Yellow water from a tap almost always comes down to one of four causes: iron or manganese in the supply, rust shed from old pipes or the water heater, natural tannins leaching from decaying vegetation, or disturbed sediment after mains work. A few minutes of simple checks will tell you which one you have.
Narrow it down at the tap
Run cold and hot separately at several fixtures. If only the hot is yellow, suspect the water heater: sediment and a corroding tank or anode will do it, and flushing the heater is the first move. If one fixture is yellow and the rest are clear, the problem is that fixture's supply line or an aerator full of rust. If everything is yellow, it is coming in with the water. Then fill a clear glass and let it stand. Water that arrives clear and turns yellow after standing is dissolved iron oxidizing in air. Water that arrives yellow and settles a layer at the bottom is already-oxidized rust. Water that arrives yellow or tea colored and stays perfectly clear with no settlement is usually tannins.
Iron, manganese and the staining they cause
Penn State Extension notes that iron and manganese carry secondary, aesthetic standards rather than health-based ones, with recommended maximums of 0.3 mg/L for iron and 0.05 mg/L for manganese. Those levels are low because both stain heavily: iron in the yellow to orange range, manganese darker, into brown and black. The treatment depends on the form. Dissolved iron on suitable water can be handled by a softener, which Penn State notes can remove it when pH is above 6.7, hardness is between 3 and 20 grains per gallon and dissolved iron is under 5 mg/L, while iron that has already oxidized fouls the resin and needs a filter instead. Higher levels usually go to an oxidizing filter bed, sometimes with air or chemical injection.
When it is the pipes or the mains, not the well
Yellow water that appears suddenly on a city supply, especially after a hydrant flush, a main break or repair work in the street, is usually disturbed sediment in the distribution system. Run a cold tap at the lowest fixture in the house for several minutes and it typically clears. If it does not, call the utility, since they will know whether they have been working nearby. Inside the house, old galvanized steel piping sheds rust as it corrodes and produces yellow water that is worst first thing in the morning, after the water has sat in the pipes overnight. Your utility's annual Consumer Confidence Report, which the EPA requires every community water system to publish and maintains a finder for, is the place to check what the supply normally carries.
Tannins, and why filters for iron do not touch them
Tannins are organic compounds leached from decaying leaves and peat, common in shallow wells and surface-influenced supplies. They tint the water yellow to tea brown, they can give it a slightly musty or earthy taste, and crucially the color does not settle out and does not deepen on standing, because there is no particle to settle and nothing to oxidize. That is why an iron filter installed on tannin water changes nothing. Tannins are usually treated by an anion exchange resin, sometimes preceded by other stages, and the design needs a proper test first. Whatever the suspected cause, test before you buy: our well water testing guide covers taking a sample that reflects the supply rather than the plumbing.
Questions people ask about yellow water from tap
Is yellow tap water safe to drink?
Iron, manganese and tannins are aesthetic problems rather than health ones, but discoloration can also mean corroding pipes, so test rather than assume and avoid drinking it meanwhile.
Why is only my hot water yellow?
That usually points at the water heater. Sediment in the tank or a corroding anode and tank lining will discolor the hot side while cold taps stay clear.
How do I tell iron from tannins?
Let a glass stand. Iron either settles out or deepens in color as it oxidizes. Tannins stay evenly tinted, clear rather than cloudy, with nothing settling.
The water went yellow overnight on city supply. What now?
Run a cold tap at the lowest fixture for several minutes to clear disturbed sediment, then call the utility if it persists, since mains work is a common cause.