Concurrent nitrate and atrazine retention in bioreactors of straw and bark mulch at short hydraulic residence times

The present laboratory study tests the hypothesis that straw–bark mulch bioreactors are capable of concurrently retaining nitrate (NO3−) and the herbicides atrazine or bentazone at short hydraulic residence times (HRT). In a 1 year column experiment at HRT of ∼4h three organic carbon sources, straw of common wheat (Triticum aestivum L.), bark mulch of pine tree (Pinus sp.) and a mixture of both materials, showed high reduction of continuously dosed NO3− (100mgL−1) with average denitrification rates of 23.4g-Nd−1m−3, 8.4g-Nd−1m−3 and 20.5g-Nd−1m−3, respectively. Under denitrifying conditions, fast and substantial retention of continuously dosed atrazine (20μgL−1) was observed with estimated dissipation times (DT50) between 0.12 and 0.49 days in the straw–bark mulch bioreactor. In parallel batch experiments, it could be confirmed that atrazine retention is based on adsorption to bark mulch and on degradation processes supplied by the organic materials as continual sources of carbon. In contrast, bentazone was not significantly reduced under the experimental conditions. While aging of materials was clearly observed in a reduction of denitrification by 60–70% during the experiment, systems still worked very well until the end of the experiment. The results indicate that the combined use of straw and bark mulch could increase the efficiency of mitigation systems, which are installed to improve the quality of drainage water before its release to surface waters. Further, the addition of these materials has the potential of parallel retention of NO3− and less mobile herbicides like atrazine, even during high flow events, as expected at the outlet of agricultural drainage systems. High removal is expected for mitigation system designed to remain saturated most of the time, whereas bioreactors that run periodically dry are not covered by this study. However, further experiments with the tested materials at technical or field scale under more realistic climate conditions need to be carried out.