Spatio-Temporal Changes of Groundwater Geochemical Background Characteristics (GGBC) in Beijing, China: Insights from All Seismic Monitoring Wells Through Multiple Analytical Methods

Establishing the groundwater geochemical background (GGBC) is fundamental for distinguishing seismic precursors from non-tectonic environmental influences. This study characterizes the spatiotemporal evolution of the GGBC in Beijing using 60 groundwater samples collected from 17 seismic monitoring wells between 2018 and 2025. Major ion concentrations, stable water isotopes (δ2H and δ18O), Self-Organizing Map (SOM) clustering, Principal Component Analysis (PCA), ionic ratios, and mineral saturation indices were integrated to analyze the hydrochemical framework for establishing the GGBC baseline. Piper diagram results reveal three main hydrochemical types: Na·HCO3 type, Ca·Mg·HCO3 type, and sulfate type (subdivided into Na·SO4 and Ca·Mg·SO4/HCO3 subtypes). Hydrogen and oxygen isotope data from 2024 indicate that recharge sources range from deep bedrock fissure water to shallow mixtures of meteoric and surface water. Gibbs diagrams, SOM, and PCA consistently demonstrate that water–rock interaction (including carbonate, silicate, and evaporite dissolution, as well as cation exchange) dominates the GGBC. Ionic ratios and saturation indices further confirm that silicate weathering and forward cation exchange are persistently widespread, while carbonate and evaporite dissolution are locally important. Notably, 12 wells of the 17 monitoring wells (71%) retained the same hydrochemical type across all campaigns in which they were sampled. This stability shows no detectable temporal correspondence with either the V-shaped pattern of seismic activity (strong–weak–strong, 2018–2025) or the regional groundwater level rise caused by water diversion projects at the available annual-to-multi-year sampling resolution. Instead, the high stability of the GGBC reflects long-term water–rock equilibrium that is resilient to minor, transient perturbations. This study demonstrates that major ions are reliable indicators of the GGBC for assessing the sensitivity of the Beijing Plain to short-term seismic or hydrological disturbances. However, the limited temporal resolution of the sampling and incomplete datasets preclude definitive conclusions regarding earthquake-related geochemical signals. To strengthen regional earthquake monitoring and forecasting capabilities, we recommend establishing a continuous, long-term, high-frequency, multi-parameter monitoring network—including stable isotopes, trace elements, and dissolved and exsolved/free gases—with high data completeness in the Beijing area.