Fate, Transformation, and Ecological Risk of Antibiotic Contaminants in the Hyporheic Zone of the Ogun River, South West Nigeria

The hyporheic zone, the dynamic transition region between surface water and groundwater beneath riverbed sediments, serves as a biogeochemical reactor where antibiotic contaminants undergo biotransformation, sorption, and photolysis reactions that fundamentally determine their ecological persistence and groundwater contamination potential, yet no study has investigated hyporheic zone antibiotic fate processes in any Nigerian river system. This study investigates the fate, transformation products, and ecological risk of seven priority antibiotic compounds, specifically amoxicillin, ciprofloxacin, sulfamethoxazole, trimethoprim, erythromycin, tetracycline, and metronidazole, in the hyporheic zone of the Ogun River, Ogun State, South West Nigeria, downstream of two wastewater discharge points. Porewater samples are collected using in-situ dialysis samplers (peepers) installed at 5, 15, 30, and 60 centimetre depths within the hyporheic zone at six transects. Antibiotic concentrations are determined by solid-phase extraction and liquid chromatography-tandem mass spectrometry. Transformation products are identified by high-resolution quadrupole time-of-flight mass spectrometry using data-independent acquisition and suspect screening against a curated antibiotic transformation product spectral library. Hyporheic zone hydraulic connectivity is characterised by streambed temperature gradient analysis and conservative tracer injections. Sediment microbial communities are profiled by shotgun metagenomics to identify antibiotic resistance gene reservoirs and biotransforming bacterial guilds. A novel hyporheic zone fate and transport model is developed coupling advection-dispersion equations with Michaelis-Menten biodegradation kinetics and Freundlich sediment sorption. Probabilistic ecological risk assessment applies species sensitivity distributions for antibiotic-sensitive aquatic organisms. Keywords: antibiotics, hyporheic zone, Ogun River, antibiotic resistance genes, ecological risk