Emissions and whatever chems are leaching in from the plastic used to bottle it. Some places do have really bad municipal water though. WHere I am, you get a bad taste around this time of year from lake bacteria because of warmer than usual summers. Makes coffee taste a little weird but nothing dangerous. So they say :/.

Good Idea to run taps for a few minutes though to flush out contaminants and lead and crap that may have built up.