Active microorganisms capable of degrading a wide variety of organic compounds of environmental concern are present in the unsaturated zone [ Thomas and Ward, 1992]. Rates of oxygen utilization and biodegradation are highly variable and subject to large uncertainties. Oxygen utilization rates have been reported to range from 3 to 12% per day [ Miller et al. , 1994] and biodegradation rates from 0.5 to 27 mg hexane equivalent/kg soil/day [ Ong et al. , 1994]. Inhibition at high organic concentrations has been observed in laboratory studies with crude oil [ Huesemann and Moore, 1994] and chlorinated hydrocarbons [ Speitel and Alley, 1991]. The significance of inhibition in field settings, however, has received limited treatment.
Biodegradation rates have been assessed with a variety of
analytical techniques. Examples include: measurement of
carbon dioxide production [ Hinchee and Arthur,
1991; Crocetti et al. , 1993;
Huesemann and Moore, 1994; van Eyk, 1994];
oxygen consumption [ Huesemann and Moore,
1994]; hydrocarbon consumption rates [ Kampbell
and Wilson, 1991]; and 
evolution
[ Baker et al. , 1994]. Biogenic carbon dioxide
production has been confirmed using 
isotope ratios [ Hinchee et al. ,
1991], however, carbon dioxide may not provide an accurate
measure of bioactivity due to potential abiotic sources of
carbon dioxide [ Hinchee et al. , 1991;
Downey and Guest, 1991]. Oxygen consumption is
considered a more reliable measure of biodegradation by
Hinchee [1994]. An in situ respirometry
test measuring oxygen utilization has been applied at over 50
sites by the U.S. Air Force [ Ong et al. , 1991;
Hinchee et al. , 1992; Kittel et al. ,
1993]. Kampbell and Wilson [1991] developed
estimates of biodegradation rates from measurements of
hydrocarbon consumption. Other researchers caution, however,
that use of off measurements from extraction wells may lead to
an inaccurate determination of bioactivity due to the averaging
effect resulting from extraction wells drawing soil gas from
uncontaminated areas as well as contaminated areas
[ Marrin et al. , 1991], or when sorption is
not considered [ Novak et al. , 1993]. Only
measurements provide conclusive evidence for
mineralization of the organic contaminants [
Baker et al. , 1994], although, this technique is not suitable
for field applications.
Several methods to enhance biodegradation rates have been examined. One approach is to provide limiting nutrients, most often nitrogen or phosphorus. Enhanced bioremediation rates have been observed in laboratory studies after applying 10 mg N/kg soil, although 40 mg N/kg soil reduced biodegradation rates by 90% [ Baker et al. , 1994], and in bench studies after the addition of a mixture of inorganic nutrients [ Hinchee and Arthur, 1991; Dupont, 1993]. Nutrient addition in field applications is typically accomplished by water flooding [ Norris et al. , 1994; Nelson et al. , 1994]. At one site the contaminated soils were excavated, mixed with an aqueous fertilizer mixture, and then returned to the excavation and compacted [ Newman et al. , 1993]. Statistically significant enhancement of biodegradation after nutrient addition, however, has not been substantiated in field studies [ Miller et al. , 1994; Miller, 1990; Dupont, 1993]. Because nutrients are typically delivered through moisture addition, and because moisture alone has been shown to increase biodegradation [ Hinchee and Arthur, 1991] it is difficult to separate the effects of nutrient addition from moisture addition [ Dupont et al. , 1991]. Moisture addition has also reduced the effectiveness of BV due to reduced air permeability [ Miller et al. , 1994]. As an example of the site specific nature of BV, biodegradation rates were found to be independent of soil moisture at Tyndal Air Force Base (AFB), Florida [ Miller, 1990].
The availability of oxygen is the most important parameter controlling biodegradation rates in BV applications. Depending on the specific hydrocarbon contaminant, stoichiometric analysis reveals that a mass ratio of oxygen to hydrocarbon on the order of 3 to 4 is required [ Hinchee and Arthur, 1991; Dupont, 1993]. ``Rule of thumb'' estimates for oxygen requirements suggest that extraction and injection pumping rates should be sufficient to exchange all air in the unsaturated zone every eight hours [ Kampbell and Wilson, 1991], or every 12 to 96 hours [ Miller, 1990]. Respiration tests at Tyndal AFB, Florida, indicate air flow rates can be minimized to maintain between 2 and 4% oxygen without adversely affecting biodegradation rates [ Miller, 1990]. Passive wells (no active pumping) have also been used to direct the supply of oxygen to contaminated areas [ Zachary and Everette, 1993]. Brown and Norris [1994] examined the use hydrogen peroxide as an oxygen source and concluded that it was ineffective in most unsaturated zone applications. Injection of pure oxygen was shown to double respiration rates [ Huesemann and Moore, 1994] although this is thought not to be a practical remediation option.