Rotten Egg Smell in Well Water: Causes and the Fix

Is it the well or the water heater?

One five-minute test answers it: fill one glass at an outside spigot or a cold-only tap the water reaches before any treatment equipment, fill a second glass of hot water at a sink, and smell both.

Hot smells, cold does not: your water heater is making the gas, and the usual culprit is the magnesium anode rod reacting with sulfate in the water. Replacing it with an aluminum-zinc rod is a plumbing job, not a well job, and no amount of well treatment will fix it.

Both smell, or cold smells: the hydrogen sulfide is coming up with the groundwater, and in most wells it is being produced by sulfur-reducing bacteria living in the well and the slime coating its walls. That is a well problem, and it is treatable. And if the smell only shows up at the first tap of the day, the morning-only smell has its own guide.

Why does well water smell like rotten eggs?

Sulfur-reducing bacteria eat sulfate in the groundwater and exhale hydrogen sulfide. They are anaerobes, so they thrive in the oxygen-poor column of a well and inside biofilm, very often the same slime that iron bacteria build. If your smelly well also stains fixtures orange, you are almost certainly looking at a shared colony, and treating one without the other is how the smell comes back.

How do you get the sulfur smell out of well water?

The treatment is a shock disinfection of the whole system: the standing water, the casing walls, the pressure tank, and the plumbing, following how to shock a well. Chemistry matters here more than in almost any other well problem, because the bacteria live inside biofilm. Chlorine burns the surface of that slime and quits at the alkaline pH most wells have. The chlorine dioxide our kit ships penetrates and strips the biofilm per its manufacturer's published data, holds its strength across the real pH range, and breaks down to simple salts instead of leaving a bleach taste to flush for days.

After the contact time, flush outside until the included residual test reads clean, then give it a week and smell again. A well that still smells took less than a full dose, and the residual test will have told you so.