Attenuation of trace organic compounds along hyporheic flow paths in a lowland sandbed stream

Abstract

Knowledge of flow paths in hyporheic zones of streams is essential for understanding different turnover processes of various chemicals, such as trace organic compounds (TrOCs). Due to the spatial heterogeneity of hyporheic zones, exact flow paths in situ are unknown and thus, pore water sampling along individual flow paths remains a major challenge. To this end, we developed and tested a novel method to enforce specific flow paths under field conditions. Therefore, U-shaped pipes of 40 cm total flow path length were installed in flow direction along a river cross section in the hyporheic zone of the urban lowland River Erpe (Germany). This allowed us to determine sediment properties and to collect pore water samples along the specific flow paths during 16 consecutive hours. The pore water was analyzed for electrical conductivity, redox parameters, and TrOCs. Diurnal electrical conductivity fluctuations in the surface water and their propagation to the end of the flow paths resulted in median residence times of 11.67 or 16.92 h for different specific flow paths. A clear redox zonation along the flow paths was found. Of the 18 TrOCs studied, five were strongly attenuated (i.e., relative attenuation to surface water > 75 %), three exhibited moderate attenuation (75 to 25 %), nine were only slightly or not attenuated (< 25 %), and one compound increased in concentration (relative formation > 25 %) along the entire flow paths. For TrOCs that showed strong or moderate attenuation, the attenuation occurred mainly within the first 5 to 15 cm. This work highlights (1) the importance of knowing the specific flow paths for a comprehensive understanding of TrOC turnover in the hyporheic zone and (2) the relevance of appropriate biogeochemical and hydrological conditions for the attenuation of TrOCs.