Assessing the influence of vegetation on reduction of pesticide concentration in experimental surface flow constructed wetlands: Application of the toxic units approach

In summer 2008, an experiment on retention of a mixture of five pesticides in the Lier experimental wetland site (Norway) was performed. Two vegetated cells with hydraulic retention times (HRT) of 280 min and 330 min and one cell without vegetation (HRT of 132 min) of 120 m2 surface area each were investigated regarding their ability to reduce peak concentrations, pesticide masses and predicted adverse effects. Discrete water, plant and sediment samples were taken and analysed. The inlet peak concentrations of the pesticides dimethoate, dicamba, trifloxystrobin and tebuconazole ranged from 18 ng/L up to 5904 ng/L. The mean reduction of peak concentration was 72% in the non-vegetated cell and up to 91% in the vegetated cells. Less than 5% of the masses were retained within the wetlands. Uptake and sorption by plants was low (up to 4%), however, higher for the vegetated cell dominated by Phalaris arundinacea L. than for the one with Typha latifolia L. as dominant plant. The toxic units (TU) approach was used to describe the potential reduction of toxicity within the wetland cells. Calculated toxicity of the substances decreased by 79% in the non-vegetated cell and by 95% in the two vegetated cells. Despite the low mass retention, the vegetated wetland system reduced the toxic effects, expressed as toxic units from values of 0.24 to 0.01, i.e. a concentration two orders of magnitude below the acute toxicity threshold, within a distance of 40 m while the non vegetated would need to be about 64 m long for the same efficiency.