How to simulate the solute transport in soil column

Hi All!
I want to simulate a solute, like NaCl, transport in a soil column. The groundwater table depth is about 20m from land surface. The soil column is about 300cm. I want to set the bottom of the column as  zero concentration gradient BC. But I don't find that kind of BC. Please tell me how to set the mass concentration BC on the bottom.

Thanks!

If there is only outflow on the bottom, you do not need any mass-transport BC. FEFLOW will allow a free advective outflow of mass.

Thank you for giving a quick answer!
According that, I have set the bottom without any mass concentration BC. But Feflow simulation result does not match or similar with the result from Hydrus. The solute concentration is 30mg/L in the top 30cm, while others are 0mg/L, at initial time. Feflow results show that the top solute cannot be flushed out completely, it seems to be steady at last. However, the result from Hydrus shows the solute concentration can decrease to zero at last.  Can you explain the reasons ? Did I set the wrong BCs ?

Thanks !