Tracking nitrate sources in the Chaohu Lake, China, using the nitrogen and oxygen isotopic approach

The Chaohu Lake is highly polluted and suffers from severe eutrophication. Nitrate is a key form of nitrogen that can cause water quality degradation. In this study, hydrochemical and dual isotopic approaches were utilized to identify the seasonal variation of nitrate sources in the Chaohu Lake and its inflowing rivers. The average nitrate concentrations ([NO₃^-]) of the lake and its inflowing rivers were 89.9 and 140.8 μmol L^-1, respectively. The isotopic values of δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^- in the lake ranged from - 1.01 to + 16.67‰ and from - 4.39 to + 22.20‰, respectively. The two major rivers had distinct isotopic compositions, with average δ¹⁵N-NO₃^- values of + 17.52 and + 3.51‰, and average δ¹⁸O-NO₃^- values of + 2.71 and + 7.47‰ for the Nanfei River and Hangbu River, respectively. The results show that soil organic ammonium and urban wastewater discharge were the main sources of nitrate in the Chaohu Lake, and nitrate assimilation was an important process affected [NO₃^-] and isotopic compositions, especially in the western Chaohu Lake. The elevated [NO₃^-] and δ¹⁵N-NO₃^- values in the western Chaohu Lake indicate the high influence of human activities. Urban wastewater discharge was the primary nitrate source in the Nanfei River and soil organic ammonium was the main source in the Hangbu River. Although nitrate from direct atmospheric deposition was low, its strong flushing effect can substantially improve riverine [NO₃^-] and nitrate loading from terrestrial ecosystem. The relatively high nitrate fluxes from the Hangbu River indicate that nitrogen loading from agricultural watershed is unneglectable in watershed nitrogen sources reduction strategies.