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| Resource Library > Technology Transfer > Programs and Initiatives > Bioventing > When To Avoid |
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| Bioventing: When to Avoid |
Successful bioventing is based on the ability to supply oxygen to the targeted areas. An inability to supply and maintain oxygen at levels in excess of 2% vol/vol will result in poor bioventing effectiveness. These criteria are best evaluated through soil gas and in situ oxygen respiration tests as per the Bioventing Test Plan and Technical Protocol and its Addendum One, Test Plan and Technical Protocol for A Field Treatability Test for Bioventing - Using Soil Gas Surveys to Determine Bioventing Feasibility and Natural Attenuation Protocol
The following list represents conditions under which one should proceed with caution, or exclude bioventing from further consideration as a candidate technology for petroleum hydrocarbons in vadose-zone soils:
- Soils are impermeable to air even at low flow rates due to year-round water saturated conditions.
- Soils are impermeable to air even at low flow rates due to very low effective porosity. Note: Bioventing has proven effective in soils with greater than 80% by weights silts and clays.
- Impacted soil volumes are too small to be cost competitive with excavation and ex situ treatment and/or disposal (Rule of thumb < 500 cubic yards).
- Forced aeration is problematic due to ground water table elevations being sustained at less than 3 feet below ground surface. (Note: Aeration via periodic tilling may be more cost-effective.)
- Inorganic contaminants of concern are present, which may require excavation of other type of treatment.
- No risk-based, regulatory, community, or institutional requirement to accomplish cleanup.
The following site-specific parameters should be measured in soil-vapor samples, and evaluated during screening prior to design or installation of a bioventing system, to determine wheter a particular site is a viable candidate for successful bioventing:
- Oxygen. The concentration of oxygen in soil vapor at an ideal candidate site for bioventing generally is below 5 percent. Low oxygen concentrations indicate that fuel-degrading microorganisms have depleted the oxygen in the subsurface, and natural diffusion of oxygen from the atmosphere to the subsurface is not sufficient to meet oxygen demand.
- Carbon Dioxide. Elevated concentrations of CO2 (greater than about 0.5 percent) indicate that this primary product of biodegradation is being produced, and that native aerobic microorganisms can be stimulated by introduction of additional oxygen.
- Total Volatile Hydrocarbons (TVH). If TVH concentrations in soil vapor are less than about 30 parts per million, volume per volume (ppm-v), fuel hydrocarbons remaining in soil are unlikely to migrate to underlying groundwater at concentrations that would adversely affect groundwater quality, and remediation of petroleum constituents remaining in vadose-zone soils probably is not warranted. This possibility should be evaluated using the Bioventing Design Tool prior to proceeding with system design. On the other hand, if high levels of TVH (greater than 1,000 ppm-v) are detected in soil-vapor samples, air injection could result in undesirable (or dangerous) vapor migration into nearby buildings or subsurface utility corridors. In this situation, an alternative technology that relies on removal of vapors (e.g., soil vapor extraction [SVE]) should be considered.
- In addition, if the moisture content of vados-zone soils is low (less than about one-half the field capacity of the soil, as determined by geotechnical testing), microbial activity may be inhibited, as microorganisms require relatively moist conditions to thrive. Under these circumstances, soil amendment via introduction of an aqueous nutrient solution may be an option.
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