Sorry, I'm trying to figure out how to use this forum. I would recommend you avoid locating the pond where treated septic tank drainage could enter. The nutrients could cause problems with nuisance vegetation and potential pathogens must be considered.
As for sealing the pond, I suggest you investigate:
www.seepagecontrol.com. I've never used the product and can't recommend it, but it is interesting and may be worthwhile to you.
Good luck.
