Redox front

The relative proportions of arsenic species in estuarine waters are more variable because of changes in redox, salinity, and terrestrial inputs (Abdullah et al., 1995 Howard et al., 1988). Arsenic(V) tends to dominate, although M. O. Andreae and T. W. Andreae (1989) found increased proportions of As(III) in the Schelde Estuary of Belgium. The highest values occur in anoxic zones near sources of industrial effluent. Increased proportions of As(III) also occur near sources of mine effluent (M. O. Andreae and T. W. Andreae, 1989). Seasonal variations in concentration and speciation have been reported in seasonally anoxic waters (Riedel, 1993). Peterson and Carpenter (1983) reported a clear crossover in the proportions of the two species with increasing depth in the Saanich Inlet of British Columbia. Arsenic(III) represented only 5% (0.10 p,g L ) of the dissolved arsenic above the redox front but 87% (1.58p,gL ) below it. In marine and estuarine waters, organic forms of arsenic are... 

Roll-front uranium Texas coastal plain Shallow groundwater Leaching of ash, transport, and precipitation at redox front... 

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. 

The decrease of uranium concentration, shown in Fig. 3b between locations 13 and 17 and in Fig. 3c between locations 12 and 14, is similar to that observed at other locations where oxidizing waters encounter a decrease in redox potential but the lack of an associated well-defined increase in A.R. is unusual. An increase in A.R. at a redox front has been observed in both sandstones (Cowart and Osmond, 1980) and in carbonates (Cowart, 1980). The decrease in dissolved uranium serves to locate uranium presently accumulating as a coating on the aquifer rocks. By recoil or selective leaching modes, the A.R. in water can be increased. Direct recoil input is a function of the present whereas selective leaching is strongly influenced by the relative amount of present. The amount of generated by such a... 

Hunter et al. (1998) used a kinetic model to simulate reactive transport and ground-water evolution upon the intrusion of landfill leachate into an oxic aquifer. Most interestingly, they found that the patterns of redox fronts are similar to what have been seen in marine sediments. Hunter et al. (1998) first developed a one-dimensional multi-component reactive transport model biorxntrn and then conducted a numerical experiment using this code. The actual model is quite complex we describe here the essentials of it so that we can see what can be done. 

ScMring, J. Schlieker, M. Hencke, J. (2000) Redox Fronts in Aquifer Systems and Parameters Controlling their Dimensions. In Schuring, J. Schulz, H. D. Fischer, W. R. Bottcher, J. Duijnisveld, W. H. M. (Eds.) Redox - Fundamentals, Processes and Applications, D Edition, Springer. 

The focus in this study is on manganese and iron, i.e. on Reactions (3A) and (4A). Oxygen and nitrate are not present in groundwater observation wells (see Section 11.2) and are already consumed in the very vicinity of the river. Sulphate and methane show no variations on the entire field site, and are thus not of interest for the study of redox fronts. 

Redox fronts exist in aquifers and can be compared to the occurrence of redox fronts in marine and limnic sediments. However the spatial scale is quite different in groundwater compared with lake or river sediments. While sedimentologists usually report fronts in the scale of cm, in groundwater these move much further and are more extended. In the Oderbruch aquifer a steady peak of Mn could be observed in a travel distance of 360 m from the river. Fe increases along the entire cross-section. A front for Fe could exist even beyond the final position in about 1000 m linear distance away from the river. 

In addition to the far- and near-field studies on natural analogues mentioned above, the studies on radionuclide migration mechanisms (e.g., solubility, specia-tion, sorption, adsorption, ion exchange, precipitation, matrix diffusion, colloid, redox front, biological activity, gas generation) are also regarded as the natural analogue studies. 

With time the redox front at the updip edge of the ore may move thousands of metres downdip, carrying the orebody with it and leaving behind alteration features in the oxidized ground. As erosion progresses, these features may be exposed at the surface or subsequently truncated in subcrop several kilometres from the location of the roll front that formed it. 

A consideration of other elements associated with roll-front deposits improves the accuracy and confidence in locating favourable trends (Fig. 9). Sulphate concentration and conductivity increase toward the redox front and then decrease abruptly owing to precipitation of iron sulphide, followed by calcium carbonate. High values of bicarbonate and selenium also contribute to identification of the zone of uranium mobilization on the oxidized side of the front. Molybdenum concentrations are normally associated with sandstone deposits, but haloes in the groundwater may be to the side of or farther down-dip than the centre of the geochemical cell. Arsenic is most valuable for its regional halo around areas of mineralization. 

Owing to slightly greater asymmetry in the curve. Eh, if properly measured, is more valuable than pH as an indicator of the direction to a redox front. The Eh and pH curves show sharp... 

At a sample. spacing of 5 km very few wells will be near a redox front. Nevertheless, with a proper model and knowledge of the hydrologic gradient the position of a front is quite interpretable. ... 

Roll-front U,V,Pb,Cu,Se, Mo,C,S Shallow-dipping, developed at hydrostatic redox fronts between impermeable shale beds epigenetic Shirley Basin and Texas Coast Plain, U.S.A. 

Oscuna Uisumi, P090S de Caldas. Brazil Natural Analogue 0.5-0.6 — 0-125 Presence of a weathered zone. Pyrite oxidation induces reduction of fluids and subsequent UO2 precipitation in the redox front. 

Here P and G are the minerals pyrite and goethite while X, F and T represent the mobile oxygen, Fe and thiosulfate species. Initially in the aquifer G = 0 and upon mobilization of F, a supersaturation with respect to goethite may occur so that G is nucleated. The interesting aspect of this situation in the present context is that the G content behind the advancing pyrite redox front can be oscillatory. 

Merino, E. and Ortoleva, P., in The Self Organization of Geo-Chemical Periodicity (book manuscript in preparation) Ortoleva, P., Auchmuty, G., Chadam, J., Hettmer, J., Merino, E., Moore, C., Ripley, E., Redox Front Propagation and Banding Modalities, Physica D (submitted for publication)... 

E., Moore, C., Ripley, E., Redox Front Propagation and Banding Modalities, Physica D (submitted for publication)... 

Under stationary-state conditions, reaction front velocities are fixed relative to the Darcy velocity of the fluid. The migration velocity of a redox front, or any other reaction front, is given by (Lichtner 1988) ... 

The migration velocity of a redox front is retarded relative to the fluid velocity when... 

Lichtner Waber (1992) used the stationary-state model to estimate the velocity of redox fronts associated with pyrite oxidation at the Osamu Utsumi uranium mine, P090S de Caldas, Brazil. For the oxidation reaction ... 

Table 3. Mineralogical constraints on the stationary-state model of redox-front migration in granites...

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