Migration, transformation and nitrate source in the Lihu Underground River based on dual stable isotopes of δ15N-NO3- and δ18O-NO3.

Abstract

Nitrate (NO3-) pollution is a common phenomenon in karst underground rivers, which are important water sources in karst landscapes. For drinking water safety and environmental protection, it is crucial to accurately identify NO3- sources and their migration and transformation processes in the Lihu Underground River. In this study, water samples of the Lihu Underground River in Guangxi were collected in May 2014, October 2014, January 2015, and July 2015, and water chemical and dual isotopic (δ15N-NO3- and δ18O-NO3-) approaches were used to evaluate the NO3- characteristics and sources in the Lihu Underground River. The concentration of NO3- in the Lihu Underground River ranged from 1.16 to 19.78mg·L-1, with an average of 9.30mg·L-1, which is more than 37% of the WHO standard (10mg·L-1). The concentrations of NO3- in the wet season (May 2014 and July 2015) were slightly lower than those in the dry season (from October 2014 to January 2015) at most sampling sites due to dilution effects. The migration and transformation processes of NO3- were analyzed by comparing the measured and calculated concentrations of NO3- in the Lihu Underground River. In the dry season (from October 2014 to January 2015), the variation in NO3- concentration upstream and midstream of the Lihu Underground River was affected by exogenous input and nitrification. From midstream to the outlet of Xiaolongdong, it is affected by self-purification factors, including physical processes, chemical processes, and biological processes. In the wet season (May 2014 and July 2015), the dilution and mixing effects were the main factors controlling the variation in NO3- concentration in the Lihu Underground River. The contribution rates of potential NO3- sources (incl. atmospheric precipitation (AP), NO3- fertilizer (NF), NH4+ in fertilizer and rainfall (NFA), soil organic nitrogen (SON), and manure and sewage (M&S)) were quantitatively evaluated by using the IsoSource model. The results showed that in May 2014, the main sources of NO3- were M&S and NF, with contribution rates of 46% and 41%, respectively. In October 2014, NO3- sources were M&S with a contribution rate of 47%, followed by NFA with a contribution rate of 31%. In January 2015, NO3- sources in groundwater were M&S, with a contribution rate of 53%, followed by NFA (34%). In July 2015, the main NO3- sources were M&S and NF, whose contribution rates were 54% and 39%, respectively.