Due to its high usage, mobility, and recalcitrant nature, atrazine is a common groundwater contaminant. Moreover, groundwaters that are contaminated with atrazine often contain nitrate as well. Nitrate interferes with the biological degradation of atrazine and makes it more difficult to use in situ biological methods to remediate atrazine contaminated groundwater. To solve this problem we used two reactors in sequence as models of in situ biobarriers; the first was a vegetable-oil-based denitrifying biobarrier and the second an aerobic reactor that oxygenated the denitrifying reactor's effluent. The reactors were inoculated with an atrazine-degrading microbial consortium and supplied with water containing 5 mg l(-1) nitrate-N and 3 mg l(-1) atrazine. Our hypothesis was that the denitrifying barrier would remove nitrate from the flowing water and that the downstream reaction would remove atrazine. Our hypothesis proved correct; the two reactor system removed 99.9% of the atrazine during the final 30 weeks of the study. The denitrifying barrier removed similar to 98% of the nitrate and similar to 30% of the atrazine while the aerobic reactor removed similar to 70% of the initial atrazine. The system continued to work when the amount of nitrate-N in the influent water was increased to 50 mg l(-1). A mercury poisoning study blocked the degradation of atrazine indicating that biological processes were involved. An in situ denitrifying barrier coupled with an air injection system or other oxygenation process might be used to remove both nitrate and atrazine from contaminated groundwater or to protect groundwater from an atrazine spill.