Modelling the Fate of Micropollutants in Rivers

How are dissolved and particle-bound micropollutants in a stream transported and transformed?

In industrialized countries, the assessment of water quality in rivers has shifted in recent years from nutrients and industrial contaminants to micropollutants, such as pharmaceuticals, daily-care products, various additives, etc., which are introduced via sewage treatment plants into the rivers at concentrations typically in the nanogram-per-liter range. Persistent organic pollutants, typically sorbed to suspended matter, also prevail at low concentration levels in rivers, either as legacy compounds or from atmospheric input. Because the input into streams is temporally quite variable and transport processes vary with discharge, thorough understanding of micropollutant dynamics require mechanistic numerical models of flow, solute transport, sediment transport, water-sediment partitioning, and degradation processes.

Within WESS, several tributaries to River Neckar have been monitored for micropollutants over the last years, among them the rivers Steinlach and Ammer in the direct vicinity of Tübingen, both of which receive water from seawage treatment plants. In addition, discharge, major ions, DOC, turbidity, and electrical conductivity were monitored. The monitored substances include pharmaceuticals (e.g., carbamazepine, diclofenac), pesticides (e.g., mecoprop, DEET), flame retardants (e.g., TCPP, TDCPP), and fragrants (e.g., OTNE, HHCB). Lagrangian sampling techniques were used in conjunction with natural and artificial tracer studies, to eliminate effects of physical transport on the change of observed concentrations.

The objective is to develop and implement mechanistic numerical models of flow, reactive solute transport and particle(-facilitated) transport for rivers Steinlach and Ammer. These models will be used to interpret the measured concentrations of dissolved and particle-associated micropollutants in the two rivers in a rigorous quantitative framework and explain differences between the rivers, eventually allowing the transfer to other smaller rivers.

Flow will be modelled by the St.-Venant equations with Manning’s parameterization of friction. The two rivers differ considerably in the complexity of the flow system: While River Steinlach is a single channelized, partially revitalized stream from the effluent point of the waste-water treatment plant in Derendingen onwards, River Ammer has a series of diversions. The hydrology of River Steinlach is considerable more dynamic than that of River Ammer.

Solute transport will be simulated by the advection-dispersion-reaction equation with transient storage in the streams and stream-systems, respectively. Tracer tests have revealed that dispersion (or in-stream transient storage) in River Steinlach can be very large at low flow, which has a major influence on observed concentration time series. The model must reproduce this behavior. Degradation processes will depend on mean water depth. For photodegradation, the differences in shading between the rivers need to be accounted for. Biodegradation of micropollutants will be accounted by pseudo first-order decay, and differences in the rate coefficients will be associated with differences in river characteristics. For suspended-matter transport, standard erosion and sedimentation parameterizations will be tested. Insights from the monitoring studies will be used to translate sediment loads to loads of persistent organic pollutants in the two rivers.

While for the dissolved micropollutants the emphasis lies on understanding the transport and transformation dynamics within the stream to an extent that allows transfer to other rivers, questions regarding long-term fate of the contaminants, their retention, and remobilization are in the focus of particle-bound micropollutant transport model.

• Liu, Y., Zarfl, C., Basu, N., Cirpka, O.A. (2019): Turnover and legacy of sediment-associated PAH in a baseflow-dominated river. Science of The Total Environment, 671, 754-764, doi: 10.1016/j.scitotenv.2019.03.236
• Liu, Y., Zarfl, C., Basu, N., Schwientek, M., Cirpka, O.A. (2018): Contributions of catchment and in-stream processes to suspended sediment transport in a dominantly groundwater-fed catchment. Hydrol. Earth Sys. Sci. 22: 3903-3921, doi: 10.5194/hess-22-3903-2018