They are turning our atmosphere into their atmosphere:

The SFPUC is aware that customers in San Francisco and throughout our service area started to report taste and odor issues with their tap water on Thursday, December 1st. We understand that there continues to be a reported "earthy/musty" taste to the water. [...]

On average, 85% of the water we provide to you, our customers, comes from the Hetch Hetchy Reservoir. The rest of the water we supply comes from local Bay Area reservoirs. All of our reservoirs deliver high-quality water to 2.6 million customers throughout the Bay Area. As part of normal operations, our customers receive both local reservoir water and Hetch Hetchy water.

Why does your water taste different now?

The week of November 28th, our operators made routine operational changes to the system to bring local reservoir water levels down in anticipation of rains. This included taking water out of San Antonio Reservoir in the East Bay, and treating it at our Sunol Valley Water Treatment Plant. We also reduced the flow of water coming from Hetch Hetchy Reservoir. That operational activity stirred up sediments in the pipeline. Complaints tracked the flow of the sediments, San Antonio algae levels were extremely low, and standard odor tests were normal. Thus, the pipeline sediment was believed to be the initial source of the taste and odor issues.

Recent chemical testing in the system has shown the presence of a dissolved compound called Geosmin that is a natural byproduct of blue green algae in the water. Geosmin is found in foods like fish, grapes, wine, and beets. This dissolved compound is not harmful from a public health standard. However, this compound can cause taste and odor issues in the drinking water supply, even if there are extremely small amounts of it in the water (parts per trillion).

Press conference.

Bay Area: Do You Know Where Your Water Comes From?

The Bay Area water system is a byzantine patchwork of agencies -- more than 50 in all -- that provides water to customers. Some are the ones you see on your water bill. Others are middlemen that provide water to local agencies at the the wholesale level.

And some of that water makes a long journey. Southern California has the reputation for tapping far-flung sources for its water needs, but the Bay Area is in the same boat.

More than two-thirds of the Bay Area's water supply comes from outside the region, which means in extreme drought years like this one, local water districts are competing with many others around the state for limited supplies.

A Hoodline commenter said:

The reservoirs in Alameda and San Mateo counties are part of the SF Department of Water & Power system, as are other reservoirs upstream between SF and Hetch Hetchy. SF sells most of the water to other agencies on multi-decade contracts while using the electricity generated to power government facilities and Muni (power was originally supposed to be for citizens of SF). Blending has gone on for many years in order to balance storage and demand throughout the system. I've generally noticed a switch shortly after a few heavy rains in fall/winter and when algae requires additional chloramine. It will be interesting to hear feedback when local SF groundwater is added to the mix for the western half of the city in the near future.

...but in a quick search, I wasn't able to find a summary of what the economics of all this are, or the status of PG&E's presumed ongoing effort to strangle Clean Power SF.

I think it was in Paolo Bacigalupi's The Water Knife where someone characterized aquifers as buried glaciers: they spent a hundred thousand years accumulating and rolling south until they got spaded under, and now we're draining them three orders of magnitude faster than that.

Previously.