Coupling of sulfate isotopes with 3D-EEM: Unveiling a new approach to tracing the origin of sulfates in groundwater around landfills.

Elevated sulfate concentrations in groundwater pose risks to drinking water safety and subsurface stability; however, accurate source apportionment remains challenging in systems influenced by mixed natural and anthropogenic inputs. This study examines a landfill-impacted aquifer in the red-bed region of southwestern Sichuan, China, with particular emphasis on the coupling between dissolved organic matter (DOM) and sulfate biogeochemical processes. A multi-tracer approach integrating hydrochemical analysis, sulphur (δ³⁴S) and oxygen (δ¹⁸O) isotopes, and three-dimensional excitation-emission matrix spectroscopy (3D-EEM) was employed. Groundwater evolution is primarily characterized by Ca²⁺ and SO₄²⁻, with hydrochemical facies transitioning from HCO₃-Ca to SO₄-Ca under landfill influence. sulfate sources display pronounced seasonal variability, with atmospheric deposition dominating in the wet season (46%) and landfill leachate in the dry season (36%). Distinct differences in DOM composition are observed among sources: landfill-affected groundwater is enriched in humic- and fulvic-like substances, where the humification index (HIX) is positively correlated with SO₄²⁻ concentrations and δ³ ⁴S values, whereas the biological index (BIX) shows a negative correlation. In contrast, groundwater influenced by gypsum dissolution is characterized by protein-like DOM, accounting for 76-88% of total DOM. By incorporating DOM fluorescence indices (HIX and BIX) as continuous covariates into a dual-isotope Bayesian mixing model (MixSIAR), dynamic source apportionment is achieved. This integration enhances source discrimination by explicitly accounting for DOM-mediated biogeochemical processes, providing a robust framework for tracing sulfate contamination in landfill-impacted groundwater systems.