Methods of adding oxygen and nutrients to groundwater using low maintenance passive or semi-passive techniques will be developed. These techniques can be used to create in situ permeable reactors that enhance bioremediation of BTEX contaminated groundwater without bringing it to ground surface for treatment, i.e. the conventional pump-and-treat approach. The in situ treatment approach should have lower operating costs than conventional remediation systems. Aeration and formation fouling control methods will be evaluated using laboratory studies, and a pilot scale in situ permeable reactor system will be constructed in the field at a site with an existing BTEX plume. The field system will be operated to generate performance data. The data from lab column studies and the field pilot system will be used to improve the design of the in situ aerators/bioreactors and to validate computer models written to predict the field performance of these systems.