Source apportionment of nitrate pollution in the reservoir basins based on dual isotopes of nitrogen and oxygen

As a critical drinking water source for the Beijing-Tianjin-Hebei region, the upper basin of Miyun Reservoir is experiencing increasingly severe nitrate pollution in its water bodies, threatening the reservoir's water quality security. This study, focusing on this area, collected surface water and shallow groundwater samples. We employed single-factor evaluation to explore the spatiotemporal characteristics of nitrogen pollution, and combined graphical analysis of hydrochemical elements and dual nitrogen‑oxygen isotopes with the Bayesian mixing model (MixSIAR) to identify nitrate sources. Results showed that nitrogen pollution was primarily nitrate-driven, exhibiting significant spatiotemporal variations. Temporally, nitrate pollution in both surface water and groundwater within the watershed was most severe during the dry season. Spatially, surface water nitrate pollution was most severe in the Tangquan River and Chaohe River, while groundwater nitrate pollution was most severe in the Heihe River. Nitrate in surface water primarily originated from sewage (43.0%) and soil nitrogen (37.8%); the contribution of sewage to nitrate in groundwater further increased (62.1%). Significant differences in pollution source contributions were observed among different tributaries: agriculturally intensive areas such as the Chaohe River, Qingshuihe River, and Honghe River are dominated by soil nitrogen, while the Miyun Reservoir area is dominated by sewage.