Think about using a pump tank and get the septic system as far a possible from the pond. I design septic systems for a living and have seen many fail. Ask yourself, would you drink sewage filtered by only 25' of soil. If not, don't build the system near the pond.
Personally I would place the septic downstream on the dam. The cost of a good pump and pump tank is less than $2k plus a couple of bucks a foot for the force main.
