Redox zones

Figure 2. Biodegradation in a groundwater plume causing different redox zones.

P 14.3 What Redox Zones Can Be Expected in This Laboratory Aquifer Column ... 

O Shea (2006) has summarized the redox steps involved in the mobilization of arsenic in natural hydro-logic systems (Table 6.2). It should be noted that in natural environments, discrete redox zones are not always observable and redox disequilibrium can occur, thus complicating arsenic mobilization and resorption (Mukherjee, 2006). 

Jacobsen, R., and Postma, D. (1999) Redox zoning, rates of sulfate reduction and interactions with Fe-reduction and methanogenesis in a shallow sandy aquifer, Romo, Denmark. Geochim. Cosmochim. Acta 63, 137-151. 

As follows from the ratios of molar concentrations and vertical gradients (Fig. 2) oxygen plays the leading role in oxidation of reduced compounds and OM, but its role decreases in the middle part of the redox zone. The role of oxygen in the lower part of the redox zone cannot be estimated because the measurement techniques require improvement. 

Distribution of dissolved Fe(II), as well as in the case of dissolved Mn(II), is characterized by the increasing of its content in the redox zone and by formation of an intermediate maximum within the dissolved Mn(II) maximum. Fe(II) is oxidized rapidly in the presence of oxygen to Fe(III) that exists as oxides and hydroxides with low solubility. The dissolved Fe(II) appears at a = 16.2 kgm-3. Its concentration increases toward the maximum (about 0.3 pM at oq = 16.5-16.6 kgm 3), and then decreases to 0.05-0.07 pM at or = 16.8 kg m-3. The deep concentrations appear to be controlled by solubility with FeS (mackinawite) or Fe3S4 (greigite), even though FeS2 (pyrite) is more insoluble and is present in the water column [71]. 

In the redox zone the role of OM in the balance of oxidizers and reductants is very important. In fact, Rozanov [75] argued that OM is the main reduc-tant in the suboxic layer. Sorokin [23] explained that OM distributions in the redox zone were due to dense bacterial populations in detrital particles and marine snow. 

The seasonal variability at the depths of the redox zone was analyzed in [21]. We found that the main differences of these winter distributions from those in the summer can be summarized as follows ... 

Density layer oq = 15.50-15.70 kg nr3. In the upper part of the redox zone, concentrations of dissolved oxygen decrease to 15-20 iM, and its vertical gradient abruptly decreases and becomes equal to that of nitrate. In this layer, nitrate, instead of oxygen, becomes the main oxidizer. Nitrate is rapidly consumed and its concentrations decrease rapidly. The reason for the decrease in the vertical gradient of oxygen is because there is a decrease in the rate of reactions that consume oxygen, probably the mineralization of OM [37]. 

Density layer = 15.85-15.95 kgm-3. In the middle of the redox zone, oxidizers diffusing from the upper layer (oxygen and nitrate) decrease to zero. This occurs simultaneously with the disappearance of reductants (ammonia, Mn(II), methane) diffusing up from the anoxic zone. A minimum of phosphate is also found here. This layer may be very thin, probably only 3-5 m, and its position may vary over the density range specified. 

Density layer oq = 16.10-16.15 kgm 3. This layer constitutes the lower part of the redox zone. The onset of hydrogen sulfide occurs just below the depths of maximum particulate manganese and iron. The reduction of Mn(III) and Mn(IV) by sulfide is very intensive [63,75] and model estimates [88] suggest these reactions can balance the hydrogen sulfide flux from below. A deeper phosphate maximum occurs about 5-10 m below the appearance of hydrogen sulfide. The vertical gradient of hydrogen sulfide increases at this depth (Fig. 2). 

Yakushev EV, Lukashev YF, Chasovnikov VK, Chzhu VP (2002) Modern notion of the vertical hydrochemical structure of the Black Sea redox zone. In Zatsepin AG, Flint MV (eds) Complex investigation of the northeastern Black Sea. Nauka, Moscow, p 119 (in Russian)... 

Figure I. Sequential redox zones within a confined aquifer oxidizing NO3") neutraV (Fe, Mn ) reducing (S )...

Particle transport Transfer between major redox zones, increased reoxidation, redox oscillation -h... 

In topsoils, the development of redox zones is directly related to the soil volumetric water content (cm of water per cm ), as this determines the composition and activity of soil biota (Eenchel et al., 1998). Water potential is affected by both solute and matrix characteristics, which subsequently affect the ecophysio-logy of microorganisms. The soil biota which... 

In summary, signihcant numbers of bacteria, detected with several different approaches, are present in landhll leachate plumes. Methanogens, sulfate reducers, iron reducers, manganese reducers, and denitrihers are believed to be widespread in leachate plumes. Microbial activity seems to occur throughout leachate plumes, although the actual activity (as measured by ATP, PLFA, and redox processes) is low compared to activity in topsoil. Several redox processes can take place in the same samples adding additional diversity to the concept of redox zones illustrated in Figure 3. 

Figure 15 Proposed redox zone distribution in two parallel transect downgradients of Grindsted Landfill (DK) based on the observed groundwater chemistry (source Bjerg et al, 1995).

Overall, this refined use of hydrogen concentrations supported the results of the bioassays and the complex system of redox zones inferred from the distribution of dissolved redox-sensitive species. The Grindsted Landfill (DK) plume is host to all of the proposed redox reactions, but also to secondary oxidation-reduction reactions involving ammonium, methane, manganese oxides, ferrous iron, and sulfides. 

Lyngkilde J. and Christensen T. H. (1992a) Fate of organic contaminants in the redox zones of a landhll leachate pollution plume (Vejen, Denmark). J. Contamin. Hydrol. 10, 291-307. 

Schumacher (1996) contended that long-term leakage of hydrocarbons can establish locally-anomalous redox zones that favour the development of a diverse array of chemical and mineralogical changes. The bacterial oxidation of light hydrocarbons can... 

---