Comment Sewer pipe to water pipe to well (Score 1) 93
That seems the most plausible path. All it takes is a hole in the sewer pipe and a hole in the water pipe and there's your path. Once in the water pipes it doesn't sound impossible for pathogens to move backwards into the aquifer. I admit the pressure gradient should work against this but it sounds more plausible than quickly transiting a "a thick layer of clay or shale" separating the sewer pipes from the aquifer.