Impact of ripple celerity on the fate of trace organic contaminants in streams

Trace organic contaminants (TrOCs) are ubiquitous in wastewater-impaired river networks, posing risks to ecosystems and drinking water resources. Hyporheic exchange, i.e, the flow of stream water into saturated sediments and back into the stream, can foster TrOC attenuation. However, the role of moving sediments and particularly the speed of the streambed motion (i.e., the celerity) in regulating the attenuation of TrOCs remains poorly understood. To quantify how increasing stream water velocity, ripple celerity, and hyporheic exchange flux (HEF) affect TrOC attenuation, flume experiments were conducted with natural stream sediments. Seventeen wastewater-associated TrOCs were spiked into the flume at four stream water velocities ranging from 10 to 40 cm s⁻¹, resulting in distinct ripple celerities and HEF. TrOC concentrations were quantified over seven days in the stream water as well as in the sediment pore water over depth to evaluate TrOC attenuation. In addition, ripple morphodynamics were quantified throughout the experiments. Several compounds, including ibuprofen, venlafaxine, losartan, valsartan, naproxen, and metoprolol, were attenuated, whereas carbamazepine concentrations remained stable. Attenuation rates generally increased with increasing stream water velocities from 10 to 30 cm s⁻¹. Further increasing stream water velocity to 40 cm s⁻¹ did not increase attenuation rates and resulted in slightly lower attenuation for some TrOCs. TrOC attenuation patterns differed from that of HEF, which increased nonlinearly with stream water velocity. HEF thus cannot be the sole explanation for TrOC attenuation. At stream water velocities above 30 cm s-1, there was likely a transition from transport-enhanced to residence-time-limited TrOC attenuation. These findings demonstrate that mobile stream beds can enhance TrOC attenuation, but increasing celerity does not necessarily yield a proportional increase in TrOC attenuation. Instead, TrOC attenuation is controlled by both transport into the streambed and residence time within it. Preprint status: This manuscript has been submitted to Water Research for peer review. This version has not yet been peer-reviewed or formally published in a scientific journal.