Technical Abstract: In the past microbial redox reactions have been the driving mechanism behind in situ bioremediations that use a carbon substrate. This is because subsurface microbial activity is generally restricted by electron (e-) donor availability and microbial activity, growth and respiration, can be stimulated by the addition of compounds that function as respiratory e- donors. The increased respiration will usually drive the system anaerobic. Under these conditions groundwater contaminants that can serve as microbial respiratory e- acceptors (i.e. nitrate, perchlorate, trichloroethylene, etc.) are often reduced to less harmful compounds. In contrast, for this study, a different driving mechanism was employed. The microbial degradation of atrazine made use of atrazine’s potential to function as a source of nitrogen. Within the biobarrier the environment that was established was one where it was the supply of nitrogen that limited microbial activity. Other growth requirements, e- donor(s) and e- acceptor(s), were available in adequate or excess amounts. An additional requirement was that the biobarriers be bioaugmented with a microbial inoculum containing microorganisms capable of degrading atrazine. For this study two reactors placed in series were used. The first was a biobarrier formed in a sand filled column by coating the sand with soybean oil as the columns were packed. The porous matrix allowed water to flow freely through the biobarriers while the oil provided a carbon rich and nitrogen poor substrate to the microbial inoculum. The second was an air injection reactor at the effluent end of the biobarrier. A simulated groundwater containing 1 mg/L atrazine and 5 mg/L nitrate-N was pumped through the columns for 22 weeks. At intervals effluents from the biobarriers were collected and analyzed for atrazine and nitrate. The results showed that the first reactor, the biobarrier, was effective at removing nitrate and the second reactor, the aerobic reactor, was effective at removing atrazine. Atrazine levels in the effluents of the 1st reactor, the anaerobic reactor, remained high throughout the study but declined with time in the 2nd reactor, the aerobic reactor, and by the 20th week of the study no detectable atrazine was present in biobarrier effluents (limit of detection < 0.005 mg/L). Such barriers, when inoculated with atrazine degrading bacteria, might be used in situ to remove atrazine from aquifers that are contaminated with both nitrate and atrazine.