Contrasting Nitrate Sources and Transport Pathways in a Connected Karst Surface Water and Groundwater System

Nitrate contamination threatens surface water and groundwater quality in karst regions, posing risks to drinking water safety and aquatic ecosystems. Strong surface water–groundwater connectivity in karst recharge areas can accelerate contaminant transport through fractures and conduits. In this study, 166 samples, comprising 100 groundwater samples and 66 surface-water samples, were collected under wet-season, normal-flow, and dry-season conditions from a typical karst recharge area in Fengshan Township, Dafang County, Guizhou Province, China. Hydrochemical analyses, dual nitrate isotope analysis, and isotope-based mixing models were integrated to evaluate potential nitrate source contributions and examine the hydrochemical factors associated with nitrate variability. Groundwater was dominated by Ca–HCO3 and mixed hydrochemical facies and exhibited relatively stable ionic compositions, whereas surface water showed more diverse facies and greater variability in total dissolved solids, SO42−, Na+, K+, and Cl−, reflecting a stronger response to external inputs and short-term hydrological processes. NO3− concentrations ranged from 0.02 to 16.24 mg/L in groundwater and from 0.00 to 41.20 mg/L in surface water, with mean concentrations of 3.07 and 4.38 mg/L, respectively. Mixing-model estimates identified manure and sewage (47%) and soil nitrogen (30%) as the leading potential contributors to groundwater nitrate, whereas manure and sewage had the largest estimated contribution to surface-water nitrate (68%). Given the overlap among the isotopic signatures of potential sources, these percentages represent probable source combinations rather than exact apportionments. The absence of consistent covariation between NO3− and Cl− indicated that nitrate transport was not controlled solely by conservative mixing but was jointly regulated by source-input intensity, rapid surface-runoff responses, conduit transport, subsurface mixing, dilution, and water–rock interactions. Statistical modeling further showed that groundwater NO3− variability was associated with the major-ion composition, whereas surface-water NO3− variability was partly explained by a multiple regression model incorporating SO42− and Cl−. Together, these findings support a conceptual source-to-transport framework involving external inputs, rapid surface-water responses, karst conduit transport, subsurface mixing, and water–rock interaction. This study provides insight into contrasting potential nitrate sources and transport processes in connected karst surface water-groundwater systems and supports pollution-source tracing, recharge-area management, and drinking-water source protection.