Submarine Groundwater Discharge Acts as a Significant Vector for Coastal Organophosphate Esters: Joint Evidence from Pollution Surveys and Radionuclide Analysis

Abstract Organophosphate esters (OPEs) are ubiquitous marine contaminants that raise global concern. While surface inputs are well-studied, the role of subsurface transport remains poorly constrained. Submarine groundwater discharge (SGD) drives seawater-groundwater interactions, yet its impact on OPEs behavior remains overlooked. This study comprehensively investigates the distribution, transport property and sources of OPEs from coastal groundwater to seawater in the Beibu Gulf, China. No statistically significant difference in Σ19OPEs levels was found between groundwater and tidal flat porewater, while both matrices exhibited significantly higher levels than seawater. The high compositional similarity, strong correlations (p < 0.01) and PCA overlap of OPEs in different waters, as well as their shared source profiles indicated the transport potential of OPEs from groundwater to seawater. Multiple statistical modeling integrating OPEs, environmental factors and DOM components reveals the land-to-sea transport pathways and biogeochemical processes of OPEs via SGD. Furthermore, radon isotope analysis verified the presence of SGD, offering evidence for SGD-mediated OPE transport. For the first time, this work integrates pollutant investigation, multiple statistical analysis, and radionuclide tracing, to identify SGD as a previously underestimated vector of coastal OPEs and elucidates its potential transport mechanisms, providing crucial support for integrated land-sea management and risk assessment.