Review of seawater–groundwater interactions in the coastal earth critical zone of the Bohai Sea

Over the past several decades, extensive investigations into seawater–groundwater interactions have significantly advanced the understanding of the hydrological and biogeochemical processes in the coastal earth critical zone (CECZ). Although seawater–groundwater interactions in the Bohai Sea region (China) have been the focus of many studies, a systematic understanding of the interactions between groundwater flow and environmental effects remains limited. This paper provides a review of coastal groundwater flow and solute transport into the Bohai Sea, a highly human-affected marginal sea. It reveals that despite large uncertainties quantified via geochemical tracers, submarine groundwater discharge (SGD) and associated solutes are comparable to the riverine inputs, which have historically been overlooked. SGD serves as a significant pathway for nutrient transport, delivering nutrients (nitrogen, phosphorus, silicon, and carbon) from terrestrial sources to coastal waters. These inputs play vital roles in biogeochemical cycles and influence dynamics related to coastal eutrophication and ocean acidification. Moreover, SGD represents a primary source of heavy metals including dissolved Mn, Zn, and Fe, thereby exerting substantial impacts on metal distribution within this region. This review synthesizes research findings regarding nutrient fluxes while elucidating controlling factors such as hydrogeological characteristics, anthropogenic activities, and climate change. Uncertainties persist in assessing SGD and its direct impacts on the marine environment due to limited availability of long-term monitoring datasets. This highlights the necessity for further research in this field. Strengthening coastal groundwater monitoring and enhancing groundwater quality assessment in these areas are imperative tasks. • The fluxes of SGD and the associated solutes into the Bohai Sea are comparable to local riverine inputs. • SGD impacts biogeochemical processes and influences coastal eutrophication dynamics and ocean acidification. • The knowledge research gaps and challenges to improve the understanding of seawater–groundwater interactions are proposed.