River Water Quality Model No. 1 (RWQM1): Case study I. Compartmentalisation approach applied to oxygen balances in the River Lahn (Germany)

A case study on the application of the River Water Quality Model No. 1 (RWQ M 1) is presented in order to illustrate the importance of modelling a sedi ment compartment for an ecologically meaningful assessment of the impact of wastewater effluents and combined sewer overflows. The focus of this case study is on the compartmentalisation approach of the RWQM 1 that makes such a description possible. In contrast to this, a strongly simplified biochem ical submodel is used that considers only oxygen and dissolved substrate. T he object of the case study is the River Lahn, a moderately polluted 5th or der stream in Germany. for which the connectivity of surface/subsurface flo ws and mass fluxes within river sediments have been intensively investigate d. The hyporheic flow between a downwelling and upwelling zone of a riffle- pool sequence has been studied with the aid of tracer experiments and conti nuous records of water constituents. High diurnal fluctuations of oxygen tr avelled to considerable depth of the sediment and oxygen in the interstitia l water decreased considerably while travelling through the riffle, Startin g with the implementation of a strongly simplified version of the biochemic al part of the RWQM 1, but with the consideration of a sediment pore water compartment in addition to the water column compartment, a calibration proc edure is performed using tracer data from the water column and the sediment . The calibrated model is then used to study the system response to wastewa ter treatment plant effluent and combined sewer overflow emissions. The mod elling approach makes it possible to quantify the sediment oxygen demand an d the spatial and temporal extent of sediment zones with oxygen depletion. However, the spatially averaged approach does not account for inhomogeneiti es in the sediment. It is shown that for this river with its alluvial coars e sediments even moderate emissions from sewerage systems may be high enoug h to drop sediment oxygen concentrations to low levels while those in the s urface flow remain close to saturation. Similarly, it is demonstrated that combined sewer overflows may cause anoxic sediment oxygen conditions for ex tended time periods. The implications for ecologically sound river water qu ality modelling and for specific quality objectives are discussed.