Simultaneous fate of trace organic compounds and dissolved organic matter in surface water and the hyporheic zone of an urban river

Rivers being influenced by treated wastewater are characterized by an altered water chemistry compared to their natural state. Downstream of the outlet of a wastewater treatment plant (WWTP), concentrations of dissolved organic matter (DOM) and trace organic compounds (TrOCs) in the receiving river are increased. As DOM might serve as a metabolic co-substrate during microbial TrOC degradation, DOM influences the attenuation of TrOCs. Due to steep biochemical gradients at the surface water - groundwater interface, the hyporheic zone is considered a hotspot for microbial activity. Therefore, turnover rates in the hyporheic zone of a stream are high in comparison to the turnover rates in the water column. The River Erpe is a sandy lowland river in the East of Berlin, Germany, which receives treated wastewater from the WWTP Muenchehofe. In order to study the simultaneous fate of TrOCs and DOM in surface water and the hyporheic zone, a field sampling campaign was conducted at a side channel of the River Erpe. Surface water samples were taken at site A and both surface and pore water samples from 25 cm sediment depth were taken at site B which is 850 m downstream of site A. The sampling interval was every three hours over 48 hours. Samples were analysed for 17 TrOCs (HPLC-MS/MS) and the molecular composition of DOM (FT-ICR MS). DOM compound classes were calculated semi-quantitatively as the percentage share of each class of the whole DOM composition. Mean concentrations of the TrOCs analysed changed by an increase of 200 % to a decrease of 29 % in the surface water between site A and B and by a decrease of 5 to 93 % in the hyporheic zone at site B. The molecular composition of DOM changed at most by a single digit percentage per compound group with the attenuation being larger in the hyporheic zone. The percentage share of two out of seven DOM compound classes significantly correlated with the concentration of at least ten TrOCs between surface water at site A and B. Such a correlation was observed for five compound classes in the hyporheic zone at site B. The present study shows that the attenuation of both TrOCs and DOM in a sandy urban river mainly takes place in the hyporheic zone but it is not capable of assuming a causal relationship between the attenuation of TrOCs and DOM.