Abstract:
In industrialized countries, the assessment of water quality in rivers remains a timely topic even though problems of eutrophication have been overcome by wastewater treatment. In particular, hydrophobic organic pollutants that typically sorb to suspended sediments (e.g., PAH), and pharmaceuticals that are not included in regular treatment of WWTPs have gained environmental concerns. These pollutants occur at concentrations of nano- to micro-grams per liter in rivers. Continuous monitoring of these compounds is time- and cost-consuming. A modelling approach is advantageous and necessary to understand the interacting environmental processes that determine the fate of micropollutants in river systems.
Sediment transport facilitates the transport of PAH, one group of micropollutants of interest in this thesis. Combining hydrological, hydraulic, and in-stream transport models can give good insights on sediment sources and transport on the catchment-scale, which is essential for investigating the fate of PAH. Therefore, I developed such an integrated sediment transport model to simulate sediment contributions from catchment and in-stream processes under different flow conditions. The characteristics of surface runoff essentially control the sediment supply from urban and rural areas. In the mainly groundwater-fed Ammer catchment, the weak rural surface runoff leads to a small rural sediment supply. By contrast, urban particles dominate the annual sediment load. The flow rate and river geometry determine the deposition and remobilization of sediments in the river. The modelled sediment trapping occurs in very mild reaches of River Ammer. I extended the integrated sediment transport model to a particle-facilitated pollutant transport model, which considers PAH interaction between water and sediment. This model allows to study the source, turnover, and legacy potential of PAH in river systems. The supply and composition of sediments determine to a large extent the PAH supply to a river. In the Ammer River, the high proportion of urban particles with high PAH content results in the dominant supply of PAH from urban areas. In steep reaches, sediment turnover governs the turnover of PAH, whereas in very mild river segments diffusion of PAH from the river bed to the mobile water is relevant and reduces PAH turnover times. PAH legacy occurs in river segments with slow sediment turnover. For River Ammer, the simulated sediment trapping reaches have acted as secondary PAH source over 10-20 years after the introduction of environmental regulations in the 1970s.
Pharmaceuticals are emitted to rivers by WWTPs due to incomplete removal. I developed a one-dimensional reactive solute transport model considering transient storage to investigate the transport and fate of these compounds in the WWTP effluent-impacted River Steinlach. The degradation processes are substantially affected by the local conditions. Carbamazepine is relatively conservative, sulfamethoxazole is only biodegradable, while metoprolol and venlafaxine undergo both photo- and bio-degradation. The flow rate influences the relative transient storage and thus pollutant removal decreases with increasing flow rates, particularly under low-flow conditions. The combination of tracer experiments and the Lagrangian sampling approach of pollutants can improve model calibration and diagnose different attenuation mechanisms.
This thesis aims at understanding major controls of transport of sediments as well as dissolved and sediment-bound micropollutants in two exemplary rivers to investigate the long-term fate of sediment-bound micropollutants (PAH) and the transport and transformation dynamics of dissolved micropollutants (pharmaceuticals). While I developed the models to meet measured data in Rivers Ammer and Steinlach, the framework is transferable to other small streams that are affected by anthropogenic micropollutants.
Pollutant