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
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
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.
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.