water quality- NH4

Hi, there are 13 water bodies including 14 branches in my model. The downstream is estuary. So branches will be influenced by tidal. There are one upstream head boundary and four downstream head boundaries. The simulation of hydrodynamic is correct。 Now I’m simulating the water quality ammonium. I set the initial condition of ammonium 0.675mg/L to all of segments and layers. The upstream head constituent concentration and the downstream head concentration are also 0.675mg/L. The sediment release rate and ammonium decay rate are set to 0. So in my opinion during simulation the ammonium’s concentration of all branches should not change be equal to 0.675mg/L. But the ammonium’s concentration of the upstream segment changes from 0.655mg/L to 4mg/L. This change mainly occurs during high tide. The ammonium’s concentration of the downstream segment is equal to 0mg/L and not change. So this is not what I thought. So what is the problem? How to reset the model? Best regards Lei