Biological treatment involves the use of micro organisms or vegetation (phytoremediation).This 
measurement is not required, but it is available to 
resolve regulatory concerns. In addition to fuels, aerobic bioventing has treated a 
variety of other contaminants, including nonhalo genated solvents, such as benzene, acetone, toluene, 
and phenol; lightly halogenated solvents, such as 
1,2-dichloroethane, dichloromethane, and chloro benzene; and SVOCs, such as some PAHs (Figure 
4).The manual contains information on bioventing 
principles; site characterization; field treatability 
testing; system design, operation, and installation; site 
closure; and techniques to demonstrate the extent and 
mechanism for contaminant removal.Low oxygen 
levels (less than five percent) 
are a good indicator that 
existing bacteria are capable 
of degrading the contam inants of concern, because 
soil gas in uncontaminated 
soil generally exhibits oxygen 
concentrations similar to 
ambient air.For sites where non-fuel 
contaminants are to be treated by bioventing, other 
factors may be considered in establishing biological 
a bioventing system has been  
soil gas monitoring to ensure 
that the site is well oxygenated, in situ respiration 
testing to monitor the progress of remediation, and 
operation and maintenance of the bioventing system.Bioventing involves supplying oxygen to 
contaminated unsaturated soils with low oxygen 
concentrations to facilitate aerobic microbial biode gradation.For example, laboratory 
and pilot-scale studies may be needed to evaluate 
effectiveness, design the bioventing system, esti mate treatment times, and demonstrate that bio degradation is the primary mechanism of removal.The injection system should be designed 
considering soil gas permeability, contaminant 
diffusion and distribution, and environmental 
factors, such as moisture content, pH, temperature, 
and electron acceptor conditions.Performance monitoring, after 
and amount of contaminant removed, oxygen 
supply, and carbon dioxide generation, as well as 
mass balances relating the three amounts, may be 
useful in establishing bioremediation as the primary 
mechanism of removal. Many 
naturally occurring microorganisms (typically, 
bacteria and fungi) can transform hazardous chem icals to substances that may be less hazardous than 
the original compounds.When building 
foundations or similar structures are close to the site, 
vacuum extraction wells, which draw air through the 
subsurface, may be used to avoid the buildup of 
contaminated, and possibly explosive, vapors in the 
building basements.Microbial bioremediation occurs under both aerobic 
and anaerobic conditions and at contaminated sites 
as either intrinsic and/or enhanced biodegradation.Typically, 
the environment is manipulated by supplying 
amendments, such as air, organic substrates, nutri ents, and other compounds, whose absence limit 
15 
treatment.A soil gas survey can 
provide useful information, especially at sites with 
relatively shallow contam ination (depths typically less 
than 20 ft).In addition, the 
soil gas survey can be useful 
in delineating the extent of 
contamination and identifying 
locations for vent wells and 
monitoring points.Microrganisms also have 
been used to alter the valence of some hazardous 
metals (e.g., Cr(VI)), thereby making them less 
hazardous and less mobile.Several plant species 
have the ability to bioaccumulate heavy metals 
found in the soil, and some tree species can 
sequester, destroy, and/or evapotranspire various 
organic compounds.The U.S. Air Force Bioventing Initiative 
and the U.S. EPA Bioremediation Field Initiative 
evaluated bioventing at 125 sites contaminated by 
petroleum hydrocarbons.At sites using extractive bioventing, the 
degree of volatilization versus biodegradation may be 
determined by measuring offgas concentrations.Finally, measurement of stable isotope ratios may be 
useful in qualitatively validating biodegradation as 
the mechanism of contaminant removal.Monitored natural attenuation 
(MNA) often relies on intrinsic bioremediation as an 
important removal mechanism.Bioventing is typically operated in air injection 
mode to alleviate low oxygen levels in the 
subsurface.