Clearly, this is not a commercial site however it provides a good description of the effect of infestation and also how they are endemic in the soil. http://www.vaxa.com/roundworms.cfm
Presumably at Mac Springs, if present, they would travel from the soil into the water supply via seepage / overland flow of water.
I note the following sentence from the info on the link: Once the eggs are ingested in the body, the roundworm larvae travels through the liver, lungs, and other organs. In most cases, these "wandering worms" cause no symptoms or apparent damage.
So, my memory does not totally fail me.
While it probably doesn't satisfy sschmez or alanjan, it at least describes circumstances generally consistent with what I posted with respect to:
1/. Being a-symptomatic at the time of drinking the water (as distinct from faecies infestation);
2/. resulting from a localised infestation of the parasites;
3/. Causing severe long term problems for humans who suffer an infestation.
Clearly this is significantly different to a generic 'human waste' problem, if one exists at Mac Springs.
Some video links here, including one (admittedly from Fox News) titled: A woman mis-diagnosed with a brain tumor in fact had worms in her brain.
I have not watched it as yet, but would seem to indicate that such things are possible.
In the end, drink at Mac Springs as you wish ...