Vadose zone

Of course the presence of a Hquid phase of hydrocarbon in a soil gives rise to vapor contamination in the vadose zone above the water table. This can be treated by vacuum extraction, and the passage of the exhaust gases through a biofilter (see above) can be a cheap and effective way of destroying the contaminant permanently. 

In Situ Air Stripping. An innovation to conventional pump and treat air stripping is in situ air stripping. Two horizontal wells are installed, one below the water table and one in the vadose zone. Air is injected in the lower well while contaminated soil vapor is extracted by vacuum through the upper well. 

A U.S. EPA study (41) showed that soil vapor extraction (SVE) is an effective treatment for removing volatile contaminants from the vadose zone. Sandy soils are more effectively treated than clay or soils with higher organic content because higher air flows are possible in sand and clays—organic soils tend to adsorb or retain more contaminants. Removal of volatiles is rapid in the initial phase of treatment and thereafter decreases rapidly thereafter-an important consideration in the design of air emissions control over the life of the project. 

R. N. Miller, "A Field-Scale Investigation of Enhanced Petroleum Hydrocarbon Biodegradation ia the Vadose Zone Combining Soil Venting as an Oxygen Source with Moisture and Nutrient Addition," doctoral dissertation submitted to the Civil and Environmental Engineering Department,... 

Lappala, E. and G. Thompson. Detection of Groundwater Contamination by Shallow Soil Gas Sampling in the Vadose Zone and Applications. In Management of Uncontrolled Hazardous Waste Sites Proceedings, Hazardous Materials Control Research Institute,Washington, D.C., 1984. 

Vadose zone Unsaturated zone of soil above the groundwater, extending from the bottom of the eapillary fringe all the way to the soil surfaee. 

As more sensitive analytical methods for pesticides are developed, greater care must be taken to avoid sample contamination and misidentification of residues. For example, in pesticide leaching or field dissipation studies, small amounts of surface soil coming in contact with soil core or soil pore water samples taken from further below the ground surface can sometimes lead to wildly inaccurate analytical results. This is probably the cause of isolated, high-level detections of pesticides in the lower part of the vadose zone or in groundwater in samples taken soon after application when other data (weather, soil permeability determinations and other pesticide or tracer analytical results) imply that such results are highly improbable. 

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. 

Soil vapor extraction (SVE) is a relatively new yet widely applied technology for the remediation of soils contaminated with volatile organic compounds (VOC) in the unsaturated zone above the water table (vadose zone). The process consists of generating an airstream through the contaminated soil subsurface in order to enhance the volatilization of organic contaminants and thus remove them from the soil matrix.913... 

Pumping of the groundwater to lower the water table and enlarge the vadose zone, with simultaneous treatment of contaminated groundwater.10... 

Phase Distribution of Organic Contaminants in the Vadose Zone... 

Organic contaminants can be present in the vadose zone in four distinct phases (Figure 14.2) ... 

FIGURE 14.2 Phase distribution of organic contaminants in the vadose zone. The solid arrows in the three-and four-phase models represent the equilibria taken into consideration in the equations of Table 14.3. 

The airflow equations presented above are based on the assumption that the soil is a spatially homogeneous porous medium with constant intrinsic permeability. However, in most sites, the vadose zone is heterogeneous. For this reason, design calculations are rarely based on previous hydraulic conductivity measurements. One of the objectives of preliminary field testing is to collect data for the reliable estimation of permeability in the contaminated zone. The field tests include measurements of air flow rates at the extraction well, which are combined with the vacuum monitoring data at several distances to obtain a more accurate estimation of air permeability at the particular site. 

The concentrations and the mass distribution of toluene in the four phases, as calculated from this set of equations, are presented in Table 14.4. As seen in the table, the major part of the toluene, i.e., 68.9%, remains in the vadose zone as free NAPL, 27.6% is adsorbed on the surfaces of solid particles, and only 3.5% is distributed between the aqueous and gas phases. Free NAPL occupies only a small part of the available pore volume, and it is not expected to disturb the movement of air through the contaminated zone. 

Concentrations, Mass Distribution of Toluene, and Volume Occupied by the Four Phases in the Vadose Zone... 

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. 

---