That was my question when I poured a bath for my daughter. The obviously yellow-tinted bath water made me realize that the water I boiled for pasta also had a yellow-ish tint. Ack! We ate pasta boiled in yellow water? I checked the Natick web site to see if there had been any hydrant flushing or other water-disturbing activities lately. No luck. It was Saturday night so the town offices were closed, so I gave the police a call to see if they had heard anything. They took my information so that an on-call water guy might call me back, but he hasn't yet.
I guess I was on my own. What next? Well, what else but Google? Google led me to I bought a duplex, the author of which had seen this problem before. He referred to water authority sites which noted that the color is most likely from small amounts of rust being disturbed. Also, they say it is completely harmless. I bet it's harmless because (1) the concentration is very low, and (2) iron is something our bodies need anyway. Might even be a good thing for pregnant women since they need higher iron amounts than most people (but don't quote me on that!) The Massachusetts Regional Water Authority has a FAQ on water quality including a paragraph about yellowish water:
MWRA water, which comes from the Quabbin and Wachusett Reservoirs, sometimes turns yellowish from low levels of dissolved iron picked up from old iron pipes in both our system and your community's water system. This mainly occurs in winter, when water usage is low, because the water sits in the pipes for a longer period of time. The water may not be aesthetically pleasing, but according to the federal Environmental Protection Agency (EPA), increased levels of iron that produce yellow water are not public health threats. Hardware stores have inexpensive products to remove rust stains from clothing and household surfaces. MWRA and customer communities replace or rehabilitate old pipes a few miles at a time, an expensive and time-consuming process. Long term solutions take time, but we are constantly cleaning and replacing as much pipe as we can.
So, it's official---yellow water is a minor issue and can be ignored except for stains it might leave on fixtures and clothes.