Lloyd Kahn (author of Shelter and many other books) is coming out with a new book on septic systems.
(http://www.shelterpub.com/_blog/DB-11-lo-res.jpg)
Here is his blog about how the poor homeowner (at least in California) is being hijacked by Health Depts. and engineers.
http://lloydkahn-ongoing.blogspot.com/2006/08/organized-slime-great-septic-rip-off_30.html
All too true. It's happening here.
They are so picky they try to get you into an engineered system and even if you know what you have and its miles from anyone else, they will require it. But a bear can take a dump in their drinking water and it would be just fine. Our town gets its water from the Merced river - right downstream from the town of El Portal flood sewer overflow area then it is stored in an open air lake with plenty of wildlife. No sh*t. :)