Zinc

Although Zn is known as an ubiquiteous element in nature, it is supposed that about 96% of its release into the global environment is the result of anthropogenic activities, like electroplating, smelting and ore processing, corrosion from alloys and galvanized surfaces as well as erosion from agricultural land (see chapters 2-3 before). It is supposed that dissolved Zn in Canadian and Scandinavian freshwaters rarely exceeds 40 |g/l. High concentrations are likely to occur in acid surface waters as a consequence of decreasing pH (acid rain) and increased atmospheric inputs. In Sweden a significant negative relationship between Zn and pH has been reported, but not as evident as in northern Sweden watercourses with about half the concentration level found in southern surface waters for the same pH. From a large monitoring campaign in 1990 including > 2000 samples from 23 large rivers and 13 small streams, highest concentrations were found to be about 60 |g/l (for pH values between 4.0-7.5). Highest Zn levels reported from the 'Nordic Lake survey' in 1995 were 55 |g/l in Finland, 427 |g/l in nonlimed Swedish lakes, 56 |g/l in limed Swedish lakes, and 139 |g/l in Norwegian lakes (see Lydersen et al., 2002). But almost 90% of all lakes investigated had concentrations < 5 |g Zn/l. Beside low pH, also SO4 can significantly increase Zn solubility and mobility in surface waters by forming soluble ZnSO4 complexes. To what extent the reduced atmospheric SO4-deposition to Scandinavian surface waters may contribute to an increasing Zn mobility remains to be answered. There are hardly reports on a significant relationship between Zn concentrations and TOC/DOC, although the data from the 1995 Nordic Lake survey seems to indicate a weak tendency of increasing Zn concentrations with increasing TOC.

Based on the 1995 survey, median concentrations of Zn in Norwegian, Swedish and Finish lakes are 1.54, 1.27, and 1.80 |g/l, respectively. Zn concentrations analysed in 360 samples from the mor layers of Swedish forest soils varied between 14 to 149 mg/kg dw (with a median value of 55 mg/kg dw). A similar survey in Norway showed a large influence of longrange atmospheric transport on Zn distribution in soils with an average value around 30 mg/kg dw in the humus layer. As for the other metals, moss analysis in Scandinavia in 1995 showed elevated concentrations in the vicinity of large point sources, such as the metal smelter in Ronnskar (Sweden) and Odda (Norway). Median concentration of Zn in Swedish groundwater is 10 |g/l, which is higher than for running (4.1 |g/l) and lake waters (1.0 in northern and 2.2 |g/l in southern Sweden).

Background concentrations are supposed to be 3 | g/l (pH > 6), and vary greatly with pH, with higher levels at lower pH. Most Zn entering surface waters precipitates into the sediment. Although Zn seems to be more associated with easily reducible (exchangeable and Mn oxide bound) and reducible sediment fractions (Fe oxide bound), in acidic (pH 5.0-5.6) lakes, more Zn seems also associated with organic fractions (likely coprecipitated with Fe-humic compounds). Sediment-bound Zn in relatively unpolluted lakes in southern, central and northern Sweden varied between 130-140, 130-380 and 59-330 mg/kg dw, respectively. Background concentrations for zinc in freshwater sediments in other European countries usually range between 12 and 150 mg/kg dw. Concerning aqueous Zn speciation, the free Zn ion coordinates with water molecules to form the octahedral Zn(H2O)62+ in the absence of inorganic and organic ligands. At pH 4-7, Zn exists in freshwaters almost completely as the aqua ion, while at pH 6 the free ion and ZnSO4 are the dominant forms. It is supposed that about 90% of dissolved inorganic Zn occurs as free ion in addition to ZnHCO3+, ZnCO3 and ZnSO4. In general, Zn is increasingly released from sediments under high dissolved oxygen, low salinity and pH.

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