Attenuation of Nitrate and Atrazine From Agricultural Runoff in a Bioretention Swale: The Importance of Season and Residence Time.

A bioretention swale was tested for its effectiveness as a best management practice for reducing nitrate (NO₃^-) and atrazine export from agricultural tile drains in the US Midwest. The system includes a combination of subsurface (anaerobic) and surface (aerobic) cells, and was designed with the expectation that these different flow paths would allow for effective transformations of the various contaminants that are normally present in drainage waters. More specifically, anaerobic conditions in the subsurface cell would support denitrification, whereas aerobic conditions in the surface cell should be favorable for atrazine degradation. The goal of this study is to identify the biogeochemical processes (sorption, microbial degradation, vegetative uptake) controlling NO₃^- and atrazine removal, and to experimentally create conditions conducive to the attenuation of these water pollutants by changing the hydraulic retention time (HRT). Starting in summer 2011, sampling of the bioswale inflow, outflow, and across the surface and subsurface cells has been conducted to assess contaminant attenuation during periods of long HRT (> 2 days) and short HRT (< 2 days). Preliminary results have demonstrated the effectiveness of the bioswale at reducing the discharge of nutrients into nearby streams. Evaluation of the bioswale is ongoing, and involves experimental variations of HRT to determine the optimal HRT for effective NO₃^- and atrazine removal during different seasons. These results will be discussed in relation to a suite of biophysical parameters (water level, temperature, conductivity and dissolved oxygen) to better characterize bioswale flow paths and underlying biogeochemical processes.