ARS | Publication request: Vadose Zone Microbial Biobarriers Remove Nitrate from Percolating Groundwater

Submitted to: Current Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 6, 2009
Publication Date: June 1, 2009
Citation: Hunter, W.J. 2009. Vadose Zone Microbial Biobarriers Remove Nitrate from Percolating Groundwater. Current Microbiology. 58:622-627.

Interpretive Summary: Microbial based barriers have become an established technique for cleansing contaminants from aquifers and nearly saturated soils but their ability to function in drier soils has not been evaluated. This study evaluated the use of barriers under well drained conditions within the 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 meter from the top of the 145 cm long columns. A fourth set of columns served as a negative control and contained no biobarrier. The positive control group’s biobarriers simulated an aquifer based barrier and were saturated while biobarriers in the other groups were allowed to drain simulating barriers that might be placed in well drained soils within the vadose zone. At intervals rainfall events were stimulated by adding water containing 20 mg L-1 NO3--N to the columns, the water was allowed to percolate through the columns, and the water that came from the columns was collected and analyzed. The biobarriers were found to be 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 to 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. The study suggests that biobarriers could be placed in the vadose zone and as such could provide a useful means of protecting surface waters and aquifers from NO3- contamination.

Technical Abstract: 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 meter from the top of the 145 cm columns. A fourth set of columns, 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 allowed to percolate through the columns, and effluents 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 to 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-.