Identification of nitrate sources and transformation in karst cave water using hydrochemistry and NO3− isotopes (δ15N/δ18O) combined with a Bayesian mixing model

Abstract

ABSTRACTNitrogen mainly exists as nitrate in aqueous environments. In karst areas, nitrate pollution caused by anthropogenic activities severely endangers aqueous systems. In order to identify nitrate sources, nitrate in the cave water of Mahuang Cave was comprehensively analysed using hydrochemistry, δ15N-δ18O isotopes and stable isotope analysis in R. The results indicate that the nitrate content in the cave water presented insignificant seasonal variations but significant spatial variations. Compared with the variation of δ15N-NO3−, the seasonal variation of δ18O-NO3− was positive (2.68‰) in the dry season and negative (−0.52‰) in the wet season. The variation of NO3− and Cl− and the isotopes of NO3− (δ15N, δ18O) suggest multiple sources of NO3− in the water. The average contribution of different nitrate sources in the karst cave water was assessed by the SIAR model: chemical fertilisers (46.29%) > soil nitrogen (30.57%) > manure & sewage (14.14%) > atmospheric precipitation (9.14%) in the dry season and chemical fertilisers (61.86%) > soil nitrogen (20.29%) > manure & sewage (12.43%) > atmospheric precipitation(5.57%) in the wet season. The contribution in dry and wet seasons was dominated by chemical fertilisers and soil nitrogen.

Keywords

• Karst water
• nitrate
• stable isotopes
• SIAR model
• Mahuang Cave