In theory, tap water is the most tightly regulated and controlled food in Europe. In practice, tap water has the highest quality in many European countries but not all. For instance, in Switzerland or Germany, scientific studies regularly find that bottled water has more contaminants than tap water.

Some people still claim bottled water tastes better. Personally, I have doubts such claims would hold up in a properly conducted double-blind test.

As for Bulgaria: My understanding is the tap water in Sofia is of high quality and safe to drink. When there, I drink it myself. Now, as for smaller towns and new settlements, I don't know what the numbers are. Honestly, I'd be at least a tiny bit skeptical. Tourism is on the rise in Bulgaria. Maybe some corners are cut here and there during construction and in the provision of basic services? I'm not saying that's necessarily true. Just that I'd err on the side of caution.

I know that in Croatia, another country which currently sees a huge spike in tourism--not unlike the gold coast in Bulgaria--public infrastructure hasn't kept up with demand. Huge flaws in basic services are well-known problems.