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Air Sparging through Horizontal Wells

Air sparging through a horizontal air sparging system can work with or without a paired soil vapor extraction (SVE) system to treat contaminated groundwater.

Horizontal wells provide a very effective mechanism for distributing air throughout the contaminated region. A horizontal air sparging system can be installed under a wide variety of conditions to induce more effective soil vapor extraction. Above the water table, the use of hot air sparging or steam injection systems can mobilize even semi-volatile contaminants like those in diesel or Bunker C fuel for extraction. Beneath the water table, air sparging will act as an “in-situ stripper” for groundwater contamination, carrying the contaminants in the vapor phase to an extraction line above and parallel to the sparge line.

In some locales, air sparging alone is not considered a complete remedy, since contaminants removed from the groundwater could be re-deposited in the unsaturated zone. For these installations, the air sparge wells are typically paired with SVE wells that capture and convey the stripped-out contaminants to a treatment facility, where they are absorbed in carbon or other media, or combusted. A well-designed, precisely placed sparge and extract system can clean up a moderately contaminated site in as little as six months!

Material Choices

The choice of materials for air sparging is quite flexible, but several design factors must be considered. First, there can be a considerable pressure drop along the length of a horizontal air sparge well, so the slot size must be properly designed. By designing the pipe as a pressure vessel, pressure drop along the length of the pipe is accommodated and air dispersion is equalized across the length of the slotted zone. In some cases, the pipe is perforated or slotted with a graduated open area — fewer, more widely spaced holes or slots near the supply end, becoming larger in extent and more densely congregated at the closed end of the well. DTD engineers have designed and installed numerous horizontal air sparging systems that have performed in the field with excellent results. For systems requiring an extremely rigorous approach, or for turn key designs that include blower parameters and mechanical and electrical design, we work with leading consultans who specialize in system design.

Sparge Line Filter Considerations

A second consideration is whether or not to encase the sparge well in a filtration system. For a horizontal air sparging system under continuous operation it is unlikely that soil fines will find their way into the perforated pipe to cause clogging. However, if the system is pulsed or cycled, depending on the native materials it is possible that plugging of the air line can occur. In most cases a double-ended installation, where the sparge lines are accessible from the surface at both ends, is preferred to permit periodic maintenance of the well. Integrated well screens with filtration components, such as DTD’s patented EnviroFlex well screens, prevent siltation within the well screen for much lower cost than pre-pack systems, and may be manufactured with any desired perforation or slot pattern to meet design criteria.

Temperature Considerations

Finally, the temperature of the air (or steam) supply must be considered. For normal air temperatures, ordinary high-density polyethylene (HDPE) pipe is sufficient for air supply. Air sparging systems that inject heated air are often specified with stainless steel pipe, which is quite expensive. Some alternatives to stainless steel include carbon steel, high-temperature polyethylene (HTPE) or CPVC—all of these materials can withstand elevated temperatures, are easier to install, cost far less, and are more readily available than stainless.

Horizontal wells can provide a very effective mechanism for distributing air throughout contaminated regions and have many flexible applications. DTD has installed air sparging systems ranging from a few hundred feet to nearly 2000 feet in length. Please contact us with your soil vapor extraction and horizontal air sparging questions today.