What are air sparging and soil vapor extraction?

Air sparging (AS) and soil vapor extraction (SVE) are mature in-situ remediation processes that can be effective for removing volatile organic compounds (VOCs) from impacted media in both the vadose and saturated zones. VOCs can exist as liquids within the soil pores, solutes in the soil moisture or ground water, adsorbed onto soil particles, and/or as vapor in the pore space and entrapped within soil particles.

Soil vapor extraction is often used as a stand alone remedy when volatile organic compound impacts to unsaturated soils are the primary focus for remediation.  By applying a vacuum to vadose zone wells, air is drawn down from the atmosphere and through the soil column, enhancing volatilization. The captured vapors are drawn to the surface for treatment and discharge, as required.

sve-diagram

Often, soil vapor extraction systems will be designed based upon vacuum distribution in the subsurface.  As vacuum distribution does not specifically translate into the flow necessary for volatilization, XDD uses both industry and proprietary models in our design process relative to pore-volume exchanges.  XDD Environmental’s design process adds certainty relative to both remedy effectiveness and also the timframe required to meet project goals, which can significantly impact the cost of the remedy

Injection of air into the impacted ground water (through air sparging wells) will mobilize the VOCs in vapor form into the vadose zone where operation of SVE wells can provide a moving air-stream to capture and treat the sparged vapors.

What chemicals are treated?
As a general rule, compounds with a vapor pressure of greater than 1 mm of mercury are potential candidates for air sparging and soil vapor extraction.  Some compounds with high aqueous solubility may be difficult to remedy with either of these technologies.  Additionally, metals may be precipitated via the geochemical changes induced by the injection or air into the subsurface.  The geochemical changes and therefore metals precipitation  may be a transient condition.

When to use air sparging and soil vapor extraction?
Air sparging and soil vapor extraction are generally cost effective remedial technologies for removing VOCs from soils.  In general cost is less sensitive to the total mass of VOCs than other technologies, such as enhanced biodegradation or in situ chemical oxidation.  Additionally, air sparging and soil vapor extraction can be installed and operated underneath buildings and other structures with minimal disruption.

How long will it take?
Depending upon the design of the system, air sparging and soil vapor extraction can take anywhere from one to more than three years.  It is important to understand the site conceptual model relative to the magnitude of impacts when trying to set acheivable goals for this technology

Design Considerations

What are the contaminants and their properties? Where is the source?

Requires good understanding site geology:

  • Variable geology may result in preferential treatment of higher-permeable soils, or poor treatment of low-permeability soils
  • Can be negatively affected by highly variable groundwater table (i.e. sucks up water)

Generally pilot-tested to determine performance capabilities

  • Injection/Extraction rates
  • Radius of influence of each well location (determines the number and spacing of wells)
  • Performance capabilities can be determined with relatively simple modeling using pilot test results

Optimization

  • Increase or decrease flow/vacuum at specific locations or areas to target specific location, areas, or interval