If there is leakage of volatile organic compounds (VOCs) above the surface or above the groundwater level, it will leave residual material in the entrained zone. These residual materials can plunge into the pore throats of coarse-grained soil by capillary forces in the form of inclusions of pure liquid phase, or adsorb to the fine-grained soil matrix for the vast majority of pore space. Volatile organic compounds will separate into pure liquid phase and vapor phase occupy the pores in the aeration zone. Both LNAPLs and DNAPLs can enter the aeration zone this way.
LNAPLs floating on the surface of the capillaries can also be separated, so vapor will be present in the entrained zone above the LNAPL lens. As the LNAPL lens moves, other parts of the vents will be contaminated with VOC vapors. VOC vapors can migrate in the aeration zone even when LNAPL is not moving.
Sampling and analysis can be used to determine whether VOCs are present in the aeration zone. Care should be taken to prevent the loss of VOCs in the sample. Usually, in order to reduce the cost of laboratory tests, several soil samples will be mixed into one sample. However, the mixing process can result in the loss of volatile materials, especially when the weather is hot.
Soil pollution caused by VOCs can be controlled by soil vapor extraction. This process is similar to groundwater extraction-treatment. Air is the medium that moves within the contaminated zone of the aeration zone. The contaminants are removed into the moving air. As with the migration of water in aquifers, air will preferentially flow from the more permeable material and bypass the less permeable area. Initially for large-scale migration. In fact large-scale migration will be faster than water because moving more volume of water than a certain volume of soil at the same time. But as the vapor flows from the fine grain area to the coarse grain area, the coarse grain area becomes the rate limiting factor of air migration. This process can be advanced by using hot air, especially when the outside air is cold.
The removal of residual NAPL from the entrained zone is not only used to treat the soil, but it also helps to repair the groundwater. The residual NAPL in the aeration zone is a persistent source of groundwater contamination because it dissolves into groundwater through precipitation infiltration.
Soil vapor extraction can also be used to treat highly volatile hydrocarbon floats. It is not removed in liquid form as the LNAPL is removed, but rather through the vapor extraction well through the soil vapor. If the water level is reduced by extraction, the uppermost part of the aquifer will become part of the aeration zone. The LNAPLs that float at the top of the edge of the tube will also drop, leaving residual material in the newly formed zonal portion. Since soil vapor extraction is more effective than extraction-treatment in removing contaminants, if the soil vapor extraction system is designed to remove air from this part of the soil, the space between the original position of the soil water surface and the new position of the groundwater table Can be more effective governance. For this reason, it is a good design practice to expand the soil vapor extraction system below the water surface. This will take advantage of the seasonal drop in water levels and declines due to pumping.
Soil vapor extraction systems can be achieved by drilling wells in a gas-bearing zone and are designed in much the same way as wells in groundwater. The lift well is about 10 feet or deeper from the surface of the dive. Well slotted plastic filter. Pumped wells and wells into the well not the same way, in order to obtain maximum air flow, the filter part of the backfill with gravel. It is important to seal the surrounding area around the top of the well about 5 feet with a solid plastic casing and seal the annular space so that the air flow will not escape outside the casing. The stripping wells are designed to extract soil vapor from the zonal zone in a circular area near the well. Ventilation wells often used in conjunction with the extraction wells. In order to extract the vapor, fresh air must be circulated in pores containing residual NAPL species. Ventilation wells must have a minimum of resistance to allow air to circulate through the entire pore volume to be repaired. The well is constructed the same as the well, except that it is not attached to the siphon head and has an open inverted U-shaped head at the top to prevent precipitation from entering it. Without the vent well, the extraction well speed must be continually changed to ensure that there will be no permanent dock area between the well and the well.
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