Excess nitrate (NO₃^-) pollution in groundwater is a global environmental concern, particularly in agricultural areas. Excess NO₃^- in groundwater can potentially contaminate surface water bodies, thereby serving as a significant source of NO₃^- contamination. Therefore, quantifying the sources and their contribution of NO₃^- in groundwater is needed for designing NO₃^- control strategies. This study selected the groundwater surrounding a riverine reservoir located in an agricultural region of the Sichuan Basin as the study area. Hydrogeochemistry, stable isotopes (δD-H₂O and δ¹⁸O-H₂O, δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-), the Bayesian isotope mixing model (SIAR), and the human health risk assessment (HHRA) were employed to analyze the sources of NO₃^- and their contribution, and to evaluate their potential risks to human health. Our investigation results showed that 61% and 40% of groundwater samples exceeded the drinking water guideline (10 mg/L) of NO₃^--N concentration, with a range of 1.24 to 42.91 mg/L and 0 to 42.96 mg/L during the high and low water-level periods, respectively. The isotopic compositions of δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^- indicated that manure/domestic wastewater were identified as the primary sources of NO₃^- in the study area. Nitrification was found to be a critical nitrogen cycling process in the groundwater, and may potentially exacerbated NO₃^- pollution. The SIAR model results revealed that manure/domestic wastewater contributed approximately 50% and 38% of total NO₃^- concentration in the groundwater during the high and low water-level periods, respectively. Based on HHRA investigation, consumption of groundwater containing high concentrations of NO₃^- may pose potential health risks to human beings.