Constructed Wetlands receiving treated wastewater (CWtw) are systems placed between Wastewater Treatment Plants (WWTPs) and receiving water bodies. In order to assess the efficiency of these systems, a 6-years project has been launched with 3 sites (full and pilot scales). A comprehensive monitoring is carried out to understand the fate of conventional pollutants (C, N, P, TSS, etc.), micropollutants and faecal contamination. One of the CWtw monitored, located in Marguerittes (Southern France), is a 2-ha full-size site made of 2 ponds. This study focuses on the CWtw performance assessment based on different methods used (including ion-selective electrodes, bathymetry and quadrats) to measure nitrogen (N) in each compartment. It gives the N mass balance over a 1-year period (2015).