Abstract

Herbicides have been detected in tile drain water, representing a point source to surface waters. This experiment aimed to evaluate the effectiveness of a woodchip denitrifying bioreactor to dissipate the herbicides acetochlor and atrazine from tile drainage. Flowing water (16.4±2.8L/min) containing either 1.8, 3.0 or 6.6g/Lacetochlor, 1.4, 2.4 or 5.3g/L atrazine, 6.5 to 14.0mg/L of NO₃-N, and less than 0.4mg/L total phosphorus was passed through a woodchip bioreactor with an average hydraulic residence time of 21.2±3.2h. Load reduction of acetochlor, atrazine, nitrate, and total phosphorus reached 70%, 53%, 47% and 78%, respectively. Herbicide metabolites were not detected in the bioreactor outflow, suggesting herbicide dissipation resulted from adsorption rather than degradation. Two mathematical reactor models, Bohart-Adams (B-A) and Yoon-Nelson (Y-N), were used to characterize the woodchip adsorption properties. The B-A model estimated that chemical breakthrough from the woodchip bioreactor would vary between 7-12days and 6-10days for acetochlor and atrazine, respectively. The Y-N model has indicated that the half-life adsorption capacity of the woodchip matrix is on average 6 days for acetochlor and 4days for and atrazine. However, breakthrough time (C=0.9C_o) at all three influent concentrations using Y-N model fell in a range of 9 to 18days for acetochlor and 8 to 13days and for atrazine, compared to the <6d breakthrough for acetochlor and atrazine observed during the bioreactor experiment. Although herbicide breakthrough was measured within days of entering the bioreactor, the cumulative mass of herbicides measured in the water flowing out of the bioreactor were substantially reduced indicating woodchip bioreactors should be investigated further as a potential tool to mitigate herbicide concentrations transported in tile drain water. These results indicate benefit for both producers and the environment by providing a potential mitigation strategy that can reduce tile drainage herbicide and nutrient loads. 

Keywords

woodchip bioreactors, acetochlor, atrazine, nitrate, total phosphorus, adsorption reactor model, herbicides, external electron acceptors, acidic-basic interactions, physical adsorption, plumbing components, herbicide experiment, enzyme-mediated reactions, anoxic conditions