Atrazine and nitrate NO3-N are two agricultural pollutants that occur widely in surface and groundwater. One of the pathways by which these pollutants reach surface water is through subsurface drainage tile lines. Edge-of-field anaerobic denitrifying bioreactors apply organic substrates such as woodchips to stimulate the removal of NO3-N from the subsurface tile waters through denitrification. Here we investigated the co-removal of NO3-N and atrazine by these bioreactors. Laboratory experiments were conducted using 12-L woodchips-containing flow-through bioreactors, with and without the addition of biochar, to treat two concentrations of atrazine (20 and 50 μg L-1) and NO3-N (1.5 and 11.5 mg L-1), operated at four hydraulic retention time, HRT, (4 h, 8 h, 24 h, 72 h). Additionally, we examined the effect of aerating the bioreactors on atrazine removal. Furthermore, we tested atrazine removal by a field woodchip denitrifying bioreactor. The removal of both NO3-N and atrazine increased with increasing HRT in the laboratory bioreactors. At 4 h, the woodchip bioreactors removed 65% of NO3-N and 25% of atrazine but, at 72 h, the bioreactors eliminated all the NO3-N and 53% of atrazine. Biochar-amended bioreactors removed up to 90% of atrazine at 72-h retention time. We concluded that atrazine removal was primarily via adsorption because neither aeration nor NO3-N levels had an effect. At 4-h retention time, the field bioreactors achieved 2.5 times greater atrazine removal than the laboratory bioreactors. Our findings thus highlighted hydraulic retention time and biochar amendments as two important factors that may control the efficiency of atrazine removal by denitrifying bioreactors. In sum, laboratory and field data demonstrated that denitrifying bioreactors have the potential to decrease pesticide transport from agricultural lands to surface waters.
Keywords: Adsorption; Biochar; Denitrification; Hydraulic retention time; Woodchips.
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