From inner-bay preservation to offshore dissipation: Spatiotemporal patterns and tracing of pharmaceuticals in Shenzhen Bay of the greater bay area, South China.

Pharmaceutical active compounds (PhACs) are increasingly recognized as contaminants of emerging concern, but their spatiotemporal accumulation patterns in coastal environments remain insufficiently understood. Here, we analyzed 24 PhACs in surface sediments and down cores collected from Shenzhen Bay to trace their residual fate along land-to-sea transport pathways. PhACs exhibited decreasing concentrations from the estuary to offshore and were continuously buried into deeper sediment layers, reaching up to 199.31 ng g-1 dry weight. Along the transport pathway, degradation processes affecting multiple PhACs were observed in addition to dilution. We fractionated the surface sediments by grain size for PhAC analysis and found that PhACs were preferentially enriched in the coarse fraction (>20 μm). Based on the observed grain-size patterns and supporting evidence from previous studies, we propose a conceptual framework for the residual accumulation of PhACs in coastal bays. Furthermore, we conducted 210Pb dating of the sediment cores and found that vertical PhAC trends can reflect documented local socio-economic transitions (e.g., population growth and aquaculture). Finally, we established the sedimentary fluxes of the 24 target PhACs and estimated that the cumulative sedimentary inventory of these compounds in the inner bay reached approximately 13.51 ± 4.05 tons over the past 60 years.