Tidal pumping controls transport of foodborne microbial pathogens between coastal groundwater and seawater

Understanding how tidal dynamics influence the foodborne transport of microbial pathogens in coastal aquifers is essential for protecting groundwater resources and public health. This study investigates the seasonal distribution and migration mechanisms of five key foodborne pathogens in a tidally influenced coastal groundwater system in Southeast Asia. By integrating field observations, microbial analyses, hydrochemical indicators, and environmental isotopes, we examine how tidal pumping, rainfall, and land-based pollution sources jointly shape microbial transport. Results reveal that in the dry season, limited tidal exchange allows land-derived pathogens to dominate, with E. coli and Salmonella showing strong correlations and association with ²²²Rn, indicating subsurface transport rather than surface runoff. In contrast, the rainy season intensifies tidal pumping and surface infiltration, enabling seawater intrusion and facilitating deeper and broader dispersal of pathogens such as Salmonella and Staphylococcus aureus. Environmental factors such as salinity, pH, dissolved inorganic carbon, phosphate, and δ¹⁸O values significantly influence pathogen survival and distribution. Differences in pathogen tolerance to salinity and redox conditions lead to varied spatial patterns: Salmonella thrives in brackish transition zones, while Staphylococcus aureus prefers fresher water; Vibrio parahaemolyticus and Listeria monocytogenes are more prevalent in deeper, mixed groundwater, linked to longer-lived isotopes ²²⁸Ra and ²²⁸Th. These findings highlight the key role of tidal processes and seasonal hydrology in regulating pathogen pathways in coastal aquifers. This study offers new insights into the mechanisms driving microbial contamination in coastal groundwater, supporting the development of integrated public health and water management policies in vulnerable coastal zones.