[Identification of Nitrate Pollution Sources Through Various Isotopic Methods: A Case Study of the Huixian Wetland]

In recent years, the ecological environment of wetlands has deteriorated under the predatory exploitation of human beings. Nitrate pollution is a major problem faced by wetland ecosystems. In this paper, Huixian karst wetland was selected as a study area to identify the main factors leading to the increase of nitrate in wetland water. The main sources of nitrate pollution in the area were determined by means of ¹⁵N(NO₃^-) and ¹⁸O(NO₃^-) isotopes. The contribution rate of all types of pollution sources was determined with the SIAR model. To explore the relationship between the spatial distribution of the nitrate concentration and the groundwater runoff conditions, the ¹³C_DIC isotopes were used to qualitatively characterize the runoff conditions of the groundwater. The results show that there are three main factors affecting the nitrate concentration in the study area:animal manure and domestic sewage, NO₃^- in chemical fertilizer, and soil nitrogen. The average contribution rates to nitrate pollution are 39.1%, 32.2%, and 28.5% respectively. The ¹³C_DIC isotope data clarify that a light ¹³C_DIC reflects better groundwater runoff conditions. The concentration of NO₃^- is generally lower, while the NO₃^- concentration corresponding to heavy ¹³C_DIC is generally higher. The condition of the groundwater runoff to a certain extent also affects the distribution of the NO₃^- concentration.