Fate of Trace Organic Contaminants in the Hyporheic Zone: A Laboratory-Scale Column Experiment

Rivers impacted by anthropogenic activities often contain numerous trace organic contaminants (TrOCs) that can infiltrate aquifers. The hyporheic zone, which is the interface between rivers and groundwater, can act as a natural attenuation barrier via sorption and biotransformation. To differentiate between these two processes, we conducted long-term laboratory column experiments using riverbed sediments from a creek downstream of wastewater treatment plants to quantify the fate of several ubiquitous TrOCs with diverse physicochemical properties. Triplicate columns were fed with either natural contaminated river water or with TrOC-free tap water to distinguish between attenuation mechanisms and to investigate potential remobilization. Mass balances and comparisons between experiments revealed a variety of attenuation behaviors. Certain hydrophobic (e.g., carbamazepine and tolyltriazole) or cationic compounds (e.g., metoprolol and sitagliptin) were retained by sediment sorption, whereas several polar anionic TrOCs remained largely mobile (e.g., hydrochlorothiazide and gabapentin), with some of them being biotransformed under prevalent reducing conditions (e.g., iopromide and oxipurinol). Overall, 10 out of 19 continuously detected TrOCs exhibited less than 25% removal over almost 300 pore volumes, reflecting limited biotransformation and the relevance of reversible sorption. These results suggest that while polluted hyporheic sediments of high organic matter content can temporarily reduce contaminant transport, their attenuation capacity is limited. Furthermore, previously sorbed pollutants may remobilize, posing long-term risks to the groundwater quality.