Pesticides in denitrifying woodchip bioreactors: Current knowledge and research needs

Extensive research has demonstrated that excessive fertilizer and pesticide application leads to contamination of soil, surface water, and groundwater. Consequently, there is an increasing need for mitigation strategies to reduce multiple agricultural contaminants before they reach receiving water bodies. Denitrifying woodchip bioreactors (DWBs) are designed primarily to remove nitrate from agricultural surface runoff. However, evidence also indicates that they can facilitate the removal of pesticides. To date, studies investigating the fate of pesticides representing several chemical classes in DWBs have been conducted at multiple scales. Adsorption onto the carbonaceous filling material has been identified as the dominant removal mechanism, with its effectiveness depending primarily on pesticide hydrophobicity and the lignin content of the wood used. A hydraulic retention time of approximately 24 h provided both high nitrate removal and effective adsorption of some pesticides. Microbial biodegradation appeared to play a minor role in the removal of most compounds, whereas the effects of adsorption on pesticide bioavailability and subsequent biodegradation remain unclear. Future research should focus on addressing critical knowledge gaps, including strategies to enhance the biological degradation of pesticides, as well as on the formation, persistence, and properties of transformation products. Furthermore, research is needed to investigate the behavior of realistic pesticide mixtures, monitor long-term saturation of aging substrates, and develop strategies for the safe disposal of contaminated spent woodchips.