Nitrogen and oxygen isotope analysis of nitrate-nitrogen pollution sources in a typical urban river

Nitrate-nitrogen (NO₃-N) pollution in urban rivers has become a water environment problem of great concern in the context of rapid urbanization. Taking the Zaohe River in Xi'an City as an example, the river water body, discharge wastewater and inlet and outlet water of wastewater treatment plants were sampled in May (dry season) and September (rainy season) in 2021 to determine the water quality parameters and to resolve their nitrate-nitrogen sources using nitrogen and oxygen stable isotope and Iso Source models. The results showed that δ¹⁵N values of nitrate-nitrogen in the Zaohe River ranged from −26.43‰ to 32.29‰ and −2.81‰ to 20.85‰ in May and September, respectively, and δ¹⁸O values ranged from −23.42‰ to 53.02‰ and −5.26‰ to 21.53‰, respectively. Manure was the main source of nitrate pollution in the Zaohe River, and the average contribution rate of different nitrate sources in the river was in the order as follows: for river water, manure > soil organic nitrogen > fertilizer > atmospheric deposition; for discharge water, manure > soil organic nitrogen > fertilizer > atmospheric deposition; for sewage treatment plant influent water, atmospheric deposition > manure > soil organic nitrogen > fertilizer; for sewage treatment plant effluent, manure > soil organic nitrogen > fertilizer > atmospheric deposition. The combined contribution of soil organic nitrogen and manure to the Zaohe River watershed was greater than 70%. Therefore, controlling the discharge of residential sewage, strengthening the construction of pipeline networks, enhancing the management of livestock and poultry manure and strengthening the supervision of land fertilization were conducive to reducing the pollution of nitrate-nitrogen in urban rivers.