Modelling Escherichia coli concentrations in the tidal Scheldt river and estuary

Recent observations in the tidal Scheldt River and Estuary revealed a poor microbiological water quality and substantial variability of this quality which can hardly be assigned to a single factor. To assess the importance of tides, river discharge, point sources, upstream concentrations, mortality and settling a new model (SLIM-EC) was built. This model was first validated by comparison with the available field measurements of Escherichia coli (E. coli, a common fecal bacterial indicator) concentrations. The model simulations agreed well with the observations, and in particular were able to reproduce the observed long-term median concentrations and variability. Next, the model was used to perform sensitivity runs in which one process/forcing was removed at a time. These simulations revealed that the tide, upstream concentrations and the mortality process are the primary factors controlling the long-term median E. coli concentrations and the observed variability. The tide is crucial to explain the increased concentrations upstream of important inputs, as well as a generally increased variability. Remarkably, the wastewater treatment plants discharging in the study domain do not seem to have a significant impact. This is due to a dilution effect, and to the fact that the concentrations coming from upstream (where large cities are located) are high. Overall, the settling process as it is presently described in the model does not significantly affect the simulated E. coli concentrations.

► Development and application of new reactive tracer model (SLIM-EC) for the fate of E. coli concentrations in the tidal Scheldt River and Estuary. ► SLIM-EC reproduces correctly the long-term median concentrations and range. ► The correct longitudinal profiles can only be found by a model considering the tides, as these have the specific effect to push polluted water upstream. ► Major source of E. coli in the domain appears to be lying upstream the domain, so for water quality improvement the efforts should be concentrated on improving the inputs upstream of the model boundaries.