What is the fate of trace metals and phosphorus in CW receiving treated wastewater? Contributions of free water, plants and soil

Constructed Wetlands receiving Treated Wastewater (CWtw) are systems downstream of Wastewater Treatment Plants (WWTPs) before the receiving water bodies. They have recently become attractive in France under the perception that they improve water quality of the WWTP effluent (e.g. removal of excess suspended solids, or sorption of some dissolved pollutants). Depending on the historical background of the site close to the WWTP, trace metals are often present in the soil. The aim of this study is to determine the fate of the main trace metals (As, Ba, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, Zn) and phosphorus (P) in the different compartments of CWtw: free water, soil and deposits (when relevant) and plants at two different sites MAG and BTT. For both sites, the daily water balance was determined using inlet and outlet flowmeters associated with a weather monitoring. Free water samples were collected at the inlet and outlet: 4 2-day flow proportional composite samples. The BTT site had silicon carbide porous suction cups (0.3 m deep in the soil) to collect 4 composite samples of pore water. The plants and soil (BTT) sampling was performed at both sites by using quadrats (0.5 x 0.5 x 0.2 m). At MAG site, only deposits were sampled as there is no infiltration. Total phosphorus (TP) and metals were analyzed in plants, deposits (MAG) and soil (BTT). A mass balance for each metal and P is estimated thanks to daily hydraulic loads, infiltration flux (BTT), metals and P concentrations in free and pore water, in the plants and in the soil (BTT).