Why does my well water smell like rotten eggs?
What causes this
Hydrogen sulfide gas in the groundwater
The classic rotten-egg odor is hydrogen sulfide (H2S) dissolved in the water itself. It is common in groundwater and is detectable by smell at very low concentrations, so a strong odor does not necessarily mean a high level.
Sulfur bacteria in the well or plumbing
Non-harmful sulfur bacteria can grow in the well, the pressure tank, or the plumbing and produce hydrogen sulfide as a byproduct. This can come and go and may appear after the system sits unused.
The water heater's magnesium anode rod
If the smell is only on the HOT water, the cause is usually a reaction between the water heater's magnesium sacrificial anode rod and bacteria in the tank — not your well water at all.
Is it dangerous?
At the low levels where you can smell it, hydrogen sulfide is mostly a nuisance and aesthetic problem and is not usually a health hazard — but it is corrosive to plumbing and fixtures, and it can be a sign that bacteria are present. Because the smell can signal bacteria, this is a case to test rather than assume.
The test that tells you
A symptom only narrows it down. To know for sure, have a state-certified lab test for:
- hydrogen sulfide
- sulfate
- coliform bacteria
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How to fix it
First find out whether it is hot-water-only: if so, have the water heater's magnesium anode rod replaced or upgraded (an aluminum/zinc anode often solves it). If the smell is on cold water too, the options are shock-chlorinating the well to knock back bacteria, installing an oxidizing or iron filter, or adding aeration to drive the gas off. Re-test for bacteria after disinfecting.
- 1
Check whether the smell is hot-water-only
Run the cold tap and then the hot tap separately. If only the hot water smells of rotten eggs, the source is almost certainly the water heater's anode rod, not your well.
- 2
Test the water for hydrogen sulfide and bacteria
If the cold water smells too, have a state-certified lab test for hydrogen sulfide, sulfate, and coliform bacteria so you know whether you are dealing with gas, bacteria, or both.
- 3
Treat the actual cause
For a hot-water-only smell, replace or upgrade the water heater anode rod. For a whole-house smell, shock-chlorinate the well and install an oxidizing/iron filter or aeration to remove the hydrogen sulfide.
- 4
Re-test after treatment
After shock-chlorination, re-test for coliform bacteria to confirm the disinfection worked and the bacteria are gone, not just temporarily suppressed.
A symptom is a clue, not a diagnosis. Only a lab test of your individual well confirms what is in your water — do not assume a symptom is definitely harmless or definitely dangerous until you have tested. The county-level USGS area estimates elsewhere on this site describe a region as a whole and cannot stand in for testing your own well.
By TapWaterData Editorial
Frequently asked questions
Is rotten-egg-smelling well water safe to drink?
At the very low levels where the smell is detectable, hydrogen sulfide is mainly an aesthetic and corrosion nuisance rather than a health hazard. But the odor can also mean bacteria are present, so the honest answer is to test for hydrogen sulfide and coliform bacteria rather than assume it is safe.
Why does only my hot water smell like rotten eggs?
A rotten-egg smell on the hot water only usually comes from the water heater, where the magnesium sacrificial anode rod reacts with bacteria in the tank to produce hydrogen sulfide. Replacing or upgrading the anode rod typically fixes it, and it is not a problem with your well water.
How do I get rid of the sulfur smell in my well water?
If it is hot-water-only, replace the water heater anode rod. If the cold water smells too, shock-chlorinate the well and install an oxidizing or iron filter, or aeration, to remove the hydrogen sulfide — then re-test for bacteria to confirm the well is disinfected.
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