What Causes a Sulfur Smell in Well Water?
A sulfur or rotten egg smell is caused by hydrogen sulfide gas (H₂S). There are three common sources: sulfur-reducing bacteria in the well or water heater, naturally dissolved H₂S in the groundwater, or a corroding magnesium anode rod in the water heater. A water test identifies which source is responsible — and the correct treatment is different for each.
The Three Sources — and How to Fix Each
1. Sulfur-Reducing Bacteria
Clue: Smell is present in both hot and cold water, or gets worse after the well sits unused
Fix: Shock chlorination of the well + whole-house iron/sulfur filter. Bacteria can recolonize without ongoing treatment.
Very common in Calvert and Anne Arundel County wells
2. Dissolved H₂S in Groundwater
Clue: Smell is consistent in cold water, not just hot; present immediately when water runs
Fix: Aeration system or oxidizing filter (greensand or Birm media). Oxidizes H₂S gas so it can be filtered out.
Common in deeper wells in Southern Maryland
3. Water Heater Anode Rod Reaction
Clue: Smell is only in hot water, not cold
Fix: Replace the magnesium anode rod with an aluminum/zinc anode. Least expensive fix — no need for a whole-house system.
Especially common when sulfate levels are elevated
Is Sulfur in Well Water Dangerous?
At the concentrations typical in residential wells, hydrogen sulfide is not a health hazard — but it is a strong indicator of bacteria or other water quality issues that should be addressed. The smell is the primary concern for most homeowners.
A water test will confirm whether bacteria or other contaminants are present alongside the H₂S.
Why Test Before Treating?
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