Geogenic Phosphorus Enrichment Patterns in Alluvial-Lacustrine versus Alluvial Aquifers: Roles of Organic Matter Supply and Degradation Pathways

Geogenic phosphorus (P) is widely documented in aquifers worldwide, and its content varies substantially across geologic environments. However, comparative investigations elucidating the mechanisms underlying disparities in geogenic P enrichment among basins with contrasting geologic settings are limited. In this study, two representative aquifer systems with contrasting P levels were investigated, including an alluvial-lacustrine aquifer in the middle Yangtze River (MYZ) and an alluvial aquifer in the middle-lower Yellow River (MYR). We observed that dissolved inorganic phosphorus (DIP) dominated the P pool in both regions and was primarily derived from organic matter (OM) mineralization, whereas markedly higher P concentrations occurred in MYZ. In MYZ, abundant OM underwent advanced methanogenic mineralization governed by thermodynamic constraints, with DIP production driven by the transformation of recalcitrant CHO+2P into labile CHO+1P. In contrast, OM mineralization in MYR remained at an initial fermentation stage under kinetic constraints, and DIP formation was mainly associated with the dephosphorylation of labile CHO+1P. Overall, OM supply, mineralization stage, and degradation pathway jointly regulated the magnitude of geogenic P enrichment across aquifer systems under different geologic settings. These findings can improve the mechanistic understanding of geogenic P production and provide a theoretical basis for managing geogenic P in groundwater.