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Dianbu River, flowing into the western part of Chaohu Lake, has been heavily polluted, and nitrogen is one of the key factors. During three periods （wet, normal, and dry）, 30 surface water samples were collected from the Dianbu River Basin as the research objects. The water chemistry, multiple stable isotopes （ δ 15 N-NO 3 - , δ 18 O-NO 3 - , and δ 15 N-NH 4 + ）, and a SIAR mixing model were analyzed not only to understand the spatio-temporal distribution characteristics of nitrogen and its influencing factors but also the sources of nitrogen. The results showed that the concentrations of TN and NH 4 + -N were higher in dry periods than those in wet and normal periods. Compared with the water quality requirements （GB/T 14848）, 56.67% of samples in the surface water TN were in the situation of Grade Ⅴ-inferior Grade V. The proportion of Ⅳ-inferiorⅤof NH 4 + -N sections was 10%. Along the main river, the concentrations of TN and NO 3 - -N mainly increased and then decreased （in the wet season and normal season） or increased （in the dry season） from upstream to downstream, mainly affected by precipitation, land use types, etc. The ratios of δ 15 N-NO 3 - and δ 18 O-NO 3 ranged from -4.61‰ to 24.39‰ and from -5.39‰ to 15.18‰, respectively, suggesting the multiple sources of fertilizer, soil organic nitrogen, and manure sewage. Essentially, slight denitrification was observed in surface water. Based on the SIAR model, in different hydrological periods, fertilizer （34.53%-52.39%） and soil organic nitrogen （26.24%-32.29%） were the major sources of nitrate in the upper basin. Manure sewage （30.10%-37.93%） was the primary source of nitrate in the middle and lower reaches of the Dianbu River Basin, followed by soil organic nitrogen （28.78%-31.21%）, fertilizer （25.89%-28.11%）, and precipitation （4.36%-11.92%）. The ratio of δ 15 N-NH 4 + in surface water during the wet period ranged from -9.16‰ to 2.48‰, mainly indicating that the sources of ammonium salt were ammonium nitrogen fertilizer and precipitation. This study will provide a useful insight for the water environmental management.

Chaohu lake is a key water body for water pollution prevention and treatment in our country. However， it has been at a higher eutrophication level recently. Here， the surface water and groundwater in the Dianbu River Basin， a secondary tributary of Chaohu Lake， were taken as the research object. In order to test the hydrochemical composition and hydrogen and oxygen isotope values of different water bodies， 30 groups of surface water samples， 36 groups of groundwater samples， 16 groups of hydrogen and oxygen stable isotope samples， and 18 groups of groundwater hydrogen and oxygen stable isotope samples were collected in August 2021 （wet season）， November 2021 （normal season）， and February 2022 （dry season）. The seasonal and spatial variation characteristics were analyzed to explore the hydrochemical characteristics and formation mechanism of water bodies by means of mathematical statistics， Piper triangular diagram， Gibbs figures， and ion ratios. The following results were obtained： ① precipitation was the main source of surface water and groundwater in Dianbu River Basin， and the evaporation fractionation effect of surface water was more significant than that of groundwater. At different periods， the surface water was more enriched with stable isotopes of hydrogen and oxygen than groundwater. The stable isotopes of hydrogen and oxygen in water showed seasonal variation， relative enrichment in the wet season， and depletion in the dry season. ② Both surface water and groundwater in the Dianbu River Basin were weakly alkaline， and the concentration of ions in surface water was significantly lower than that in groundwater. Ca 2+ and Na + were the main cations in surface water， Ca 2+ was the main cation in groundwater， and the dominant anion in all water was HCO 3 - . The hydrochemical typology of surface water was mainly HCO 3 ·Cl-Na·Ca， and that of groundwater was mainly HCO 3 -Na·Ca. ③ The concentrations of the main hydrochemical indexes of surface water and groundwater showed certain seasonal and spatial differences. From the wet season to the dry season， the concentrations of TDS， K + ， Na + ， Ca 2+ ， Mg 2+ ， Cl - ， and SO 4 2- in surface water showed an increasing trend on the whole. The concentrations of Na + ， Ca 2+ ， and Mg 2+ in groundwater showed little change but increased slightly， whereas the concentrations of Cl - ， SO 4 2- ， and NO 3 - showed an increasing trend on the whole. The concentrations of Cl - ， SO 4 2- ， and NO 3 - in the water showed relatively large seasonal fluctuations. From upstream to downstream， the concentrations of the main hydrochemical indexes in surface water first decreased and then increased， among which the concentration of NO 3 - increased the most. The concentrations of the main hydrochemical indexes of groundwater in the direction of runoff changed little overall， but the concentration in the discharge area was higher than that in the recharge area. ④ The formation of hydrochemical characteristics of the water was mainly controlled by water-rock interaction but was also influenced by human factors. The water-rock action was mainly the weathering dissolution of silicate rock， salt rock， and carbonate rock. Man-made pollutants such as sewage from a sewage treatment plant， domestic sewage， and feces had obviously changed the hydrochemical characteristics of the local water. ⑤ Compared with that in 2016， the concentration of NO 3 - in surface water showed a certain degree of reduction. The nitrogen pollution control work carried out by the local government had achieved certain results， but it was still necessary to strengthen the pollution prevention and control of sewage and feces in the downstream of the Dianbu River， some tributaries （such as the Dingguang River and Maqiao River）， and some residential areas.