Analyses Of Sources And Transformation Of Nitrogen As A Contaminant In The River And Lake Water In Nanhe River System And Its Corresponding Lake Area In The Western Region Of The Taihu Lake Basin

The water and sediments of river and lake in Nanhe river system and its corresponding lake area in the western region of the Taihu Lake basin were sampled during four seasons from 2013 to 2014. The forms of inorganic nitrogen and their isotopic characteristics of the samples were analyzed and the ratio δ15N (‰) was used to identify the sources of nitrogen pollution and the biochemical mechanism of nitrogen cycle in the region. Dual isotope method of nitrogen and oxygen with inorganic ion in the water were used to further verify nitrogen pollution sources. The results showed that: ① NO3-N and NH4+-N were the main forms of inorganic nitrogen in the rivers entering the lake while NO3--N was the main form of inorganic nitrogen in the lake water. ② During dry spring and winter NO3--N in the rivers were mainly from fertilizers and, to a much less extent, from mineralized soil organic nitrogen according to the range of the 815N-NO3-(%o) values while during summer and autumn it came from sewage and, to a much less extend, from mineralized soil organic nitrogen and fertilizers. The conclusion Verified by dual isotope method of nitrogen and oxygen with inorganic ion in the water was consistent with it. Sewage was the major source of ammonium nitrogen in the rivers according to the range of the δ15N-NH4+(‰) values. ③ The mode of occurrence of nitrogen in the lake water was mainly affected by nitrification of the lake water and denitrification of the sediments based on combined analyses of NO3--N, NH4+-N,δ15N-NO3-, and δ15N-NH4+ of the water and sediment samples.