The Redox Interface

Because reductants are present in most water-saturated soils or sediments and oxygen is relatively unavailable, the Eh declines as the water moves into the subsurface. The decline may be from oxic to anoxic sulfidic or nonsulfidic levels (Table 11.5). The rate and extent of Eh decline with distance from the surface depends on the availability and reactivity of sediment organic matter and other reductants. In the sediments of flooded rice paddies, wetlands, estuaries, and shallow lakes, which may be especially rich in fresh organic matter, the redox front or intetface (also termed a redox barrier or boundary by some), which is the zone of abruptly changing Eh values, may be only a few millimeters or centimeters thick. 

Groundwater aquifers in older carbonate and sandstone rocks may have been leached of any reactive organic matter and of Fe(II) and sulfide minerals through time. In such systems, the paucity of reductants may create a redox interface that spans hundreds of meters to several kilometers. For example, Langmuir and Whittemore (1971) found that Eh , gradually drops from 250 mV to -100 mV over a flow distance of about 6 km in the Potomac-Magothy-Raritan aquifer of coastal plain New 

Jersey. Because of a paucity of reactive organic matter, redox conditions do not become sulfidic. Over the 6 km, nitrate drops from 3 to 5 x 10 to 10 M, whereas ferrous iron increases from about 10 to 10 M (Langmuir 1969b). 

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Quantification of Trace Metal Fluxes across the Redox Interface Caused by Vertical Turbulent Mixing... 

The vertical transport of Cd, Cu, Pb, Zn, and Co across the redox interface is a result of their adsorption to the oxidized species of Mn(I V) and Fe(lll), their adsorption to particles of biogenic and lithogenic origin, and by turbulent mixing and difftision of the dissolved species. 

Internal particulate trace metal fluxes were obtained from an experimental sediment trap flux study (Pohl et al., 2004) (Section 13.4), and internal dissolved trace metal fluxes across the redox interface were considered as estimated above. 

About 25% of Cd, 12% of Cu, and 8% of Zn, which were supplied by rivers and atmosphere, were eliminated by vertical turbulent mixing across the oxic-anoxic interface and subsequent sulfide precipitation. For Pbjiss, the flux at the redox interface was negligible. Comparing the sum of the internal fluxes (dissolved and particulate) in relation to the total... 

The diffusive flux of Fediss across the redox interface has been calculated n.Tpmol/m day (Pohl and Hennings, 2005). How far this dissolved Fe(ll) from deepwater sources crosses the stable halocline and reaches the euphotic zone and contributes to the fertilization of cyanobacteria is unknown. 

Redox-sensitive elements such as As, Mo, Se, and V, are soluble along with U(VI) in oxidized groundwaters where they occur as oxyanions. However, in the reduced waters at the redox interface of a roll front or other sedimentary U deposit, they are precipitated nearby along with U(IV) in insoluble minerals (cf. Wanty et al. 1987). 

Vertical distributions of dissolved Ba and total (dissolved+particulate) Pu, Am and Th in Framvaren Fjord all show increased concentrations with depth (Falkner etal., 1993 Roos etal., 1993). Ba cycling was dominated by its uptake into particulate matter associated with productivity in surface waters, followed by its regeneration at depth or in the sediments. Microbiological activity near the redox interface likely promotes the breakdown of settling particulate matter and the release of barite just above the 02/H2S interface (Falkner etal., 1993). Complex formation with dissolved organic carbon (DOC) is believed to be the main cause for the observed behavior of Pu, Am and Th (Roos etal., 1993). The distributions of these elements were not examined within the regions near the 02/H2S interface and the associated microbial layer. 

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