Hello, good morning. I am working on a model of a coastal wetland with numerous tributaries. The outflow from the lake to the sea is controlled by gates. The inflows I have introduced in the model as discharges while the outflows as open boundary. Both inlets and outlets are known. When I run the model I get warnings about the Courant number, which is slightly greater than 1.

However, now I want to study the distribution of a pollutant in the lake. When I enter the loads I get many warnings; the most worrying one is "No convergence in DIFU for resp. constituent nr. & tstep: 1 8088" and the concentrations become negative from then on. I don't quite understand where the problem lies, as the model reflects the hydrology of the lake very well.
I am attaching all the inputs to my model in a zip file if anyone would like to take a look at it. Any suggestions are appreciated.