All "emerging" pollutants, except heavy metals.Certainly things like microplastics, pesticides, glyphosates, “PFAS”, heavy metals, etc are present in some water supplies.
Heavy metals are pesky, in that the limits have been lowered numerous times over 5 decades.
Trihalomethanes? Newspaper fodder. Carcinogenic.....till you actually look at the limits, and can even understand what you are dealing with.
These are always one of my favorite to use as an example. When i was dealing with this, if memory serves, double the limit at that time, youd have to drink 1000gallons a day, to increase your cancer risk by fractions.
The number one dangerous thing in your water today, if your water source is subject to any wastewater discharge, at any point. Pharmacueticals. There are few processes in use that can deal with them, and we only recently started to look at them.