Anoxic hypolimnion

In anoxic hypolimnion samples collected from Lower Mystic Lake, MA, hexachloroethane was abiotically transformed into tetrachloroethylene via reductive elimination and to pentachloro-ethane via hydrogenolysis. Tetrachloroethylene accounted for 70% of hexachloroethane in unaltered lake water and 62% in filter-sterilized water after 10 d. Trichloroethylene and pent-achloroethane accounted for <1 and 2% in unaltered lake water and filter-sterilized water, respectively. Disappearance rate constants for hexachloroethane were 0.33/d for unaltered water and 0.26/d for filter-sterilized water. At least 80% of the hexachloroethane disappearance in unaltered water was abiotic in origin due to the reactions with naturally occurring aqueous polysulfides, H2S and (Miller et al, 1998a). 

The CRS AVS ratio reflects the trophic state of a lake (cf. Urban (28), Table V). This observation may be explained by the preceding kinetic considerations. The high organic matter supply in eutrophic lakes leads to an intensive mineralization rate by both iron- and sulfate-reducing bacteria. However, reoxidation of Fe2+ to ferric oxide or of sulfide to sulfate does not take place because an anoxic hypolimnion prevents penetration of oxygen. Therefore FeS can build up, but the sediment becomes depleted with respect to reactive iron. 

Cycles of Trace Elements in a Lake with a Seasonally Anoxic Hypolimnion... 

Figure 1. Schematic representation of the processes influencing the cycles of trace elements in Lake Greifen. During stagnation the lake is divided into an oxic epilimnion and an anoxic hypolimnion. Three different stages can be distinguished in time and space ...

This chapter discusses the chemical mechanisms influencing the fate of trace elements (arsenic, chromium, and zinc) in a small eutrophic lake with a seasonally anoxic hypolimnion (Lake Greifen). Arsenic and chromium are redox-sensitive trace elements that may be directly involved in redox cycles, whereas zinc is indirectly influenced by the redox conditions. We will illustrate how the seasonal cycles and the variations between oxic and anoxic conditions affect the concentrations and speciation of iron, manganese, arsenic, chromium, and zinc in the water column. The redox processes occurring in the anoxic hypolimnion are discussed in detail. Interactions between major redox species and trace elements are demonstrated. 

The surface area of Lake Greifen is 8.5 X 10 m2, and the volume is 150 X 106 m3 its average depth is 17.7 m, with a maximum of 32.2 m. The residence time of water is 1.1 years. Thermal stratification lasts about from May to December, and lake overturn usually takes place in December-January. An anoxic hypolimnion develops during summer stagnation from about June to December. 

The processes occurring in the hypolimnion and in the epilimnion have to be considered separately. The appearance of As(III) in the anoxic hypolimnion is in qualitative agreement with the thermodynamic redox sequence, because it appears together with Fe(II) and S(-II). The pe calculated from the As(III)/As(V) couple at 30-31 m on October 19, 1989, is in agreement with pe calculated from Fe(II)/Fe(III) (pe 0), but is higher than indicated by the presence of S(-II). 

The oxidation state of redox-sensitive trace elements such as As(III)/ As(V) and Cr(III)/Cr(VI) is thus affected by the redox conditions, as indicated by the occurrence of major reduced species. Kinetic control of the redox reactions plays an important role. As(III) appears in the anoxic hypolimnion in agreement with the thermodynamic redox sequence together with Fe(II) and sulfide, although the reduction of As(V) is incomplete under these conditions. Whereas the reduced As(III) species can clearly be observed in the... 

The speciation of arsenic in lakes does not always follow thermodynamic predictions. Recent studies have shown that arsenite predominates in the oxidized epilimnion of some stratified lakes, whereas arsenate may persist in the anoxic hypolimnion (Kuhn and Sigg, 1993 Newman et al., 1998 Seyler and Martin, 1989). Proportions of arsenic species can also vary according to the availability of particulate iron and manganese oxides (Kuhn and Sigg, 1993 Pettine et al., 1992). Sunlight could promote oxidation in surface waters (Voegelin and Hug, 2003). 

Kuhn, A., Johnson, C. A., and Sigg, L. (1994) Cycles of Trace Elements in a Lake with a Seasonally Anoxic Hypolimnion. In Environmental Chemistry of Lakes and Reservoirs, L. A. Baker, Ed., Advanced Chemical Services, vol. 237, pp. 473-457. 

Both sediment traps (at 15 and 28 m) are below the productive layers. From July to November 1990, oxygen was absent from the hypolimnion below 10-m depth, so that both traps were within the anoxic hypolimnion. The differences in composition between the traps are relatively small (Table I). The organic C and P content, as well as the concentrations of the other elements, are generally very similar at both depths. Differences in the manganese content appear during certain time periods, such as during May-June 1990, wher... 

Tipping, E. and Woof, C. (1983b). Elevated concentrations of humic substances in a seasonally anoxic hypolimnion Evidence for co-accumulation with iron. Arch. Hydrobiol. 98, 137-145. 

Xue, H. B., R. Gachter L. Sigg, 1997. Comparison of Cu and Zn cycling in eutrophic lakes with oxic and anoxic hypolimnion. Aquat. Sci. 59 176-189. 

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