Impacts of surface water-groundwater interactions on DOM dynamics revealed by fluorescence and isotopic tracing

The sources and transport of dissolved organic matter (DOM) in rivers are highly complex due to the variability in surface water-groundwater interaction (SGI); however, the link between SGI and DOM dynamics remains poorly understood. The objective of this study is to investigate the spatiotemporal variability and controls of DOM in the surface water and groundwater of a large river (area: 134,766 km²), i.e., the Wei River Basin (WRB) in China. Specifically, the effects of SGI on DOM sources and transport under drought and dam-regulated conditions are examined using fluorescence excitation-emission matrix spectroscopy coupled with parallel factor analysis (EEM-PARAFAC) and stable isotopes (δ²H-H₂O, δ¹⁸O-H₂O). Three fluorescence components were identified: microbial humic-like (C1), protein-like substances (C2), and terrestrial humic-like (C3), each showing distinct seasonal patterns corresponding to SGI variability. During the high-flow season, the dominant form of SGI was surface water recharging groundwater, contributing to 46 % of aquifer recharge and substantially enhancing the transport of fresh and humified DOM (C1 and C3) from surface water to groundwater. In contrast, the low-flow conditions decoupled surface water-groundwater exchange and amplified the contribution of fertilizer-derived DOM in groundwater, reaching up to 43 %. Notably, drought and dams critically influence DOM pathways by altering groundwater recharge and discharge. This study underscores the pivotal role of SGI in DOM transport and demonstrates the value of integrating optical and isotopic tools to unravel biogeochemical processes under changing environmental conditions.