Seasonal nitrate input drives the spatiotemporal variability of regional surface water-groundwater interactions, nitrate sources and transformations

Addressing the nitrogen geochemical cycle, accompanied with hydrological processes, influenced by anthropogenic nitrogen inputs provides new insights on policymaking for water resource management and aquatic ecosystems protection. However, there is currently a lack of further understanding on the source contributions of nitrate (NO₃^–) and transformations under explicit hydrological conditions. Due to their spatiotemporal heterogeneity, the impact of seasonal human activity changes on them has not been thoroughly elucidated. This study combines hydrogeochemical analysis, multiple stable isotopes (δ¹⁵N-NO₃^–, δ¹⁸O-NO₃^–, δ²H-H₂O, and δ¹⁸O-H₂O), statistical methods, and a Bayesian isotope mixing model (MixSIAR) to investigate surface water-groundwater interactions, nitrate sources and cycling processes in a hilly region of China. Results showed that groundwater hydrochemical types, governed by water–rock interactions and anthropogenic activities, shift seasonally, from HCO₃-Ca·Mg in the dry season to Cl·SO₄-Ca·Mg in the wet season, particularly in areas with high nitrate concentrations. Chemical weathering in groundwater was driven by the combined dissolution of silicate and carbonate rocks. Groundwater contributed 73.5 % to surface water, with the rate of 0.20 mm/day in March. Total interflow was 407.03 mm from May to October. Nitrification emerged as the dominant nitrogen transformation process in groundwater, while denitrification was localized, primarily occurring during the wet season. The major sources of NO₃^– in groundwater were soil nitrogen (SN; 48.4 ± 17.5 % in the dry season and 30.5 ± 11.6 % in the wet season) and manure and sewage (M&S; 41.0 ± 17.7 % in the dry season and 54.8 ± 11.7 % in the wet season). The spatial distribution of M&S contributions corresponded to elevated Cl^- and SO₄^2- concentrations, particularly at sites with high nitrate levels during the wet season. These findings reveal that anthropogenic nitrogen inputs significantly influence the spatiotemporal variability of groundwater hydrochemical types, nitrate sources, and nitrogen transformation processes.