<p>Accurate analysis of nitrate (NO₃⁻) distribution and sources is essential for controlling non-point source pollution in watersheds. In the Jinxiuchuan River Basin, a key drinking water source for Jinan, potential NO₃⁻ pollution in surface water poses risks to human health and the local ecosystem. Surface water samples were collected in January (dry season, <i>n</i> = 10), May (normal season, <i>n</i> = 10), and July (wet season, <i>n</i> = 11). The spatiotemporal distribution and sources of NO₃⁻ in the complex surface water environment of this basin were analyzed by integrating physicochemical parameters, land use, and δ¹⁵N/δ¹⁸O-NO₃⁻ data using IsoSource and SIAR models. The results showed higher NO₃⁻ concentrations in January (5.35 ± 0.78 mg/L) and July (5.11 ± 1.21 mg/L) than in May (1.93 ± 1.35 mg/L). Spatially, higher concentrations were observed downstream and in the upper tributaries in January and May; in July, higher concentrations were found upstream and in the upper tributaries. NO₃⁻ sources correlated strongly with land use types. Nitrification dominated in all seasons. Temporally, the main sources were sewage (43.0%) and manure (24.2%) in January, manure and sewage (72.3%) and soil (22.0%) in May, and soil (56.4%) and fertilizer (34.0%) in July. Spatially, the primary difference occurs in May, when the contributions of manure and sewage exceed those of soil in upstream tributaries. The sensitivity of the SIAR model correlates with the isotopic composition of the sources. This study will provide a scientific foundation for the prevention and management of NO₃⁻ pollution in the Jinxiuchuan River Basin and a reference for similar studies in karst regions.</p>