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 (delta N-15(NO3) and delta O-18(NO3)) combined with stable water isotopes (delta O-18(H2O) and delta H-2(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, delta O-18(H2O) and delta H-2(H2O) ranged from -9.37 parts per thousand to -8.68 parts per thousand and from -65.29 parts per thousand to -59.37 parts per thousand, respectively, exhibiting isotopic depletion in wet season and enrichment in dry season. Moreover, in the effluent, delta N-15(NO3) and delta O-18(NO3) ranged from 10.56 parts per thousand to 16.60 parts per thousand and -4.86 parts per thousand to -0.35 parts per thousand, 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 delta N-15(NO3) in the effluent was influenced by the N source instead of the seasonal variation, although delta O-18(NO3) exhibited a high dependence on the seasonal effects, influenced by delta O-18(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 (delta N-15(NO3) and delta O-18(NO3)) and stable water isotopes (delta O-18(H2O) and delta H-2(H2O)) as tracers can help evaluate the potential impact of N sources on water systems.