Soil vapor extraction (SVE) is a method used to clean up pollution in the area above groundwater, where soil is not fully saturated with water. This process removes harmful, volatile substances by moving them from the soil into the air. Once these substances are in the air, they are collected through special wells and sent to systems above ground for treatment. SVE works similarly to a method called "pump-and-treat," which is used to clean groundwater. This technique is especially effective for substances that evaporate easily, such as certain types of chlorinated solvents and hydrocarbons. The U.S. Environmental Protection Agency (EPA) considers SVE a well-established and reliable method for cleaning up polluted soil.
SVE configuration
Soil vapor extraction (SVE) is a method used to clean polluted soil. It uses machines called vacuum blowers and wells to pull air through the ground. This air carries harmful vapors from the soil, which are then collected and cleaned above the ground. SVE systems can use natural air movement or special wells to bring air into the ground. When air is pulled through the soil, it helps move pollutants from the soil’s water, solid parts, and non-water parts into the air. Air movement is important, but areas with too much water or uneven soil layers may slow this process. In some cases, like when natural processes are used to clean the soil, SVE systems can rely on natural air pressure changes to work.
SVE is a useful method for cleaning soil above the groundwater level. It uses standard wells and common equipment, such as blowers and tools to treat the air. The process causes little damage to the ground, mainly requiring the installation of wells and some aboveground tools. Depending on the type of pollution and the soil’s structure, SVE can clean large areas of soil at a reasonable cost.
The air pulled from the soil by SVE must be cleaned before it is released back into the environment. Cleaning is usually done for air, but liquid condensation must also be handled, as it can sometimes be useful. Many methods are used to clean the air, including burning the vapor (thermal oxidation), using materials to trap pollutants (like activated carbon), and using natural processes to break down the pollution. The most common methods are burning the vapor and using activated carbon to absorb the pollutants. The choice of method depends on the type of pollutant, how much is in the air, how much air needs to be cleaned, and cost.
SVE effectiveness
The success of SVE, or how quickly and completely it removes pollutants, depends on several factors that affect how pollutants move into the air. SVE effectiveness is influenced by the properties of the pollutants (such as Henry’s Law constant, vapor pressure, boiling point, and adsorption coefficient), the temperature underground, the characteristics of the soil in the vadose zone (like grain size, moisture content, permeability, and carbon content), the unevenness of the underground area, and the force that moves air through the soil (the pressure gradient). For example, a small amount of a highly volatile pollutant, like trichloroethene, in a uniform sand with high permeability and low carbon content (which means the pollutant does not stick to the soil) can be removed quickly using SVE. However, in an uneven vadose zone with clay layers containing a less volatile pollutant like naphthalene, the process may take longer or need improvements to SVE. Problems with SVE include tailing and rebound, which happen in areas with less air movement (such as low permeability or high moisture areas) or pollutants that are less volatile or more likely to stick to soil. Recent studies at U.S. Department of Energy sites have looked at how layers of soil and areas with low permeability affect SVE operations.
Enhancement of SVE
Improvements to make SVE more effective may involve directional drilling, pneumatic and hydraulic fracturing, and thermal methods like hot air or steam injection. These methods help increase gas movement through the ground, particularly in areas where the ground is less porous. Thermal methods, like injecting hot air or steam, raise the soil's temperature, making contaminants more likely to vaporize. Injecting hot dry air can also remove moisture from the soil, which helps gases move more easily through it. Other thermal methods, like electrical resistance heating or radio-frequency heating, can also be used to heat the soil and remove contaminants. However, these are usually considered separate from SVE improvements and might use vacuum systems or other ways to collect the gas.
Design, optimization, performance assessment, and closure
Implementing SVE as a solution requires several steps: planning the system, running it, improving its performance, checking how well it works, and ending the process. Several guides provide details about these steps. The EPA and U.S. Army Corps of Engineers (USACE) guides explain the overall process for planning, running, improving, and ending an SVE system. The Air Force Center for Engineering and the Environment (AFCEE) guide focuses on improving the system but includes limited information about ending the process or meeting cleanup goals. The Pacific Northwest National Laboratory (PNNL) guide adds more details about improving the system, changing to another method, or ending the process.
Designing and running an SVE system is generally simple, but challenges often arise from underground soil conditions and the location of pollution. Over time, the system may remove less pollution due to natural limits in how quickly pollution moves or is removed. Checking the system’s performance is important to decide if it should be improved, stopped, or replaced with another method. Evaluating rebound (when pollution levels rise temporarily) and mass flux (how much pollution moves through the system) helps assess performance and guide decisions.
Related technologies
Several technologies are connected to soil vapor extraction (SVE). As mentioned earlier, some soil-heating methods used to clean polluted soil, such as electrical resistive heating and in situ vitrification, need a part that collects soil gas. This part can be SVE or a surface barrier, like a hood. Another related method is bioventing, which adds oxygen or other gases underground to help living organisms break down pollution. A different technology called in situ air sparging cleans groundwater pollution by injecting air into the water. The air is then collected using SVE wells.