Soil Vapor Extraction & AIR SPARGING
Generalized Process Diagram – Soil Vapor Extraction/Air Sparging
What is soil vapor extraction?
Soil vapor extraction, known as SVE, is the most frequently selected method for treatment of contaminated soil at superfund sites, brown-fields, refineries, gas stations, dry cleaners, and grain elevators. It is a relatively simple process that physically separates contaminants from soil. As the name suggests, SVE extracts contaminants from the soil in vapor form. Therefore, SVE systems are designed to remove contaminants that have a tendency to volatilize or evaporate easily. SVE removes volatile organic compounds (VOCs) and some semi-volatile organic compounds (SVOCs) from soil beneath the ground surface in the unsaturated zone (that part of the subsurface located above the water table). By applying a vacuum through a system of underground wells, contaminants are pulled to the surface as vapor or gas. Often, in addition to vacuum extraction wells, air injection wells are installed to increase the air flow and improve the removal rate of the contaminant. An added benefit of introducing air into the soil is that it can stimulate bioremediation of some contaminants.
SVE is sometimes called in situ volatilization, enhanced volatilization, in situ soil venting, forced soil venting, in situ air stripping, or soil vacuum extraction.
What is air sparging?
Used alone, soil vapor extraction cannot remove contaminants in the saturated zone of the subsurface, the water-soaked soil that lies below the water table. At sites where contamination is in the saturated zone, a process called air sparging may be used along with the SVE system. Air Sparging means pumping air into the saturated zone to help flush (bubble) the contaminants up into the unsaturated zone where the SVE extraction wells can remove them.
For air sparging to be successful, the soil in the saturated zone must be loose enough to allow the injected air to readily escape up into the unsaturated zone. Air Sparging, therefore, will work fastest at sites with coarse-grained soil, like sand and gravel.
As with SVE, an added benefit of air sparging is that it provides an oxygen source that helps stimulate the bioremediation of some contaminants. Bioremediation is an innovative treatment technology that uses microorganisms, such as bacteria that live in the soil or groundwater to break down contaminants into harmless substances. Air Sparging also can be a quick and effective treatment for VOCs in groundwater.
As with all of PES’s remedial response applications, site specific data is collected during the subsurface investigation phase and is coupled with the results of on-site bench scale pilot testing to form the basis of the remedial design/planning. A few of the established site parameters include: soil classification/lithology, contaminant concentration/type/distribution (vertically and horizontally), hydraulic conductivity, groundwater elevation/seasonal variation, vacuum/pressure distribution, groundwater physical/chemical characterization including temp/dissolved oxygen/biologic oxygen demand/etc., and contaminant removal during technology application. Our geologic, engineering and technical staff has performed numerous SVE/AS designs and subsequent implementations.
Soil Vapor Extraction, Air Sparging, and Reactive Barrier/Advanced Oxidation
Project Name: Highway Service Area
Client: NYS Agency/Major Oil
Location: Central, New York
Impact Characterization: Release of 20,000-40,000 gallons of gasoline, resulting in an adverse impact to the subsurface soil and groundwater regimes. Sensitive receptors included a regulated wetland area.
Technical Overview: PES initiated its site involvement with a third party technical evaluation of the existing remedial systems (conventional pump and treat) via the implementation of stepped aquifer pump testing. A three dimensional computer assisted data/aquifer evaluation was performed that resulted in the recommendation for alternative contaminant mitigation methodologies. Pilot testing and remedial feasibility analyses were performed for sparging and soil vapor extraction methods and the alternative remedial approach was tailored to provide substantial financial relief for the project as well as provide accelerated contaminant mitigation.
Based on the collected site-specific pilot test data, a design for a complete upgraded remedial system was developed. The design incorporated the unique application of directional horizontal drilling methods to allow retrofit application of sparging and soil vapor extraction (horizontal) wells beneath the operating service area. Directional drilling methods were utilized to assure minimal disruption of normal service area activities. Horizontal air sparge and ventilation wells were utilized to mitigate contaminant source areas as well as develop an oxygen rich reactive aeration barrier to prevent continued plume propagation and down gradient impact to sensitive wetland locations.
Processed off-gas treatment for the vapor extraction system was accomplished utilizing catalytic incineration equipment (Falco-300). Approximately 12,000 gallons of gasoline hydrocarbons were recovered during the first 10 months of operation of the retrofit remedial system. The ineffective conventional pump and treat system was decommissioned resulting in a significant reduction in the annual operational cost.
Site Progression Specifics: Work at this spill site was initiated in 1997. After approximately six years of successful operation, residual contaminant concentrations approached closure levels. A subsequent release by Mobil Oil Corporation in July of 2003 resulted in the need to reevaluate the site for a second remedial corrective action. In response, dormant remedial infrastructure was reactivated. Due to the limited aerial impact of the second release event, additional targeted remedial methodologies (advanced oxidation) were deployed at the site that included source area injection using a hydrogen peroxide/surfactant mixture. The periodic targeted injection events as well as the application of conventional air sparge and soil vapor extraction methods continue.