Abstract: The increasing urban sewage discharges have severely threatened the quality of surface water. Baiyangdian is the largest freshwater lake in the North China Plain, which plays a critical role in flood control, micro-climate regulation, improvement of ecological environment and development of aquaculture and tourism in the region. However, with rapid economic development and population growth in recent years, its' pollutant load has increased rapidly. Also eutrophication of water body caused by high nitrate concentration has created a significant problem. Excessive nitrate in water not only causes eutrophication, but also threatens human health. Therefore, the investigation of the sources of nitrate pollution and transformation in Fuhe River-Baiyangdian Lake area is important in order to provide a reference for the management of eutrophication and water quality. For the period 2008-2016, the variation in hydrochemical type as well as sources of nitrate in Fuhe River and Baiyangdian Lake surface water systems were investigated by the combined use of hydrochemistry with isotopes (d²H, d¹⁸O and d¹⁵N). The environmental behavior of NO₃^- along the river was evaluated using Cl^- as the standard reference to estimate the dilution and mix of different waters in the study area. 1) If variation in measured NO₃^- was similar to calculated NO₃^-, then it implied that dilution and mix effects were the controlling factors for the change in NO₃^- in the river. 2) If the measured NO₃^- was smaller than calculated NO₃^-, it implied that apart from dilution and mixing, other processes such as biochemical reactions influenced the change in NO₃^-. 3) If also measured NO₃^- was larger than calculated NO₃^-, it implied that there was excess NO₃^- entering the river compared with Cl^-. The results showed that in September 2008, NO₃-d¹⁵N in Fuhe River was higher than 10‰, with a range of 2.07‰-18.49‰ in July 2014. Domestic waste water from nearby villages was the dominant source of nitrate in Fuhe River. However, in June 2009, the range of NO₃-d¹⁵N was -3.7‰-4‰ and discharge of industrial waste water had a significant impact on nitrate concentration in the river in June 2009. In Baiyangdian Lake, the ranges of NO₃-d¹⁵N for September 2008 and July 2014 were 5.8‰-11.7‰ and 3.31‰-12.53‰, respectively. However, the range of NO₃-d¹⁵N for June 2009 was -3.8‰-0.7‰. Domestic and industrial waste water in Fuhe River were the main source of nitrate pollution in Baiyangdian Lake. For 2008-2014, the proportion of Cl^- and SO₄^2-decreased gradually and the discharge of industrial wastewater and domestic sewage was controlled. In 2009, NO₃^- concentration in industrial waste water exceeded 50 mg·L^-1. Then in 2014 and 2016, NO₃^- concentration fell below the standard. The spatial variation in nitrate concentration along the river was mainly affected by dilution, extra input of sewage and denitrification. In June 2011, the variation in nitrate concentration along the river was mainly affected by extra input of sewage. In June 2009, July 2014 and June 2016, denitrification impacted the decrease in concentration of nitrate in Fuhe River when dissolved oxygen (DO) was less than 2 mg·L^-1.