[Analysis of Nitrate Sources in Different Waters of a Karst Basin]

To identify the sources and transformation processes of nitrate in surface water and groundwater in a karst basin, water samples were collected in the Songbai Mountain Reservoir basin during the normal and dry seasons. The spatio-temporal distribution, sources, and transformation processes of nitrate in the waters were analyed using a hydrochemical and stable isotopic (δ¹⁵N-NO₃^-, δ¹⁸O-NO₃^-, and δ¹⁸O-H₂O) multi-tracing approach. The contribution rates of different nitrate sources in surface and groundwater were estimated based on the SIAR model. The results showed that NO₃^--N and NH₄⁺-N were the main species of dissolved inorganic nitrogen in the waters. The over standard rate of NO₃^--N in groundwater was 7.89% in the normal season and 16.67% in the dry season. Temporally, the nitrate concentrations of waters in the dry season were higher than those in the normal season. Spatially, the nitrate concentrations of groundwater around dryland areas (from the Kailun River to the Songbai Mountain Reservoir) were higher than those of paddy fields (Ganhe River), and the nitrate concentrations of surface water in dryland and construction sites (Kailun River) were generally high. Nitrification was the dominant process in the waters. The nitrate in the waters mainly came from soil organic nitrogen, manure/sewage, and chemical fertilizers; their contribution rates to nitrate were 36.7%, 34.7%, and 28.6% for surface water and 39.9%, 34.9%, and 25.2% for groundwater, respectively. Nitrate pollution in the waters was mainly affected by agricultural activities and the discharge of sewage; appropriate control measures such as water and fertilizer regulation for farmland and treatment of rural sewage should be strengthened.