This paper is the result of environmental geological survey engineering.

Objective Nitrate pollution in surface water is a global environmental issue, therefore identifying the source and transformation of nitrate is fundamental to improving water quality.

Methods To accurately identify the sources and transformation processes of nitrate in surface water in north of Yantai city, hydrochemistry and dual isotopes of nitrate nitrogen−oxygen technologies were used to sample and analyze surface water from different river basins, including Dagujia River, Xin'an River, Qinshui River, and Han River. And the Bayesian isotopes mixing model MixSIAR was also used to calculate the contribution rates of various nitrate sources.

Results (1) The surface water in the study area is weakly alkaline, with NO₃⁻−N concentration significantly higher than NH₄⁺−N and NO₂⁻−N concentrations, making NO₃⁻−N is the primary nitrogen pollutant in the surface water of the study area. Only the NO₃⁻−N concentration in Xin'an River (NO₃⁻−N<10 mg/L) fell within the national health standard for drinking water. (2) Nitrification dominates the nitrate transformation process in the surface waters in the study area, while denitrification playing a negligible role. (3) The output results of the MixSIAR model show that the contribution rates of nitrate sources in surface water in the study area are as follows: sewage and manure (51.1%)>soil organic nitrogen (23.9%)>chemical fertilizer (22.9%)>atmospheric deposition (2.1%).

Conclusions Overall, sewage and manure have the highest contribution rate to nitrate sources in surface water in the study area, followed by soil organic nitrogen and chemical fertilizer. Sewage and manure from anthropogenic inputs are the main sources of nitrate pollution in the study area. Therefore, reducing discharge of sewage and manure into rivers is key to controlling pollution.