Much of the microbial activity in nature takes place at interfaces, which are often associated with redox discontinuities. One example is the oxic/anoxic interface where polluted groundwater interacts with the overlying vadose zone. We tested whether microbes in the vadose zone can use synthetic chemicals as electron donors and thus protect the overlying air and buildings from groundwater pollutants. Samples from the vadose zone of a site contaminated with chlorobenzene (CB), 1,2-dichlorobenzene (12DCB), and 1,4-dichlorobenzene (14DCB) were packed in a multiport column to simulate the interface of the vadose zone with an underlying groundwater plume. A mixture of CB, 12DCB, and 14DCB in anoxic water was pumped continuously through the bottom of column to an outlet below the first sampling port to create an oxic/anoxic interface and a capillary fringe. Removal to below the detection limits by rapid biodegradation with rates of 21 +/- 1 mg of CB . m(-2) . d(-1), 3.7 +/- 0.5 mg of 12DCB . m(-2) . d(-1), and 7.4 +/- 0.7 mg of 1.4 DCB . m(-2) . d(-1) indicated that natural attenuation in the capillary fringe can prevent the migration of CB, 12DCB, and 14DCB vapors. Enumeration of bacteria capable of degrading chlorobenzenes suggested that most of the biodegradation takes place within the first 10 cm above the saturated zone. Biodegradation also increased the upward flux of contaminants and thus enhanced their elimination from the underlying water. The results revealed a substantial biodegradation capacity for chlorinated aromatic compounds at the oxic/anoxic interface and illustrate the role of microbes in creating steep redox gradients.