Spectral and molecular insights into the variations of dissolved organic matter in shallow groundwater impacted by surface water recharge

Dissolved organic matter (DOM) represents one of the most active elements in aquatic systems, whose fraction is engaged in chemical and biological reactions. However, fluorescence, molecular diversity and variations of DOM in groundwater systems with the alteration of surface water recharge remain unclear. Herein, Excitation-emission matrix (EEM) fluorescence spectroscopy and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) combined with principal component coefficients, parallel factor analyses (PARAFAC) with two‒dimensional correlation spectroscopy (2D-COS) were applied in this study. EEM data reassembled for principal component analysis (PCA) highlighted differences in tryptophan-like peak between groundwater collected parallel to the river (PR) and those taken vertical to the river (VR). PARAFAC have identified six components, i.e., microbial-related humic substances (C1 and C6), protein-like substances (C2 and C5), and terrestrial humic-like substances (C3 and C4). In the PR direction, variations of fluorescence components were dominated by terrestrial humic-like substances, while microbial humic-like substances predominated in the VR direction, as revealed by 2D-COS analysis. FT-ICR MS data showed a similar DOM molecular evolution trend in groundwater. Specifically, biodegradable molecular formulae decreased with a diminishing contribution of river water to groundwater recharge. This decrease was accompanied by a decrease in O₃S and O₅S components, which highlight the influence of anthropogenic river water on groundwater DOM characteristics. Groundwater DOM variations were attributed to the influx of bioavailable and low-oxidized components and the release of terrestrial humic-like substances during river water recharge processes. This study contributes valuable insights into the transformations of DOM in groundwater systems influenced by surface water recharge, enhancing our understanding of the interplay between surface water and groundwater quality.