Abstract: To investigate the enrichment characteristics and underlying mechanisms of phosphorus (P) in groundwater within the river lateral interaction zone, this study conducted hydrochemical and phosphate oxygen isotope (δ¹⁸O_p) analyses on three lateral interaction zones (Jiangling, Jianli, and Honghu) within the Four Lake Basin. The results revealed the widespread presence of significant P enrichment zones in the groundwater of the interaction zones, with the locations of enrichment differing between the unconfined and confined aquifers. Hydrochemical analysis indicated that P enrichment in the Jianli section is primarily driven by organic matter mineralization. In contrast, in the Jiangling section, it is more closely related to mineral dissolution/release. The Honghu section appears to be regulated by a combination of both processes. The δ¹⁸O_p data confirmed the crucial role of microbially mediated reductive dissolution of P from iron oxides in the enrichment of P in the confined aquifer. The spatial variation and dominant mechanisms of P enrichment in the interaction zones are regulated by hydrological processes. More intense hydrological dynamics enhance the role of organic matter in P enrichment, shifting the enrichment zone closer to the Yangtze River.