Hydrogen Sulfide in Well Water — Fixing the Rotten Egg Smell

That unmistakable rotten egg smell from your well water is hydrogen sulfide gas. It is unpleasant, corrosive to plumbing, and alarming — but it is almost always fixable. Here are the 3 causes and the right fix for each.

3 Causes of Rotten Egg Smell — Each Has a Different Fix

Cause 1: Magnesium Anode Rod in Water Heater (Hot Water Only)

If only your hot water smells like rotten eggs and your cold water is fine, the cause is almost certainly the magnesium anode rod in your water heater. The rod reacts with sulfur compounds in the water to produce hydrogen sulfide gas.

Fix: Replace the magnesium anode rod with an aluminum/zinc combination anode rod ($20–$40 at hardware stores). This eliminates hot-water-only sulfur smell in the vast majority of cases — immediately and permanently.

Cause 2: Sulfur-Reducing Bacteria in the Well

If both hot and cold water smell, sulfur-reducing bacteria living in the well casing, pump, or water softener are the likely source. These bacteria feed on sulfates in the water and produce H2S as a byproduct.

Fix: Shock chlorinate the well (same process as for coliform bacteria). Pour 1–2 quarts of unscented bleach into the well casing, run all faucets until you smell chlorine, let sit 12–24 hours, then flush. This kills the bacteria and often eliminates the smell permanently. If smell returns within months, a continuous treatment system is needed.

Cause 3: Naturally Occurring Sulfur in the Aquifer

In some regions, groundwater naturally contains high sulfate levels. Shock chlorination provides only temporary relief. The smell returns because it is coming from the water source itself, not from bacteria.

Fix: An oxidizing filter using air injection or hydrogen peroxide injection oxidizes and removes H2S before it reaches your tap. Cost: $800–$2,500 installed. This is the permanent solution for naturally high-sulfur water.

Is Hydrogen Sulfide Dangerous?

At typical residential well concentrations (under 1 mg/L), hydrogen sulfide in water is unpleasant but not a health hazard for drinking. At higher concentrations it can cause nausea. The primary concerns are the corrosive effect on copper plumbing and appliances, and the bacterial contamination that may accompany it.

If You Have a Water Softener

Water softeners create low-oxygen environments that sulfur bacteria love. If the smell is worse after periods of no water use or started after installing a softener, sanitize the softener resin with a bleach solution per the manufacturer's instructions. Increasing the regeneration frequency also reduces bacterial growth in the resin tank.