Topography-driven hydrochemical evolution in semi-arid watershed: Isotopic and seasonal insights into surface water-groundwater interactions

a tributary of the Baiyangdian Watershed, from mountainous headwater to low-lying wetland, North of China Hydrochemical evolution and surface water-groundwater (SW-GW) exchange govern water quantity and quality, but their dynamics across heterogeneous landscapes are poorly quantified. This study aims to elucidate the spatiotemporal patterns and driving mechanisms of SW–GW interactions by combining hydrochemical and stable isotope tracers (δ 2 H and δ 18 O) with a novel coupled isotopic fractionation and lc-excess model. Results showed GW exhibited significantly higher total dissolved solids (1158.4 mg L −1 ) and total hardness (506.3 mg L −1 ), exceeding SW by factors of 2.2 and 1.6, respectively. Forward model demonstrated rock weathering as the dominant solute source, with carbonate weathering gaining prominence upstream. GW exhibited higher mean NO 3 - concentrations (39 mg L −1 ) than SW (18 mg L −1 ), with source analysis indicating a downstream shift from predominantly soil/sewage inputs to increased manure contributions. Spatially, TDS, Na + , Cl - and SO 4 2- increased by 1.2, 2.1, 2.8 and 1.4 - fold for SW and 2.5, 6, 6.2 and 3.7 - fold for GW along the sampling sites. Seasonally, SW showed decreased TDS, Cl - , and SO 4 2- post-monsoon, while GW displayed upstream concentrations rise post-monsoon, whereas downstream concentrations declined. Coupled isotopic fraction modeling and lc-excess analysis showed that the mean evaporation fractions of SW and GW were 12 % and 13 %, while wetlands exhibiting enhanced evaporation (SW: 20 %; GW: 26 %). This work establishes a mechanistic link between natural weathering, anthropogenic forcing, and seasonal hydrology in driving SW-GW hydrochemical evolution within semi-arid basins, providing critical insights for contamination mitigation and water-resource sustainability. • GW TDS/TH exceeded SW by 2.2 and 1.6 times, and shifting to hard-saline downstream • Mean GW NO 3 - doubled SW, with sources shifting downstream from soil/sewage to manure • Post-monsoon, SW TDS/Cl - /SO 4 2- decreased, while upstream GW concentrations increased • Mean evaporation fractions were 12 % (SW) and 13 % (GW), boosted to 20 %/26 % in wetland