The Rhine is a manmade impoundment and therefore is regulated differently than natural lakes. There maybe wetlands along the perimeter, however that does not disqualify discharging into the Rhine. BG should be able to use the Rhine to control the quantity of stormwater while simultaneously purchasing nutrient credits to address the water quality issues, assuming that the Rhine does not meet the current regulations on water quality treatment.
The bigger issue with the Rhine is that some of it's water does make it to the Chesapeake Bay. The Chesapeake Bay does have some of the most unique and strictest regulations on it, which is why we need the buffer on the Rhine, despite it not being connected. The Rhine also taps into the natural water table of the area, adding into the protections that are needed.