Vadose conditions

Another complexity is that pore-water flow in soils commonly occurs under unsaturated or vadose conditions. This is shown in Figure 8(a) for the Luquillo regolith in which fluid saturation varies between 65% and 95% over a depth of 7 m (White et al., 1998). In such a case, the hydraulic conductivity (Equation (11)) is strongly dependent on the soil moisture content (Stonestrom etal., 1998). Experimental conductivities produced for cores taken from the Luquillo regolith (Figure 8(c)) decrease between 2 and 4 orders of magnitude with less than a 30% decrease in moisture saturation. This is caused by the entrapment of air within the pore spaces, which physically obstructs the movement of water. 

Figure 9. Time trends showing rates of chromate loss and ferrous and ferric ion production at 25°C in pH 3 suspensions of magnetites naturally weathered under (A) oxic vadose conditions (LA River) and (B) anoxic groundwater conditions (Kesterson Reservoir). Horizontal dashed lines are initial Na2Cr04 concentrations (0.06 mM) (adapted from ref 10),...

Leaching of nuclides implanted into adjacent minerals has been suggested for the supply of Rn into the vadose zone. Where there are intermittent undersaturated conditions, i.e., in soils or rocks where the water table lowers seasonally, the low stopping power of air allows atoms ejected from minerals to be implanted across pore spaces. These atoms will then be available for leaching... 

Modeling of the transport of the long-lived nuclides, especially U, require knowledge of the input at the water table as a boundary condition for aquifer profiles. There are few studies of the characteristics of radionuclides in vadose zone waters or at the water table. Significant inputs are likely to occur to the aquifer due to elevated rates of weathering in soils, and this is likely to be dependent upon climatic parameters and has varied with time. Soils may also be a source of colloids and so provide an important control on colloidal transport near recharge regions. 

A moisture ranging between 25 and 85% of complete saturation is considered to be adequate for soil bioremediation.12 In many cases, the soil moisture in the vadose zone is below or at the lower end of this range, so the addition of water is often needed to maintain good operating conditions. 

May cause a lateral spread of dissolved or separate phase contaminant plume Contamination may be transferred from groundwater to die vadose zone Has limited applicability at sites with confined aquifers Low soil permeability or other heterogeneous conditions may reduce effectiveness... 

In an SVE system, the primary mechanism for contaminant removal from the soil to the vadose zone is the volatilization of contaminants present in the pure or adsorbed phase onto soil into the vapor phase, as the vapor phase is continually extracted. The property that shows the extent to which this transfer can take place during SVE is vapor pressure, which provides an indication of the extent to which each contaminant will partition between the liquid phase and the vapor state at equilibrium conditions. Generally, a contaminant with a greater vapor pressure more readily volatilizes than one with a lesser vapor pressure. 

Oliver D.D., Brockman F.J., Bowman R.S., Kieft T.L. Microbial reduction of hexavalent chromium under vadose zone conditions. J Environ Qual 2003 32 317-324. 

Above the water table, groundwater can also occur in perched aquifer conditions. In these instances, groundwater occurs in relatively permeable soil that is suspended over a relatively low permeability layer of limited lateral extent and thickness at some elevation above the water table. Perched groundwater occurrences are common within the vadose zone high-permeability zones overlie low-permeability zones of limited lateral extent in unconsolidated deposits. However, perched conditions can also occur within low-permeability units overlying zones of higher permeability in both unconsolidated and consolidated deposits. In the latter case, for example, a siltstone or clay stone overlies jointed and fractured bedrock such that groundwater presence reflects the inability of the water to drain at a rate that exceeds replenishment from above. 

Martin, J. P. and Koener, R. M., 1984a, The Influence of Vadose Zone Conditions on Ground-water Pollution, Part I Basic Principles and Static Conditions Journal of Hazardous Materials, No. 8, pp. 349-366. 

Bioremediation as a remediation procedure within the vadose zone can take several forms including bioventing as previously discussed. When conditions of low or inadequate permeability exist, the site may not be suitable for ventilation, and the contaminants may be best degraded by anaerobic reactions. 

Contaminant volatilization from subsurface solid and aqueous phases may lead, on the one hand, to pollution of the atmosphere and, on the other hand, to contamination (by vapor transport) of the vadose zone and groundwater. Potential volatihty of a contaminant is related to its inherent vapor pressure, but actual vaporization rates depend on the environmental conditions and other factors that control behavior of chemicals at the solid-gas-water interface. For surface deposits, the actual rate of loss, or the pro-portionahty constant relating vapor pressure to volatilization rates, depends on external conditions (such as turbulence, surface roughness, and wind speed) that affect movement away from the evaporating surface. Close to the evaporating surface, there is relatively little movement of air and the vaporized substance is transported from the surface through the stagnant air layer only by molecular diffusion. The rate of contaminant volatilization from the subsurface is a function of the equilibrium distribution between the gas, water, and solid phases, as related to vapor pressure solubility and adsorption, as well as of the rate of contaminant movement to the soil surface. 

Cells R, Cox L, Hermosin MC, Cornejo J (1996) Retention of metamitron by model and natural particulate matter. Intern J Environ Anal Chem 65 245-260 Chaney RL (1989) Toxic element accumulation in soils and crops protecting soil fertility and agricultural food chains. In Bar Yosef B, Barrow NJ, Goldschmid J (eds) Inorganic contaminants in the vadose zone Springer, Heidelberg, pp 140-159 Charlatchka R, Cambier P (2000) Influence of reducing conditions on solubility of trace metals in contaminated soils. Water, Air Soil PoUut 118 143-167 Chien SH, Clyton WR (1980) Application of Elovich equation to the kinetics of phosphate release and sorption in soils. Soil Sci Am J 44 265-268... 

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