Air sparging (AS) is a mature in-situ remediation processes that can be effective at removing volatile organic compounds (VOCs) from impacted media in the saturated zone (and because you need an SVE system to capture the vapors, also the vadose zone). VOCs can be liquids within the soil pores, solutes in ground water, adsorbed onto soil particles, and/or vapor entrapped between soil particles.
Injection of air into groundwater (through air sparging wells) will partition and transport VOCs in vapor form into the vadose zone where operation of SVE wells can provide a moving air-stream to capture and treat the impacted vapors. Without an SVE system, the vapor are likely to impact either vadose zone soils or potentially the indoor air of nearby structures.
An additional benefit of air sparging is that the injection of air into the groundwater table will likely increase redox conditions, resulting in the potential for increasing the rate of bioremediation of petroleum compounds. Alternatively, and increase in redox conditions may temporarily prohibit the reduction dechlorination of chlorinated solvents in the saturated zone.
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. Some compounds with high aqueous solubilities (e.g., 1,4-dioxane) may be difficult to remedy with air sparging. Additionally, metals may be precipitated via the geochemical changes induced by the injection of air into groundwater. The geochemical changes and metals precipitation may be a transient condition.
When to use air sparging?
Air sparging 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. As both air sparging and SVE require vapor flow through soil, lower permeability soils can reduce the cost effectiveness of these technologies.
How long will it take?
Depending upon the design of the system, air sparging 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 achievable 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 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 testing results
Optimization
- Increase or decrease flow at specific locations or areas to target specific location, areas, or intervals
- Varying injection rates and locations to prevent the air sparging system from pushing or moving the contamination into unimpacted areas.
More information on XDD Environmental’s soil vapor extraction (SVE) experience and services.
Additional references:
Air Sparging Design Paradigm, 2002