Actually the MTBE was simply more easily detected by smell if the groundwater had come from a source where underground retail storage tanks had leaked MTBE-oxygenated gasoline.
The portion of the fuel which does not evaporate can sink with gravity until it rests upon a water table within range.
The vast majority of the wells contaminated by leaking gasoline went largely undetected until MTBE was widely introduced to gasoline according to the Clean Air Act of 1990. When compromised retail tanks in use started to recieve gas containing MTBE, it still took a while to seep down into some people's water just like the plain gasoline had been doing, before they started to notice since the MTBE has a characteristic non-hydrocarbon smell of an ether.
MTBE itself is far less toxic than the hydrocarbons it had replaced in the fuel.
Technically, by experts not considered dangerous to health in the trace amounts found in the contaminated water, just bad taste.
Physicians have treated patients using pure MTBE with therapeutic effect, with side-effects that would be expected also from the more traditional USP Ethyl Ether.
https://medical-dictionary.thefreedictionary.com/MTBE
The natural components of gasoline like the benzene are recognized as far worse but they are not so easy to taste.