How to set up a "background concentration" for 3D transport modelling ?

I am trying to model the leakage of a tailings storage deposit facility in a valley using chloride as transport specie. The problem I have is that I can not find an (easy) way to force the minimum cloride concentration to 200mg/l at all stress periods and everywhere as it is considered as the background concentration in this area.
Would you have any suggestion how to address this issue ?

Loic

I'd typically not force this concentration, but set it as an initial concentration everywhere and fix the concentration also at inflowing boundaries.

I have tried your suggestion but the problem is that recharge (fresh water) is occuring on the whole model and so after a while the initial concentration goes below the background concentration by dilution.
If I put some fixed background concentration BC everywhere (but the contaminant sources), I won t be able to observe my plume distribution ????

For recharge with a specified concentration you should use the 2nd kind boundary condition for mass transport all over the top of the model. However, this REQUIRES THE USE OF  THE DIVERGENCE FORM OF THE TRANSPORT EQUATION to obtain the correct boundary fluxes. Please have a look at the White Paper "About the difference between the convective form and the divergence form of the transport equation".