Seasonal variations in stable nitrate isotopes combined with stable water isotopes in a wastewater treatment plant: Implications for nitrogen sources and transformation

A multi-isotopes approach involving the use of stable nitrate isotopes (δ¹⁵N_NO3 and δ¹⁸O_NO3) combined with stable water isotopes (δ¹⁸O_H2O and δ²H_H2O) as tracers can help identify the nitrogen source and understand the transformation process in a river water system. In this study, we identify the potential impact of the N source in the effluent discharged from a wastewater treatment plant (WWTP) on the surrounding stream and clarified the seasonal variations in the isotope values of the effluent and stream water from 2019 to 2020 in South Korea. In addition, we investigate the factors that led to the seasonal variations of the stable isotopes and evaluated the relationship between the uncertainty of the contribution ratio of the N sources and isotopic variations in the river water system. To examine the potential impact of the inputs of N from the WWTP, samples were obtained from the stream (up and down with respect to the WWTP) and WWTP (influent and treated effluent) once a month. For the monthly effluent samples, δ¹⁸O_H2O and δ²H_H2O ranged from –9.37‰ to –8.68‰ and from –65.29‰ to –59.37‰, respectively, exhibiting isotopic depletion in wet season and enrichment in dry season. Moreover, in the effluent, δ¹⁵N_NO3 and δ¹⁸O_NO3 ranged from 10.56‰ to 16.60‰ and –4.86‰ to –0.35‰, respectively. To clarify the seasonal variation in the effluent, high-resolution samples of the influent and treated effluent were obtained every day in September 2019. It is noted that δ¹⁵N_NO3 in the effluent was influenced by the N source instead of the seasonal variation, although δ¹⁸O_NO3 exhibited a high dependence on the seasonal effects, influenced by δ¹⁸O_H2O. When estimating the contribution of the effluent using isotope values to the water system, uncertainty could arise due to these seasonal variations. Overall, a multi-isotope approach involving the combined use of stable nitrate isotopes (δ¹⁵N_NO3 and δ¹⁸O_NO3) and stable water isotopes (δ¹⁸O_H2O and δ²H_H2O) as tracers can help evaluate the potential impact of N sources on water systems.