Subterranean estuaries (STE) are a continuum from the land to the sea where groundwater interacts with seawater. This biogeochemically dynamic environment shape the fate of nutrients and chemical contaminants before their delivery into coastal waters. These environments, often overlooked, particularly under urbanized beaches, are increasingly exposed to anthropogenic pollutants. In particular, pharmaceuticals and personal care products (PPCPs) raise concerns due to their persistence and potential ecotoxicological impacts. On the other hand, environmental gradients and coalescence of evolutionary divergent microbiomes puts forward STE as a hot spot of microbial diversity, with a higher probability of encountering microbes able to modulate chemical contaminations. This study aims to characterize microbial community dynamics and their potential for PPCP degradation in a subterranean estuary of a highly urbanized Mediterranean beach in Marseille, France. By coupling in situ field sampling with controlled laboratory experiments, we assess the influence of salinity gradients, redox conditions, nutrients and PPCPs on microbial diversity. Using flow cytometry, quantitative PCR and metabarcoding, distribution and taxonomic diversity of heterotrophic prokaryotes and fungi are evaluated to locate the microbiological reaction field and its environmental drivers. In parallel, microcosm experiments are performed to evaluate biodegradation potential of STE communities for different PPCPs (two antibiotics (azithromycin, erythromycin), an antidepressant (fluoxetine) and a beta-blocker (sotalol), or one UV filter (octocrylene)). Preliminary results highlighted a wide salinity gradient under this beach, ranging from 0.7 to more than 38 g.L-1 at the end of the dry season as well as during winter. This salinity gradient seemed to structure microbial assemblages with microbial hotspots at intermediate salinity (between 8 and 10 g.L-1). First results on laboratory experiments show significant stimulation of underground community growth in the presence of octocrylene at a concentration representative of values observed in summer bathing areas, and in just one day.