Can vegetated drainage ditches be effective in a similar way as constructed wetlands? Heavy metal and nutrient standing stock by ditch plant species

Agricultural runoff impacts rivers and lakes on a global scale, while in developing countries (e.g., China), rural domestic wastewater is often an additional source of pollution. Scholars and policy makers are now promoting the use of agricultural vegetated ditches (VDs) as a potential supplemental management practices for nutrients and pesticide reduction from agricultural sources; however, information on the efficacy of these systems for the treatment of rural wastewater loaded with nutrients and heavy metal(loid)s (HMLs) removal and standing stock (SS) in aquatic plants growing there is rare or unknown. These shortcomings in our understanding of these systems must be addressed if they are going to serve further as an effective tool for purifying rural domestic sewage laden with concentrations of HMLs. To address this knowledge gap, the present study deals with SS of ten HMLs in ten plant species growing in a VD receiving raw rural domestic wastewater, with the intention to revealing an appropriate period for ditch plant harvesting to maximize HM removal. Mean reduction efficiencies amount to TN (60.6%), NH₄-N (63.3%), NO₃-N (48.1%), TP (58.5%) and PO₄-P (51.9%), with favorable treatment performance during the summer season. Concentrations of nutrients typically declined gradually with distance along the length of the VD. Furthermore, the annual average reduction amounted to Ni (50.6%), Cu (56.1%), Cr (63.3%), Zn (79.3%), Cd (67.5%), Pb (80.1%), As (60.3%), Fe (52.6%), Al (19.8%), and Mn (24.3%). Plant uptake and sediment retention were the major contributors for nutrient mitigation within the vegetated ditches, with subsequent bacterial interaction and transformation of pollutants. Accumulation of HMLs varied among species and plant parts. Overall, SSs of most HMLs in plant species were greater in either stems or roots than leaves, with maximum values recorded in SSs of Al, Fe, and Mn. Based on the seasonal fluctuation of the accumulated HMLs in different plant species, the approach of harvesting plant biomass in order to effectively target HMLs in August or early September is recommended. Overall, reported results suggest that VD is an effective ecosystem to mitigate HMLs and nutrients from raw rural domestic wastewater in a similar way as constructed wetlands, noting the potential incorporation into local best management practices.