ABSTRACT
We used soil microcosms to assess the potential of aerobic microorganisms to degrade petroleum hydrocarbon vapors in the unsaturated zone. Figure 1 schematizes the aerobic biodegradation process in the unsaturated zone. Alexander (1994) suggests that the reaction is distinguished in most cases by the biologically catalyzed conversion of complex organic substrates to simpler inorganic products. Biodegradation requires
1. An active population of microorganisms with the enzymes needed to catalyze the reaction
2. Enzymatic access to the chemical bonds of the petroleum hydrocarbon substrate 3. A supply of nutrients and a terminal electron acceptor
Since microorganisms are water-borne, the reactions must occur in the soil moisture of the unsaturated zone. Volatile substrates can reach the organisms in the unsaturated zone by evaporating through the soil gas to the air/water interface. The substrate must then sorb to the interface (Hoff et aI., 1993) or dissolve into bulk moisture (Baehr, 1987) containing an active population of organisms with proper enzymes. Oxygen must also diffuse down soil gas concentration gradients and partition into the soil moisture in order to accept electrons donated by the hydrocarbon substrate under aerobic conditions. Carbon dioxide is created by aerobic degradation and must diffuse out of the soil moisture and partition into the soil gas. Indeed, vapor concentration gradients of hydrocarbon, carbon dioxide, and oxygen may be used to compute biodegradation rates under field conditions (Hinchee and Ong, 1992; Ostendorf and Kampbell, 1991).
