That sulfur rotten egg smell in your well water is hydrogen sulfide gas. Here are the 4 causes and the most effective fixes — from shock chlorination to filtration.
The sulfur rotten egg smell is hydrogen sulfide gas (H2S) dissolved in your water. It is produced by sulfur-reducing bacteria that live in low-oxygen environments — including well water, water heaters, and water softeners. It can also come from naturally occurring sulfur compounds in the aquifer.
Sulfur-reducing bacteria colonize the well casing, pump, and surrounding aquifer. They feed on sulfates in the water and produce hydrogen sulfide as a byproduct. The smell is present in both hot and cold water throughout the house.
Fix: Shock chlorination. Pour 1-2 quarts of unscented household bleach into the well casing. Run faucets until you smell chlorine at every tap. Let sit 12-24 hours. Flush until chlorine smell is gone. This kills the bacteria and often eliminates the smell permanently. If it returns within months, a continuous chlorination system or oxidizing filtration is needed.
Some groundwater naturally contains high sulfate levels regardless of bacteria. The smell is consistent, present year-round, and shock chlorination provides only temporary relief.
Fix: Oxidizing filter. An iron/sulfur filter using air injection or hydrogen peroxide oxidizes and removes H2S before it reaches your tap. This is a permanent solution for naturally high-sulfur water. Cost: $800-2,500 installed.
If only your hot water smells like rotten eggs, the magnesium anode rod in your water heater is reacting with sulfur in the water to produce H2S. The cold water may smell fine.
Fix: Replace the magnesium anode rod with an aluminum/zinc combination anode rod ($20-40). This typically eliminates hot-water-only rotten egg smell completely and immediately.
Water softeners create a low-oxygen environment that sulfur bacteria love. If the smell started after installing a softener or is worse after periods of no water use, bacteria in the softener resin tank may be the source.
Fix: Sanitize the softener with bleach solution per the manufacturer's instructions. Set the softener to regenerate more frequently to reduce stagnation.
💡 Get a water test first. A basic water test ($30-75) confirms whether bacteria are present and measures H2S levels. This determines whether shock chlorination or filtration is the right approach.
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