In the 1980's, I knew some farmers that were having ponds built on their land by the USDA Soil Conservation Service. The purpose was to keep eroding farm soil out of water courses.

Was your pond built by the gov't, or did some former owner just think a pond would be nice? If it was the gov't, visit the local office (now, I believe, the National Resources Conservation Service) and ask what to do, now that 30 years have passed and the quality is deteriorating.

If some amateur just bulldozed himself a pond, why don't you get rid of it? What does it do, besides produce duckweed and mosquitoes? Maybe someday a kid will drown in it.