Permeability vadose zone

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. 

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. 

Soils and vadose zone information, including soil characteristics (type, holding capacity, temperature, biological activity, and engineering properties), soil chemical characteristics (solubility, ion specification, adsorption, leachability, cation exchange capacity, mineral partition coefficient, and chemical and sorptive properties), and vadose zone characteristics (permeability, variability, porosity, moisture content, chemical characteristics, and extent of contamination)... 

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... 

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. 

The system must be operated properly so the vadose zone does not become completely saturated with water, thus reducing the effective permeability to the point that gases and vapors cannot be recovered. When the soil-heating process has progressed to the extent that gaseous steam reaches the recovery wells, all the water within the soil zone has been vaporized. At this juncture, S VE becomes the primary removal mechanism. A blower or vacuum pump can be used to induce airflow in the subsurface, which will facilitate the removal of any remaining residual liquids. 

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. 

The duration of the remediation is dependent on the soil type, water content, and the nature of the contaminants. The HRUBOUT process cannot remove metals from soils. Polychlorinated biphenyls (PCBs) cannot be totally removed. The in situ HRUBOUT process is designed for removing contaminants from the vadose zone, (i.e., the zone between the surface aud the water table). Low permeability lowers system effectiveness and raises remediation costs. Soils with variable permeabilities may cause uneven delivery of air to contaminants. VOC removal rates may be reduced by high organic content in the soil because soil orgauics have a high VOC-sorption capacity. 

The vadose zone soils at the New Mexico State Highway and Transportation Department (NMSHTD) District 2 Maintenance Patrol Yard in Artesia, New Mexico (Artesia Yard) consist primarily of massive to poorly stratified silty clay, clay, or clayey silt. Soils of such low permeability (<10 10 square centimeters) are generally not amenable to the use of SVE (U.S. EPA 1995). However, as this case history shows, operation of a high-vacuum SVE system, with periodic monitoring and adjustment, can be very successful in removing sizeable secondary sources of petroleum hydrocarbons (both PSH and residual soil contamination) from the subsurface. 

Permeability in the vadose zone tends to be higher in finer sediments because flow occurs preferentially along grain surfaces rather than the centre of large pores. Finer sediments have more surfaces on which vadose flow can occur (Palmquist Johnson, 1962 Hillel, 1980 Jury et ai, 1991 Mozley Davis, 1996). If cementation is limited by the supply of Ca " and/or HCO3" to the precipitation site, vadose cements should occur preferentially in the finer sediments (Mozley Davis, 1996). 

Even where it is not occluded, the mineral surface may not be reactive. In the vadose zone, the surface may not be fully in contact with water or may contact water only intermittently. In the saturated zone, a mineral may touch virtually immobile water within isolated portions of the sediment s pore structure. Fluid chemistry in such microenvironments may bear little relationship to the bulk chemistry of the pore water. Since groundwater flow tends to be channeled through the most permeable portions of the subsurface, furthermore, fluids may bypass many or most of the mineral grains in a sediment or rock. The latter phenomenon is especially pronounced in fractured rocks, where only the mineral surfaces lining the fracture may be reactive. 

The migration of gas becomes an important consideration in certain geoenvironmental engineering applications when the gas content in a porous medium is sufficiently high such that the gas phase is continuous. Examples of such apphcations include the removal of VOCs and SVOCs from the imsaturated or vadose zone above the water table in the subsurface via the gas phase and the minimization of oxygen influx or radon efflux from engineered covers for tailings disposal applications. In the former case, removal efficiency is improved as gas permeability increases, whereas in the latter case, the objective is to minimize the gas permeability and therefore minimize the gas flow. 

The mounding of groundwater is essentially the measure of water table encroachment into the vadose zone. Other measures of the radius/zone of influence are possible, including the measurement of VOC vapors in the vadose zone, the increase of dissolved oxygen, the extent of tracer gases that may have been included in the air injection process, and the increase of overall soil gas pressure within the vadose zone. Caution should be exercised, however, when using vadose detection methods because differences exist between the permeabilities of the saturated zone and the vadose zone. Any type of measurement that requires... 

A soil-gas permeability test is conducted. This determines if air can be injected at sufficient rates to aerate the vadose zone. The actual test involves injection and withdrawal of air, measuring changes in subsurface pressures at specific distances from the injection point. 

Cementation in the phreatic zone occurred preferentially in zones of high primary permeability, whereas vadose cementation occurred principally in association with soil development. Pedogenic carbonates may have served as nucleation sites for later phreatic cementation, leading to complex zones of mixed pedogenic and phreatic cements. 

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