Elucidating nitrate sources in a karst watershed: Integrating isotope techniques, hydrochemical methods, and Bayesian...

Karst-dominated watersheds, characterized by complex interactions between surface water and groundwater systems, face significant challenges in managing nitrate (NO₃^-) pollution. Despite the critical importance of this issue, the contributions and driving factors of NO₃^- pollution sources in karst watersheds remain poorly understood. This study presented a comprehensive assessment of NO₃^- sources in the Puzhehei watershed, a distinctive karst landscape, by integrating multiple analytical approaches: stable isotope analysis (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-), hydrochemical data, and a Bayesian isotope mixing model. Our findings revealed that nitrogen fertilizers (NF) constituted the predominant source of NO₃^- pollution in surface water, contributing over 30 %, followed by soil nitrogen (SN), manure/sewage (M&S), and aquaculture (AQ). In contrast, groundwater pollution was primarily influenced by M&S and AQ, accounting for 29 % and 26 % of the total contribution, respectively. During the dry season, agricultural practices, particularly irrigation, enhanced NO₃^- contributions from NF and SN sources, collectively accounting for approximately 60 %. Conversely, the wet season facilitated NO₃^- mobilization from soils and fertilizers, leading to downstream migration and a shift in primary pollution sources toward M&S and AQ. Additionally, seasonal tourism and related human activities may significantly shape NO₃^- source dynamics during high-flow periods. These insights provide crucial guidance for developing targeted water quality management strategies in karst regions to effectively mitigate NO₃^- pollution and protect water resources.