Nitrate is one of the most prevalent inorganic pollutants in groundwater systems. Its concentration directly affects the safety assessment of groundwater quality. To scientifically identify nitrate sources in strontium-rich mineral water and facilitate the protection of mineral water resources, this study selects Chengde City, Hebei Province, as the study area. Nitrate source apportionment was quantified using the MixSIAR model, with uncertainties assessed via cumulative probability distributions. The results show that the nitrate concentration in strontium-rich mineral water of the study area ranges from <0.003 to 70.2 mg/L, with a coefficient of variation of 1.24, indicating high data dispersion. The nitrate sources present a mixed pollution characteristic dominated by natural processes and supplemented by human activities. Nitrification dominates biogeochemical processes of strontium-rich mineral water. Soil nitrogen is the primary contributor to nitrate in mineral water, with an average contribution rate of 70.4%, followed by manure; synthetic fertilizers and rainwater together account for less than 1% of the total. Uncertainty analysis shows that contributions of rainwater, synthetic fertilizer, manure, and soil nitrogen to nitrate in strontium-rich mineral water remain stable (UI₉₅ < 0.15), verifying reliable results. This study provides guidance for protecting mineral water quality and sustainable resource exploitation.