived from these sources is influenced by the following factors:
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Water chemistry. Some elements are more easily mobilised at lower pH, and many metals form complexes with organic matter, leading to higher concentrations in areas with high total organic carbon (TOC; Groenenberg et al., 2015)
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Hydrology. Short residence times and/or high water fluxes mean reduced contact time between water and soil, leading to lower metal concentrations. Sedimentation rates are also important, as a high proportion of the total metal load is transported in solid phases (Vink and Peters, 2003)
Key factors believed to determine surface water concentrations for the eight metals of interest are summarised in Table 1.
| Element | Key factors in Norway |
|---|---|
| As | Strongly influenced by LRTAP |
| Cd | Strongly influenced by LRTAP. Concentrations higher in acidified regions |
| Cu | Primarily determined by bedrock geology, except close to the Russian border, where local industrial sources dominate. LRTAP not a dominant factor. Some old mining regions exhibit high concentrations |
| Cr | Concentrations primarily determined by bedrock geology and TOC. Not strongly influenced by atmospheric deposition |
| Hg | Primarily influenced by LRTAP. Strong affinity with organic matter |
| Ni | Primarily determined by bedrock geology, except close to the Russian border, where local industrial sources dominate. LRTAP not a dominant factor |
| Pb | Strongly influenced by LRTAP. Strong affinity with organic matter. Concentrations higher in acidified regions |
| Zn | Strongly influenced by LRTAP, but with local geological effects too. Some clear point sources in mining areas. Concentrations higher in acidified regions |
Table 1: Key factors influencing metal concentrations in Norwegian surface waters. LRTAP, Long-range, transboundary air pollution. Summarised from Skjelkvåle et al. (2006)