Acetochlor and atrazine dissipation in a woodchip denitrifying bioreactor: A comparison of experimental results with model estimates

Authors: RANAIVOSON, ANDRY - University Of Minnesota; Rice, Pamela; MONCRIEF, JOHN - University Of Minnesota; Feyereisen, Gary; DITTRICH, MARK - University Of Minnesota

Submitted to: International Journal of Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/1/2019
Publication Date: 8/14/2019
Citation: Ranaivoson, A., Rice, P.J., Moncrief, J.F., Feyereisen, G.W., Dittrich, M. 2019. Acetochlor and atrazine dissipation in a woodchip denitrifying bioreactor: A comparison of experimental results with model estimates. International Journal of Hydrology. 3(4):286-306.

Interpretive Summary: Herbicides have been detected in tile drain water, representing a potential 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 ± 2.8 L/min) containing either 1.8, 3.0, or 6.6 ug/L acetochlor; 1.4, 2.4, or 5.3 ug/L atrazine; 6.5 to 14.0 mg/L of NO3-N; and less than 0.4 mg/L total phosphorus was passed through a woodchip bioreactor with an average hydraulic residence time of 21.2 ± 3.2 h. Load reduction of acetochlor, atrazine, nitrate, and total phosphorus reached 70%, 53%, 47%, and 79%, 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 7-12 d and 6-10 d 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 d for acetochlor and 4 d for and atrazine. However, breakthrough time at all three influent concentrations using the Y-N model fell in a range of 9 to 18 d for acetochlor and 8 to 13 d for atrazine; compared to the less than 6 d breakthrough observed with 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 was substantially reduced; indicating woodchip bioreactors should be investigated further as a potential tool to mitigate herbicide concentrations in tile drain water. This research will benefit both farmers and the environment by providing a potential mitigation strategy that can reduce loads of herbicides and nutrients transported to non-target location though tile drainage. These results indicate benefits for both producers and the environment by providing a potential mitigation strategy that can reduce tile drainage herbicide and nutrient loads.

Technical Abstract: Herbicides have been detected in tile drain water, representing a potential 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 ± 2.8 L/min) containing either 1.8, 3.0, or 6.6 ug/L acetochlor; 1.4, 2.4, or 5.3 ug/L atrazine; 6.5 to 14.0 mg/L of NO3-N; and less than 0.4 mg/L total phosphorus was passed through a woodchip bioreactor with an average hydraulic residence time of 21.2 ± 3.2 h. Load reduction of acetochlor, atrazine, nitrate, and total phosphorus reached 70%, 53%, 47%, and 79%, 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 7-12 d and 6-10 d 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 d for acetochlor and 4 d for and atrazine. However, breakthrough time at all three influent concentrations using the Y-N model fell in a range of 9 to 18 d for acetochlor and 8 to 13 d for atrazine; compared to the less than 6 d breakthrough observed with 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 was substantially reduced; indicating woodchip bioreactors should be investigated further as a potential tool to mitigate herbicide concentrations in tile drain water. These results indicate benefits for both producers and the environment by providing a potential mitigation strategy that can reduce tile drainage herbicide and nutrient loads.