Water Quality · In brief
When the Tap Smells Like Rotten Eggs
Well water that smells like rotten eggs usually carries hydrogen sulfide, sometimes made inside the water heater; a sulfur smell in the air rather than at the tap means leave and call.
By the Ranch House Water desk · Updated · 3 min read
A sulfur smell that rises only when a faucet runs is almost always hydrogen sulfide dissolved in the water. Penn State Extension notes that the human nose picks it up at levels well under 0.5 mg/L; it can also leave yellow or black greasy stains on fixtures. PG&E adds a similar sulfur-like odor to natural gas on purpose, so the first job is working out which one you have.
Water smell or gas?
A smell that comes from the water shows up at the sink or in the shower while the tap runs, and only there. A smell anywhere else is treated as gas. The California Public Utilities Commission lists other gas leak signs, such as hissing or roaring, blowing dirt, bubbles in standing water and dead plants, and says to leave the area on foot before calling.
Only in the hot water?
Then look at the water heater. Penn State Extension explains that a sulfur smell confined to hot water often comes from chemistry inside the tank, where a magnesium rod can turn sulfate into hydrogen sulfide. A licensed plumber can look at the rod and the tank; gas burner, valve and venting work belongs with a licensed professional. Our story on water heaters and hard water covers the heater itself.
In both hot and cold?
Then the hydrogen sulfide is most likely coming from the groundwater. It has no drinking water standard because it makes water unpleasant long before it could do harm, per Penn State, but a new or changed smell is still a reason to test. Have an accredited lab check bacteria and the regular round first; testing your well water explains what to order. The state's well-owner booklet lists oxidation, activated carbon filtration, shock chlorination, ion exchange and ozone as treatments. Shock chlorination of a well is covered in well disinfection; pump and wiring work during it belongs with a licensed well or pump contractor.
Sources
- Penn State Extension — Hydrogen Sulfide (Rotten Egg Odor) in Water Wells — detection level, stains, no standard, water heater cause
- California Public Utilities Commission — Gas emergency response — leak signs; leave on foot first
- State Water Board GAMA — A Guide for Private Domestic Well Owners (PDF) — hydrogen sulfide treatment options
- PG&E — Gas Safety — leave, avoid ignition sources, call 911 then PG&E