Microbial biobarriers are an established technique for cleansing contaminants from aquifers. This study evaluated their use under well-drained conditions within the vadose or unsaturated zone. Three sets of sand filled columns, the positive control, field-capacity, and sub-field-capacity groups, contained biobarriers formed by mixing sand with sawdust and soybean oil. The biobarriers were positioned 1 m from the top of the 145 cm columns. A fourth set of column, the negative control, contained no biobarrier. The positive control group's biobarriers were saturated while biobarriers in the other groups were allowed to drain. At intervals water containing 20 mg l(-1) NO3--N was applied to the columns, the water was allowed to percolate through the columns, and the effluents were collected and analyzed. The biobarriers were highly effective at removing NO3-. NO3--N in the effluents from the field-capacity, sub-field-capacity, and positive control groups averaged 0.4 +/- 0.1, 0.6 +/- 0.1, and 0.8 +/- 0.1 mg l(-1), respectively, during the final weeks of the study while effluents from the negative control group averaged 17.9 +/- 0.4 mg l(-1). The barriers removed NO3- even when the water content was in the 20-40% pore filled space range. During the 12-week study the field-capacity barriers lost 5.6% of their organic content while those in the sub-field-capacity group lost no detectable organic matter indicating that the barriers contained sufficient substrate to last for several years. Vadose zone biobarriers could provide a useful means of protecting surface waters and aquifers from NO3-.