Fig. 1 The location of sampling sites

 

To understand the nitrogen sources and fate in the surface water of Tianjin, the concentrations of nitrogen and δ¹⁵N-NO₃ ⁻ were analyzed in different types of waters. Mostly, NO₃ ⁻ was the dominant species of DIN (Dissolved Inorganic Nitrogen), although NH₄ ⁺ was the main species in certain samples, such as sewage.  The δ¹⁵N-NO₃ ⁻ values ranged from −5.5 to +28.6‰.  The water chemical and isotopic results suggested that domestic sewage and agricultural activities were the two main sources of nitrate in surface waters.  In addition, the nitrogen isotopic compositions were significantly influenced by nitrification, ammonia volatilization and denitrification.