The odorous and strange-tasting tap water that's plagued some San Francisco households over the past two weeks may still be flowing, but officials now say they know the source, and are taking steps to mitigate the situation.

Yesterday, the SFPUC announced that an organic compound called geosmin is to blame for the musty odor and muddy taste some locals have noticed.

Geosmin is responsible for the earthy flavor of beets and the unmistakable scent of rain after a dry spell. But when it comes to the city's tap water, it appears to have been a byproduct of blue-green algae.

"Initially, we thought that it was sediment," said Suzanne Gautier of SFPUC. "But because the issue persisted and was more associated with odor than color, we did a more thorough examination." She noted that the find was surprising, given that algae normally flourishes in the warmer summer months.

The problem arose after the agency switched up its mix of water sources in a routine operational change, anticipating our current spate of rainfall. It decided to use less water from the Hetch Hetchy Reservoir, and more from local reservoirs.

Gautier says that the geosmin-flavored water likely comes from the San Antonio Reservoir in Alameda County. So, as of yesterday, the SFPUC has stopped pulling water from the reservoir.

In terms of local pipelines, "we anticipate that we will have completely cleaned this out in 4-5 days," said Gautier, noting that different areas of the city may see different timelines.

Although some readers and commenters have reported stomach pains and diarrhea they believe could be linked to the strange-tasting water, Gautier says that there's no clinical evidence of geosmin causing any sort of health issue. She says the water is also safe for pets to drink.

A fact sheet released by the SFPUC yesterday includes a statement from the San Francisco Department of Public Health:

"Earthy or musty tastes and odors are the result of an algal bloom. Although PUC’s modern treatment plants remove algae, the presence of compounds related to algae will remain in the water. The metabolites that remain may result in earthy or musty tastes and odors in the water. The two most common metabolites found are geosmin and 2-methylisoborneal (MIB). The human senses of taste and smell are extremely sensitive, and can detect them in the water at concentrations as low as 5 parts per trillion. These compounds are harmless to humans, and pose no health risk when water containing them is consumed or when people are exposed to their odor.”

According to the SFPUC's fact sheet, the winter algae issue is "extremely uncommon" and unexpected. "Now that we know it can happen, we will test for these naturally occurring dissolved chemicals in the water supply, in addition to the extensive routine testing that we already do."