Tracking nitrate sources in agricultural-urban watershed using dual stable isotope and Bayesian mixing model approach: Considering N transformation by Lagrangian sampling. (15th December 2021) - Ryu, Hui-Seong
Abstract: A dual isotopes approach and the Bayesian isotope mixing model were applied to trace nitrogen pollution sources and to quantify their relative contribution to river water quality. We focused on two points to enhance the applicability of the method: 1) Direct measurement on the end-members to distinguish "sewage" and "manure" which used to be grouped in one pollution source as their isotope ranges overlap; 2) The Lagrangian sampling method was applied to consider the transport of nitrogen pollutants in a long river so that any fractionation process can be dealt with in the given Bayesian modeling framework. The results of the analysis confirmed the NO3 − isotope composition in the river of interest to be within the range of NO3 − with origins in "NH4 + in fertilizer", "Soil N", and "Manure and sewage" pollution. This suggests that nitrogen pollution is mostly attributed to anthropogenic sources. The δ 18 O NO3 value follows the range +2.5∼+15.0‰, implying that NO3 − in the river is mainly derived from nitrification, and possible nitrification in groundwater or waterfront other than surface water. The ratio of the concentration of δ 15 N NO3 to that of δ 18 O NO3, and the corresponding regression equation indicates that the denitrification effect in surface water was insignificant during the study period. From the results of the contribution ratio of each source, improving the water quality of the discharge from the sewage treatment plants was proved to be the key factor to reduce nitrogen pollution in the river. Graphical abstract: Image 1 Highlights: Origins of nitrogen sources in the Yeongsan River in Korea were traced. Dual isotope approach and the Bayesian isotope mixing model were used. N transformation by Lagrangian sampling was considered. Denitrification effect in surface water was insignificant during the study period. Improving discharge from sewage treatment plants key to reduce nitrogen pollution.