Dirichlet and Robin Boundary problem

Dear Sir or Madam,

Sorry to disturb you. I am setting up a very simple 1-D contaminant transport model. And I was wondering if I just apply the boundary water with a injection concentration C0 to the first cell and do not make a big volume factor. What kind of the boundary types is the TOUGHREACT model ? Boundary 1 or Boundary 2 ? How can I simulate this two different boundaries? Thank you !

Best regards,

Lang