I have read this whole thread with interest and agree with Ridgerunner wholeheartedly. A few weeks or months ago, I started a thread asking if anyone had ever allowed their ducks to swim in their sewage lagoon. For those in parts of the country where these are not common, here in the midwest, a common solution to rural household waste is to dig a "lagoon" to which all water from the house is pumped. This solution is usually utilized where the soil is clay and a septic tank is not appropriate.
The first people who responded to my thread had the gut reaction "ewwww....no way", but as we talked it all through, we came around to realizing its not so gross after all. One of the participants realized that close to her house is a large network of similar lagoons that service towns rather than just a household. They have become a haven for all kinds of birdlife, and the city has even created walking paths to allow visitors to walk and view the ducks and other waterfowl in their "natural" environment.
All of that brings me around to suggesting to the OP that the best way to tell if the water is safe is to look and see if other life is flourishing in it. In my own lagoon, the frog life is incredible. Walk down to it and they all go hopping into the water and the combined effect is a huge splash. Frogs wouldn't live on it if it were toxic. I did ultimately decide not to put my ducks on the lagoon but not because I feared for their safety. Rather, I feared that the clay banks of the lagoon would not hold up to the traffic of the ducky claws going up and down it. If I find a way to solve that problem, it will definitely be something I consider.
Your post reminds me of the way London was during the great Cholera epidemic. I read an interesting book about the era called "the ghost map". It covers Doctor John Snow and the general conditions at that time. Apparently it was common to have raw sewage lakes in back yards and basements. Personally it sounds like a recipe for disease. What happened in London was that a baby caught Cholera and the diaper washing water was dumped into the cess pool behind the house. The water from the cess pool had been seeping into the well water for a while with no noticeable taste or problems. Once the Cholera bacteria was introduced, it wiped out everyone using the surrounding well pumps.
You tube has the TEDD TALK about it. search for
Steven Johnson: A guided tour of the Ghost Map
Its an interesting video
Riki