concentration in solute trasnport model

Hi all,

I have a 2D model, coupled Fluid flow with solute transport in order to simulate the nitrate concentration inside my domain.
I have soil column, which the left angle, is river and in the right is ditch. So I imposed two concentration boundary condition. Assuming that there is a flux from left and right inside the domain. Is it correct to impose this type of boundary condition for solute transport?

And when my model run, I calculated the "Nitrate concentration" inside the domain and it seems that sometimes the concentration is negative! Why did I get negative concentration for my species?

Please let me know if my questions are not clear.

Attached my model
Thanks