Chemical profiling of surface water and biota in protected marine harbours impacted by combined sewer overflows

Few studies exist that focus on contaminants of emerging concern (CECs) in transitional and coastal waterbodies. This study presents chemical profiling of two protected marine harbours on the South coast of the UK sampled in 2022. Across 21 sites, 105 unique compounds were detected (0.05 ng L^-1 ––1798 ng L^-1, median: 11 ng L^-1) in water samples and biota, including 67 pharmaceuticals, 29 pesticides and nine recreational drugs. There were significant differences between campaigns with increased chemical numbers and concentrations that coincided with increased rainfall and combined sewer overflow (CSO) discharges. The comparison with CSO discharges revealed that they were an important source for loading of specific chemicals with concentrations increasing for some cases by three-orders of magnitude. High relative risks were estimated for sites sampled during recorded CSO discharges for five compounds with risk quotients (RQs) ranging from 1.1 up to 9.3, with the highest risk from the neonicotinoid, imidacloprid. To understand the exposure in biota, six species; one macroalgae (Fucus vesiculosus) and five fauna (Hediste diversicolor, Patella vulgate, Crassostrea gigas, Carcinus maenas, Echinogammarus marinus) were analysed (n = 5/species) at a CSO-impacted site. Between eight to 18 compounds were detected with Fucus vesiculosus (seaweed) showing the highest accumulation with mean cumulative burdens reaching up to 343 ± 71 ng g⁻¹. Surface water contamination did not correlate with body burdens. Overall, the work highlights the complexity of the chemical space present in a transitional waterbody showing dynamic contamination patterns that are further influenced by tide, rainfall and salinity. CSOs demonstrated an important but compound specific role for CEC input and pulsing into receiving waters.