When pesticides are used in arable watersheds, residues are usually found in the recipients. However, small constructed wetlands (CWs) in ﬁrst and second order streams can reduce the loss of pesticides, since water puriﬁcation processes are stimulated. This paper presents the results of adding 13 pesticides to a CW in Norway. The relative retention increased between 0 and 67% for the pesticides ﬂuroxypyr, bentazone, dicamba, mecoprop, propiconazole, MCPA, dichlorprop, linuron, fenpropimorph, metalaxyl, metribuzin, metamitron and propachlor. In many cases, the CW reduced the peak concentrations to values regarded as non-toxic for aquatic life, even though the wetland covered less than 0.4% of the watershed surface area, and the average hydraulic load often was above 0.8 m d–1. Possible retention factors were adsorption to soil particles and organic matter, sedimentation of particles, low or high redoxpotential, and biodegradation of nitrogen-rich pesticides. However, the retention processes are complex, and are not fully understood.
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