Co-Occurrence and Vertical Distribution of Antibiotics and Microplastics across Multiple Environmental Media in Typical Subtropical Estuaries.

The land-sea interface is a key zone for contaminant transfer among surface water, sediments, the vadose zone, and groundwater, yet the co-occurrence of antibiotics and microplastics across these media remains poorly resolved. We investigated surface water, groundwater, sediment, and borehole-core samples from the Jiulong River Estuary and Dongxixi River Estuary, China. Antibiotic contamination was relatively more pronounced in the Jiulong River Estuary, whereas microplastic abundance, particularly in sediments, was higher in the Dongxixi River Estuary. In borehole XTZ02, both contaminants showed shallow subsurface enrichment within their overlapping depth range; microplastics > 300 μm occurred only in shallow intervals, consistent with size-selective subsurface redistribution. Total antibiotic concentrations were not significantly correlated with total microplastic abundance, and RDA identified no statistically significant overall environmental control. These findings highlight matrix-dependent contaminant behavior across the estuary-vadose zone-groundwater-sediment continuum.