Floodwater pH

Floodwater pH in wetlands is usually regulated by the photosynthetic activity of periphyton communities and submerged macrophytes. Depending on the alkalinity of water, the pH values can increase to about 9 during the day and decrease to near neutral levels during the night. These diel changes are directly linked to the absence or presence of carbon dioxide. The possible sources of carbon dioxide in floodwater are as follows  

The possible sinks of carbon dioxide in floodwater are as follows  

FIGURE 4.16 Diurnal variations in pH of water in selected aquatic systems containing water hyacinth (Eichhornia crassipes [Mart] Solms), cattails (Typha latifoUa L.) and Egeria (Egeria densa), and control (no macrophytes but contained algae). (Reddy and Patrick, 1984.) 

The pH chauges iu wetlaud soils siguificautly iuflueuce hydroxide, carbonate, sulfide, and silicate equilibria. These equilibria regulate the precipitation and dissolution of solids, the sorption and desorption of ions, and the concentrations of nutritionally significant ions or substances such as AT+, Fe, H2S, H2CO3, and undissociated organic acids. 

When acid sulfate soils are flooded, reduced Fe can combine with sulfide and form insoluble compounds, finally resulting in mineral formation (see Chapters 3 and 10 for details). Pyrite formation can also occur in swamp sediments. When oxygen is introduced into these systems by draining, sulfides are oxidized to sulfates biologically. This lowers the pH. The pH of these soils can drop as low as 3.0, where Thiobacillus may not function. These soils are also called cat clays—predominant in most of the coastal areas of the world. 

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Flooded superheaters are treated with amine to raise the pH of the floodwater to 8.8 to 9.2. 

The floodwater often has a high pH as a result of CO2 removal by photosynthe-sizing organisms, favouring NH3 volatilization. 

Figure 3.3 Calculated diurnal changes in the pH and concentrations of carbonate species in ricefield floodwater for sinusoidally varying [H2CO3 ] with (a) [Aik] = lOmM, (b) [Aik] = 0.5 mM. The free CO2 concentrations are in mgL to be consistent with Figure 3.2...

An important practical problem in ricefields is the loss of N fertilizer through volatilization of NH3 from the floodwater. Loss of NH3 is sensitive to the pH of the floodwater, and hence is intimately linked to the dynamics of dissolved CO2 (Bouldin and Alimago, 1976). To quantify this it is necessary to consider the simultaneous transfers of CO2 and NH3 across the air-water interface and their coupling through acid-base reactions. There is an equation of type (3.33) for the flux of NH3 across the still air layer and, as for the dissolved CO2 and carbonate species, the flux across the still water layer is... 

Figure 8.9 Profiles of urea-N, ammoniacal-N and pH with depth following broadcast application of urea on ricefield floodwater, and the corresponding rates of NH3 volatilization (calculated with the model of Rachhpal-Singh and Kirk, 1993a,b)...

The pH values of floodwater and underlying soils and sediments can be summarized as follows ... 

In soils, the pH of soil pore water tends to decrease with depth. The pH values are usually above 7 at the soil-floodwater interface and decrease to native soil pH at a lower depth. Some examples of soil pore water pH as a function of depth in selected Florida s wetland soils are shown in Figure 4.15. High pH values at the soil-floodwater interface are due to the photosynthesis activity of algae. 

The thickness of the aerobic layer can be determined by measuring Eh as a function of depth redox potentials show sharp gradients at the soil-floodwater interface. Laboratory studies have indicated complete disappearance of Oj at Eh <300 mV (pH = 7.0). This Eh value was used as a boundary between aerobic and anaerobic layers. A simple technique to determine redox profiles as a function of depth is described by Patrick and DeLaune (1972). This method involves a special motor-driven assembly that advances a platinum electrode at a rate of 2 mm h" through a soil profile. Redox potential is recorded continuously on a recorder or a data logger. Examples of Eh profiles are shown in Figure 6.22. 

Wetland soils and aquatic sediments are uniquely characterized by aerobic and anaerobic interfaces at the soil-floodwater interface or in the root zone of wetland plants (see Chapter 4 for details). Aerobic oxidation of Fe(II) and Mn(ll) is restricted to the thin aerobic layer at the soil-floodwater interface or in the root zone. Thus, the extent of aerobic oxidation of Fe(ll) and Mn(ll) is dependent on the flux of dissolved species from anaerobic soil layers to aerobic zones. At circumneutral pH, concentrations of dissolved Fe(ll) and Mn(II) are very low, thus restricting flux into aerobic portions of the soil. At this pH level, the majority of Fe(II) and Mn(ll) compounds are present as immobile solid phases such as FeCOj, MnCOj, FeS2, Fe(OH)2, and Mn(OH)2. These compounds can be oxidized only when the water table is lowered, thus exposing top portion of the soil profile to aerobic conditions. 

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