We won’t agree that the Feds have not only the right, but the duty to regulate toxic substances (water is not inherently toxic - but local, state, and fed regulation of water quality to assure public health is also a responsibility of the government). Your arguments have been specious at best. The Fed regulates alcohol, tobacco, as well as all legal and illegal drugs. They also regulate processing of foods. Public safety is an area the Feds have regulatory power. Simply because you don’t like this fact does not make it wrong for the Feds to hold regulatory power over them.
Yes, water is inherently toxic. Too much and you dilute the electrolytes but let's not get into science since this is a fun political thread while I watch AMC.
If I recall correctly (and I read it a lot), the word duty does not occur in the Constitution. What basis did you have that this is ok?