Brettski,my own septic field is 35 feet from our swimming pond. There has been no seepage, leakage, or transfer of water from our septic system into the pond. Think about what goes into your septic tank. Used steak, potatoes, toothpaste and some laundry detergent. Then, microbes break it down, and gray water makes its way into the lateral lines, where soils absorb it. There is no fear from what you put into the septic tank, unless you flush pcb's or something else toxic. The fear comes from microbes harmful to human health. As mentioned above, e. coli is the main concern. If I were in your shoes, and didn't have peace of mind, I would further test the perc rates of your soils, and move the field further from the pond. One client of mine built a septic system in clay soils and chose to excavate a much larger ditch, backfill with many yards of crushed rock, add his lateral line pipe, add more rock, then fill the ditch with topsoil. He has the prettiest grass in the yard, shaped like the letter "F". You can see where it obviously starts, and where it ends. His family doesn't use lots of water. I suspect if we were raising 13 kids and running lots of stuff through the system, I would pay closer attention to it.
On another note, in 27 years in the pond management business, I have yet to come across a pond where septic fields have caused any identifiable problems. That's not to say ponds won't get e. coli. That can come from sources other than human ones, as well. But, I have not seen one where the septic system was an issue.
I did come across one situation years ago, in a larger community lake, where a number of houses were built, and one guy ran his gray water straight into the lake. There was a localized bluegreen algae bloom with some strange looking filamentous algae, too. It took about three minutes to see the pipe at the edge of the lake. Community enforcers took care of that, pronto. They tested the entire lake via random samples, and this one guy's indiscretions didn't damage the lake.
If I'm in your shoes, I put in the system, as far away from the pond as practical, and have confidence in it. I vote with the advice you got from the county.
