Fate of emerging organic contaminants in the hyporheic zone of an anthropogenically impacted stream

Rivers and streams worldwide are increasingly impacted by emerging organic contaminants (EOCs) as a result of wastewater treatment plant (WWTP) effluents discharges and human and industrial activities. Within this context, the hyporheic zone (HZ), which is the interface between surface water and groundwater, is often regarded as a critical compartment for EOCs attenuation. This is due to processes such as sorption onto soil organic and mineral phases, as well as biotransformation mediated by the diverse microbial communities present in such environments. However, the distinction between these two attenuation pathways is frequently hindered by the highly variable hydrochemical conditions encountered in field studies. To address this issue, we conducted a series of laboratory experiments designed to replicate the natural conditions of the HZ of a heavily polluted stream in the Hessian Ried, Germany.

The experimental setup consisted of a set of three different column experiments, each performed in triplicate. To achieve this, we collected nine undisturbed soil cores of 25 cm from the riverbed of the Landgraben, a stream impacted for decades by industrial and domestic WWTP effluents. The experiments differed in the feeding solution. For the first set of columns, we used real river water, collected every two weeks, stored refrigerated and replenished every three days to avoid changes in chemical composition. For the second group we spiked the inflow water with a cocktail of five pesticides not detected in the river water but commonly used in the area for pest control, to investigate their fate in the HZ. Finally, for the last set of triplicates, we used tap water free of EOCs as inflow water to characterize desorption processes. Samples were regularly collected from the inflows and outflows of all columns to generate breakthrough curves of EOCs over a total duration of 300 pore volumes, with flow rates adjusted to replicate residence times observed in the field. A total of 28 EOCs were analyzed using LC-MS/MS, covering a broad spectrum of physicochemical properties, including ionic speciation and polarity, which are key factors controlling the fate of EOCs in soils.

Our results showed that many of the analyzed compounds are highly mobile in the HZ and not attenuated. This is attributed in some cases to high polarity (e.g., candesartan, gabapentin, hydrochlorothiazide, valsartan acid) and in others to the saturation of sorption sites (e.g., metoprolol, sitagliptin). Only a few compounds exhibited evidence of transformation (e.g., diatrizoic acid, iopromide, sulfamethoxazole). Compounds with medium polarity and with negative or neutral speciation were slightly attenuated, primarily through sorption (e.g., carbamazepine, diclofenac, irbesartan, 1,2,3-benzotriazole). Overall, our findings suggest that the HZ of a long-term polluted stream is capable of mitigating only a small fraction of EOCs, posing a significant risk to surface and groundwater bodies.