Today, scientists, environmentalists and health experts are baffled by the extent to which consumers think bottled water is healthier than tap. They note that public water supplies are more tightly regulated than the springs that supply bottled water company. And one of the most highly-rated waters for taste is New York City tap water.
But this was not always the case. A nice piece from Edible Geography notes that, for a long time, the taste of tap water changed with the seasons. In the summer, bacteria and algae would proliferate in reservoirs, tainting the taste of water. The article even notes that there were scientists who would study the correlation between tastes in tap water and varieties of microorganisms present. One variety was associated with a smell the researchers called "pig-pen smell."
Today, filtration and additives keep the taste of water relatively consistent throughout the year. Now, experts say that irregular tastes are much more likely to be related to problems with a home piping system than to reservoir flora and fauna.