I really hate to let you know? But all water supplies-- bottled or otherwise (the sole exception is distilled) contain minute traces of mercury.
Meaning? All Food Sources do too. No exceptions.
This is because mercury is everywhere. Literally everywhere. If you are typical? And do not eat top-predator fish (i.e. Tuna, Bass, etc) every day?
You contain about as much mercury as an average sample of farm dirt. Maybe a wee bit more, as humans are Top Predators too.
You ain't getting away from this stuff--- it's everywhere on earth, in everything you drink and in every bite you eat.
Don't worry about it: We all evolved to have trace amounts in our bodies.... just like all other mammals, in fact...
Trace amounts are unavoidable, honestly. BUUUUT, you can put fancy filtration on your water and greatly limit your intake. (which is what I do, main concern being lead from water piping... the fact that it also eliminates mercury and others is just a bonus) Personally, I avoid all the heavy metals as much as possible all of them are linked to health problems. An occasional tuna sandwich or other seafood isn't going to compare to the intake you can get through water via constant exposure/consumption. You drink several liters of water a day your couple of oz of tuna isn't really gonna make a long term difference. The amount of Omega 3 from the fish would outweigh the tiny amounts of mercury in comparison, as far as your health. I still wouldn't eat it frequently, but once a week wouldn't make a difference at all.