Recently I have noticed a brown discoloration in water from my taps under certain circumstances.
I have a shallow well water supply with minimal gravity feed and a shallow well jet pump with a pressure tank. The supply line from the well to the pump is ABS. The plumbing around the pump is mainly lead-free brass. There is a copper distribution manifold supporting PEX distribution lines to the fixtures.
I first noticed the brown discoloration after one of the brass fittings (a shutoff valve) began to leak. At around the same time, hot water taps began spitting and spurting as if there was air in the lines. This happens immediately after the shutoff valve has been opened (and there is an accompanying sound of bubbles in the lines when this happens). I expect the leaky fitting is letting air into the line after enough water leaks from behind it to depressurize that section of water line. Typically the pump also operates at around the same time the valve is opened (since it has to re-pressurize the line and then I generally go use water from the taps the line leads to).
The discoloration is most noticeable when the fitting is spurting and letting air out. If I leave a tap full-open for a minute or so then it seems to clear all the air and the discoloration goes away - until the next time the leaky shutoff is operated.
A water test two years ago resulted in an iron level of 0.226 mg/L and a total dissolved solids of 169 mg/L.
New information: I have replaced the leaking shutoff valve. The taps no longer spit and spurt and there is no longer any visible discoloration in the water. After replacing the shutoff valve I flushed the pressure tank but I didn't observe any discoloration in the water coming from that process. Rubbing my finger on the inside of some non-brass metal fittings (not exactly sure what they are) I had open as part of the replacement process, I found something of a color similar to what I had seen in the water. This all seems to point towards rusting fittings to my untrained eye. I'm a bit surprised there would be any fittings in a modern plumbing system that can rust, though.
What causes this discoloration?