Molecular characterization and environmental response of dissolved organic matter in reserve quiescent groundwater wells of the North China plain: Insights from spectroscopy and mass spectrometry

Abstract

Dissolved organic matter (DOM) plays a critical role in aquatic ecosystems. However, the characteristics of DOM in groundwater source wells and interactions with environmental factors remain poorly understood. This study investigated the spectral properties, molecular composition, and environmental drivers across vertical groundwater gradients in Shijiazhuang using spectroscopy, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICRMS), multivariate statistics and molecular network analysis. Three components were identified: two humic-like substances (C1, C3) and one protein-like component (C2) Humic-like substances exhibited significant vertical stratification, with bottom groundwater DOM showing higher humification and autochthonous characteristics. Multivariate statistical analysis indicated that NO₃⁻-N and dissolved oxygen (DO) were keystone factors influencing the vertical differences of DOM. Surface-layer DOM was driven by dissolved total phosphorus (DTP), pH, DO and NO₃⁻-N, while the bottom layer was jointly regulated by pH, total phosphorus (TP), total nitrogen (TN) and NO₃⁻-N. DOM components correlated significantly with fluorescence index (FI), humification index (HIX), chemical oxygen demand (COD_Mn) and dissolved total nitrogen (DTN). FT-ICRMS analysis revealed that DOM molecular composition was dominated by CHO (38.71 %–52.07 %) and CHON (22.30 %–34.44 %) compounds, with lignin-like (LIG) (60.91 %–80.56 %) serving as the core molecular formulae. Redundancy analysis (RDA) identified that TN, DO, and NH₄⁺-N were key drivers regulating the DOM molecules distribution. Furthermore, molecular network analysis demonstrated that LIG molecular formulae played a crucial role in the network, significantly enhancing the chemical stability of the DOM molecular network. These findings elucidate DOM dynamics in groundwater systems at a molecular scale, providing critical insights for resource protection and risk management.