Fate of agrochemicals in wood chip denitrifying reactors and their impacts on wood chip microbial ecology

Subsurface tile drainage systems have contributed towards increasing agricultural production, but have also contributed towards water pollution by rapidly transporting excessive nutrient and agrochemicals to surface water and ground water. One of the pollution control strategies is to treat the tile drainage water or the contaminated subsurface water with denitrifying bioreactors. Wood chips have been used in denitrifying bioreactors, providing organic carbon and attachment surface area for denitrifiers. The focus of this research is to investigate fate of agrochemicals in wood chips from the in situ reactors and their potential effects on denitrification and the denitrifiers. The selected agrochemicals for study are atrazine, enrofloxacin, monensin and sulfamethazine. Partition coefficients of atrazine, enrofloxacin, monensin and sulfamethazine were determined by single–point sorption experiments by using wood chips from an in situ reactor. Of the four chemicals tested, enrofloxacin had the highest partition coefficient (Kow) while sulfamethazine had the lowest. Atrazine and monensin had moderate sorption coefficients. In addition, partition coefficients for the four chemicals for wood chips were larger than the partition coefficients for soils obtained close to the in situ reactor. Freundlich distribution coefficients (Kf) for isotherm studies for the four chemicals were in the order of (highest to lowest): enrofloxacin \u3e monensin \u3e atrazine \u3e sulfamethazine. Desorption hysteresis were found for enrofloxacin, atrazine and sulfamethazine when the wood chips were desorbed by water. For monensin, the desorption aqueous phase concentrations were larger than the adsorption aqueous phase content. A possible reason for the larger desorption concentration was that the monensin adsorbed onto wood chips were on the eternal surface of the wood chips due to its larger molecular structure which allowed monensin to be easily desorbed. Only 5% of enrofloxacin, 14% of monensin, 23% of sulfamethazine and 25% of atrazine were recovered from the wood chips after two desorption and an acetonitrile–water extraction indicating the strong binding of the chemicals onto wood chips. Degradation studies with atrazine, enrofloxacin, and sulfamethazine onto wood chips indicate that a large majority of the chemical mass was removed from the aqueous phase within the first 48 hours followed by a slow removal over time. Dissipation rates were estimated using the availability-adjusted first-order degradation model. Disappearance of sulfamethazine was slower than disappearance of enrofloxacin and atrazine. No impact on denitrifiers as measured by the denitrification potential assays, most-probable-number (MPN) and nosZ1 copy number was found for atrazine at an initial concentration of 5 mg L-1. The MPN was reduced under enrofloxacin treatment after 2 days of the incubation; however, at the end of the experiment the denitrifier MPN was similar to control treatment MPN. Sulfamethazine was found to initially impact the denitrification (both MPN, nosZ1 copy number and denitrification potential) but after 5 days the denitrification potential assays, most-probable–number (MPN) and nosZ1 copy number were found to be similar to that of the control