Branches and generic constituent mass balance

Hi, I attended the workshop in June (very well done) and have a question about an implementation of the model. The reservoir has one main branch with a tributary input, one withdrawal, and 2 side branches that don't receive inflow or withdrawals, but are basically isolated coves on side of the main branch. Problem: I put a generic constituent in the tributary inflow to Branch 1 (initial condition in the reservoir = 3ppm, trib inflow to Branch 1 100pmm). Branch 2, which doesn't receive inflows at all and is at the other end from the tributary input, shows high concentrations (60ppm) of the generic constituent by day 2. The branches are connected with an internal head boundary condition, where the downstream segments of BR2 and BR3 have a segment on BR1 as their downstream boundary condition. Branch 1 is not assigned any upstream or downstream boundary conditions, and is governed entirely by the tributary inflows and withdrawals. Any idea why I show high concentrations of Generic constituent 1 in Branch 2, even though it doesn't receive any inputs and is at the other end of the reservoir from the tributary input? Should there be some sort of internal BC on Branch 1, or is it enough to give downstream internal BCs for BR2 and BR3? Thanks for your help, Trent