A rotten egg smell from the tap is one of the most common questions homeowners in Washington bring up about their water. The smell is usually hydrogen sulfide, a gas that dissolves into water and can be noticed by smell at low levels. Where it comes from changes the fix, so it helps to sort out the source before deciding anything.
What hydrogen sulfide is
Hydrogen sulfide is a naturally occurring compound. In groundwater it can come from sulfur-bearing minerals in the rock the water moves through, or from bacteria that produce sulfide as a byproduct. In shallow wells, decaying organic matter can add to the problem. Metal parts in a well system can also react with dissolved sulfide and leave dark tarnish on fixtures.
Deeper bedrock wells are the ones most often associated with this smell in and around Washington. Water from those depths can carry dissolved gases and minerals that shallow water usually does not, so a well drilled into bedrock may smell while a nearby shallow well does not.
First, rule out the water heater
Many Washington homeowners who notice a rotten egg smell find it only at the hot tap. A water heater with a magnesium anode rod can produce a similar smell in hot water, especially when the water sits in the tank. Check the cold kitchen tap and the cold bathroom tap. If only the hot side smells, the water heater is the likely source, and the well or municipal supply may be fine. A plumber or water heater technician can inspect the anode rod.
Questions to answer about your water
- Does the smell show up on cold water, hot water, or both?
- Is it strongest at the first draw after water has sat in the pipes?
- Did it start after a new well, a pump repair, heavy rain, or flooding?
- Does it show up in every room, or only at one fixture?
- Is the water clear, or is there dark staining or sediment in sinks and toilets?
Write the answers down. A written record helps a lab or a water professional narrow the cause much faster than a general description.
Testing before you choose a fix
Smell alone is not a test. A home can have a detectable odor with no other problems, and a home can have a lab-confirmed problem with little smell at all. Choose a laboratory certified to test drinking water, and ask for hydrogen sulfide or sulfide testing along with bacteria testing. A private well is not checked by anyone on a schedule. Nobody tests it but the owner, so this job falls to you.
Sampling instructions matter. Some sulfide results depend on how quickly the sample reaches the lab, and the lab will give you a container and a timing rule. Follow the lab's steps exactly, and collect from a cold tap that you have run for a while so the sample reflects the well rather than water sitting in a pipe.
If your well draws from bedrock, ask the lab about naturally occurring radium and nitrate as well. Those are tested separately, and a sulfide test does not answer them. After any flood or high-water event, ask for a bacteria test, since flooding can affect well water in the area.
What a lab result tells you
A result is a starting point, not a diagnosis. If sulfide is present, the next question is how it should be handled, and that depends on the well, the pressure system, the strength of the odor, and whether other compounds are also present. Ask the testing lab to explain the report in plain terms, and keep a copy with your well record.
Treatment approaches to discuss
Several approaches are commonly used for sulfide in well water, including aeration, oxidizing filters, and chemical treatment followed by filtration. Each works differently, and each has tradeoffs in maintenance, space, and what other compounds it also affects. Bring your lab report and your answers from the checklist above to a qualified water professional, and ask them to explain why they recommend one approach over another for your well.
Keep the source in view
A rotten egg smell that comes and goes after rain or a pump service is worth logging. Write down the date, what you noticed, and whether it was hot or cold water. Retest after any major change to the well, the pump, or the pressure system, and after any flood in the area. Odor is an early signal, and a short written record makes the next conversation with a lab or professional much easier.