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N Speciation Under Varying Oxidation-Reduction Conditions

Figure 1 shows how the concentrations of some redox-sensitive species vary with depth in a lake sediment (The graphs are from Morel and Hering 1993 using data from Wersin et al. 1991. I'm not sure how to handle this.). The sediment surface is in contact with oxic lake water. The concentrations of NO3- and SO42- decrease rapidly with increasing depth while the concentrations of NH4+ and HS-+H2S increase. At the sediment surface, O2 becomes depleted due to aerobic respiration. Other oxidants, such as NO3- and SO42-, become more important in deeper sediments. The sequence of reactions cannot always be observed because of practical sampling and analytical limitations.

Figure 1. Concentrations of redox-active species in interstitial water as a function of depth in the sediment (cm) and in overlying water. The sediment-water interface is at a depth of 0 cm. Samples from Lake Greifen, Switzerland. Data from Wersin et al., 1991.

Figure 2 shows depth profiles of some redox-sensitive species in a stratified fjord (Emerson et al. 1979). Detectable concentrations of O2 are found in the upper mixed layer (from the surface down to 130m). At greater depths other oxidants, including NO3-, SO42-, and Fe and Mn oxides, are used in microbial respiration.

Figure 2(a). Oxygen and hydrogen sulfide (SH2S = H2S + HS- + S2-) distribution in Saanich Inlet waters in July 1977 (Sta. B). The oxygen-hydrogen sulfide boundary is at 130 m.

Figure 2(b). Nitrogen species in Saanich Inlet in July 1977 (Sta. B).

Figure 2(c). Iron and manganese in Saanich Inlet in July 1977 (Sta. B). The inset shows the closely spaced results derived from the pump cast.

Figure 3 shows schematically how concentrations of some redox-sensitive species vary laterally in shallow ground water from three aquifer systems (Edmunds et al. 1984). EH is the Pt-electrode potential, which qualitatively indicates redox conditions. In all three aquifer systems there is a sharp redox boundary. Up-gradient from the boundary O2 and NO3- are detectable and NH4+, HS-+H2S, Fe2+, and Mn2+ are undetectable. Down-gradient from the redox boundary O2 and NO3- are undetectable and NH4+, HS-+H2S, Fe2+, and Mn2+ are detectable.

Figure 3. Spatial variation of NO3-, NH4+ , and related solutes in shallow ground water (Edmunds et al. 1984).      click to enlarge graphic


References Cited:

Edmunds, W. M., D. L. Miles, and J. M. Cook. 1984. A comparative study of sequential redox processes in three British aquifers. In E. Eriksson, ed. Hydrochemical Balances of Freshwater Systems, Proceedings of the Uppsala Symposium. IAHS-AISH, Pub. 150.

Emerson, S., R. E. Cranston, and P. S. Liss. 1979. Redox species in a reducing fjord: equilibrium and kinetic considerations. Deep-Sea Res. 26A:859-878.

Morel, F. M. M. and J. G. Hering. 1993. Principles and Applications of Aquatic Chemistry. New York:Wiley.

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