Implication of two in-stream processes in the fate of nutrients discharged by sewage system into a temporary river

The aim of this study was to better understand the fate of nutrients discharged by sewage treatment plants into an intermittent Mediterranean river, during a low-flow period. Many pollutants stored in the riverbed during the low-flow period can be transferred to the downstream environments during flood events. The study focused on two processes that affect the fate and the transport of nutrients, a physical process (retention in the riverbed sediments) and a biological process (denitrification). A spatial campaign was carried out during a low-flow period to characterize the nutrient contents of both water and sediments in the Vène River. The results showed high nutrient concentrations in the water column downstream of the treated wastewater disposal (up to 13,315 μg N/L for ammonium and 2,901 μg P/L for total phosphorus). Nutrient concentrations decreased rapidly downstream of the disposal whereas nutrient contents in the sediments increased (up to 1,898 and 784 μg/g for total phosphorus and Kjeldahl nitrogen, respectively). According to an in situ experiment using sediment boxes placed in the riverbed for 85 days, we estimated that the proportion of nutrients trapped in the sediments represents 25% (respectively 10%) of phosphorus (respectively nitrogen) loads lost from the water column. In parallel, laboratory tests indicated that denitrification occurred in the Vène River, and we estimated that denitrification likely coupled to nitrification processes during the 85 days of the experiment was significantly involved in the removal of nitrogen loads (up to 38%) from the water column and was greater than accumulation processes.