Well Water Smells Like Rotten Eggs
That sulfur smell is hydrogen sulfide gas. It is rarely dangerous, but it corrodes pipes, stains fixtures and sometimes points to bacteria in the well.
Likely causes
- Sulfate-reducing bacteria in the well or pressure tank
- Hydrogen sulfide in the aquifer
- Water heater anode rod reaction
- Iron bacteria
Tests to order
- Hydrogen sulfide
- Sulfate
- Total coliform
- Iron and manganese
Treatment options
- Shock chlorination
- Aeration or oxidizing filter
- Activated carbon for low levels
- Replace the water heater anode
Where the smell comes from
Hydrogen sulfide gas forms when sulfate-reducing bacteria feed on sulfate in the water. They live in the well, in the pressure tank and very often inside the water heater, where the magnesium anode rod gives them exactly what they need. It can also come straight from the aquifer in areas with shale, coal or gypsum. The smell is strongest first thing in the morning and at hot taps.
Find out which one you have
Run the cold tap and the hot tap separately. Smell only at the hot tap means the water heater. Smell at both taps that fades after the water runs a minute points to the well or pressure tank. Smell that never fades means the aquifer itself. A lab can measure hydrogen sulfide, but it has to be sampled on site because the gas escapes fast, so ask for a field test or a preserved bottle. Order coliform and iron at the same time, since the same conditions grow iron bacteria.
Fixing it, cheapest first
If it is the water heater, replace the magnesium anode with an aluminum-zinc or powered anode, about $40 to $200. If it is the well, a shock chlorination knocks the bacteria back for months and costs under $30 to do yourself. For a permanent fix at higher levels, an aeration system or an oxidizing filter (manganese greensand or catalytic carbon) strips the gas before it reaches the house. Plain activated carbon works for very low levels but clogs quickly at anything above 1 ppm.
Is sulfur smell in well water dangerous?
At the levels found in most wells it is a nuisance, not a health risk. The real cost is corrosion of copper and brass, black stains on silverware and a shortened water heater life.
Why does only my hot water smell?
The anode rod inside the heater reacts with sulfate and bacteria to make hydrogen sulfide. Swapping the anode almost always ends it.
Will a water softener remove the smell?
No. Softeners trade hardness for sodium and do nothing for gases. Some make it worse by giving bacteria a place to grow in the resin bed.