They’re probably talking about fracking. It can (allegedly) cause contaminants to get into the ground water people drill their wells into, and in bad cases, you can literally get a small burst of fire if you hold a lighter to a running tap (there’s debate on whether it’s actually caused by that, or some other pollution source, hence allegedly). In any case, it’s not the water burning of course, it’s the pollutants, and no matter what causes it, that water is horribly contaminated and definitely harmful for human use. There are videos of it, search up things along the lines of “fracking tap water burn”.

There’s also the infamous case in Hawai’i of jet fuel from a US military base getting into the public drinking water. Affected mainly Indigenous Hawaiians and got many people seriously ill. The US government spent a disgusting amount of effort covering it up (and almost no effort cleaning it up), and it’s still not resolved to this day despite them swearing up and down it is.