Field-based assessment of in-stream contaminant fate

Abstract

Water quality of rivers worldwide is affected by the increasing use of organic mi-cropollutants for human purposes. Most pollutants enter rivers via urban areas and wastewater treatment plants either freely dissolved, or attached to particles and may undergo transformation processes during their transport in rivers. There is still a lack of a holistic assessment of relevant processes, as well as comprehensive and representative field-based studies that describe and quantify the attenuation of these compounds in rivers. This thesis tackles this knowledge gap and aims to bet-ter understand processes that determine the in-stream contaminant fate. Processes potentially relevant for the fate of organic micropollutants under field-conditions are summarized in this thesis in an integrated way including on the one hand hydrological aspects such as transport processes, groundwater and tributary inflow, and hyporheic exchange, as well as reactive processes comprising sorption, biodegradation, volatilisation and photodegradation. State of the art measurement approaches to capture these processes are compiled. The abundance of relevant processes under field-conditions and the fact that they take place simultaneously clearly shows the need for an appropriate sampling strategy to disentangle attenua-tion processes of pollutants in rivers. For this purpose, a field study was conducted aiming at the identification of relevant water fluxes that contribute to the river discharge since they may lead either to an in-stream dilution or concentration increase of pollutants. Different potentially useful groundwater tracers were methodologically compared for this purpose. The results illustrate that the proper choice of tracer is crucial to quantify and localize ground-water inflows. In addition, these results clearly evidence the need for a holistic un-derstanding of water fluxes in catchments, with respect to the water quality of in-flowing water especially in terms of the in-stream chemical status and thus pollutant turnover processes in rivers. On this basis, the goal of a following field study was the quantification of repre-sentative transformation rate constants of selected organic micropollutants as they are transported downstream from the wastewater treatment plant (selected indicator substances). The same water parcels were compared at two sequential reaches to investigate relevant environmental factors that may differ between both reaches and thus influence the fate. The results demonstrate the important role of photolysis for the attenuation of dissolved organic pollutants despite the generally comparably low reactivity in the selected river system. The latter is, inter alia, attributed to the an-thropogenic character of the river that leads to a fast travel time. A holistic assessment of organic micropollutant transport in rivers compiles equally the dissolved and particle-bound pollutants which is the reason why selected hydro-phobic compounds associated to suspended particles were traced through the catchment during high discharge conditions. The hydrophobic character of these compounds allows the identification of particle origin in the river. Moreover, the in-teraction between particle transport and the sediment storage is identified as im-portant mechanism for particle associated pollutant transport in rivers. In summary, the holistic approach of this thesis gives insights into relevant process-es that determine the fate of organic micropollutants in fluvial systems. This work provides a sound basis for future field-based fate studies that aim to quantify atten-uation processes in rivers.