Understanding the co-occurrence patterns and geochemical evolution pathways of complex geogenic groundwater contamination: A case study of the Ganjiang River Delta

• Redox condition and OM availability govern the formation and evolution of CGCG. • Mn and I released via abiotic/biotic path in sub-oxidation/reduction conditions. • GC evolution splits into two pathways based on OM availability in reducing areas. • Crystalline Fe-oxides dissolution cause the release of GCs in deep reducing area. • Enhancing AuDen may represent a viable in-situ remediation strategy for CGCG. The study investigates the complex mixture of geogenic contaminants (GCs) in the groundwater of the Ganjiang River Delta via hydrochemical, isotopic, and multivariate statistical methods, focusing on their co-occurrence patterns and evolutionary mechanisms. Fe, Mn, As, NH 4 + -N, S(II), I, and P were identified as the main GCs, with strong correlations observed between Mn and I, whereas Fe, P, As, and NH 4 + -N were closely associated. Under weakly reducing conditions, the reductive dissolution of I-bearing Mn(IV)-oxides caused the co-occurrence of I and Mn. As the reducing environment intensifies, the evolution of GCs diverged into two geochemical pathways based on organic matter (OM) availability. In organic-rich areas, the reductive dissolution of As-bearing Fe(III)-oxides coupled with As(V) reduction led to moderate concentrations of Fe and As. Subsequently, the consumption of labile OM triggered Mn(II)-dependent autotrophic denitrification (AuDen), which lowered Mn and I concentrations in a strongly reducing environment. In contrast, in organic-poor areas, the reductive dissolution of Fe(III)-oxides induced by iron ammonium oxidation, along with secondary As(V) adsorption, caused the decoupling of As and Fe. This was followed by Fe(II)-dependent AuDen, which decreased Fe and related GCs concentrations in a strongly reducing environment. Under deeply reducing conditions, both pathways were dominated by the crystalline Fe(III)-oxides-mediated degradation of refractory OM, leading to the co-release of GCs. This study highlights the controlling roles of redox conditions and OM in the formation and evolution of complex geogenic contaminated groundwater (CGCG). Our results also suggest that enhancing AuDen may be a promising in-situ remediation strategy for CGCG.