The role of rice fields and constructed wetlands as a source and a sink of pesticides and contaminants of emerging concern: Full-scale evaluation

Urban, industrial, and agricultural development in river basins has resulted in the pollution of estuarine and coastal ecosystems with a great amount of organic microcontaminants (OMCs) such as pesticides and contaminants of emerging concern (CECs). This study takes the Ebro Delta as a case study to assess the increase or reduction of 25 OMCs in rice fields and one 86 ha constructed wetland (CW). Bentazone and MCPA were the most abundant pesticides in the rice-field drainage water, with a peak concentration of 21,318 and 938 ng/L respectively, whereas the greatest CEC concentrations were found for caffeine, benzotriazoles, and bisphenol A (20- 71 ng/L, on average) in the rice irrigation water. Pesticide concentration increased after the irrigation water passed through the rice fields (from 102 to 1973 ng/L, on average), but CECs present in the irrigation water decreased by 37% (from 14 to 10 ng/L, on average). A mass balance study showed that the CW was capable of reducing OMCs by 67%. Risk assessment analysis showed that the cumulative hazard quotient for Daphnia magna, green algae, and fish was greater than 1 during several sampling campaigns for the rice-field drainage water, but the CW was capable of reducing it by 60-63%, resulting in values below 1, which indicates that the risk was not significant. The results thus indicate that rice fields reduce CECs, but increase pesticides, whereas the use of CWs seems to be a feasible nature-based solution to reduce the discharge of OMCs into estuarine and coastal areas.