Why does my well water smell like rotten eggs?

That smell is hydrogen sulfide, a colorless gas². In a home on a well it can come from four places: the rock and groundwater themselves, sulfur bacteria living in the well or the plumbing, the water heater, or, rarely, pollution¹. Most people start to notice it at around 0.5 to 1 milligram per liter².

Each source has a different fix, and some of them cost nothing. So before you buy a filter, spend five minutes finding out where the smell starts.

Find the source: the five-minute faucet test

Minnesota’s Department of Health suggests a simple test¹. Do it after the water has been sitting for a few hours, for example first thing in the morning or when you come home:

1. Run the cold water at a kitchen or bathroom tap and smell it. Then do the same with the hot water.
2. If only the hot water smells, the problem is almost certainly the water heater.
3. If you have a water softener, compare a tap that runs through it with one that doesn’t, such as an outdoor spigot. If only the softened water smells, look at the softener.
4. If both hot and cold smell, let the cold tap run for a few minutes. If the smell fades, sulfur bacteria in the well or plumbing are the likely cause. If it doesn’t fade, the hydrogen sulfide is probably in the groundwater.

What you notice	Likely source	Where to start
Only hot water smells	Water heater	Anode rod or heater disinfection (below)
Only water from the softener smells	Water softener	Ask the installer or maker how to disinfect it
Hot and cold smell, fades after a few minutes	Sulfur bacteria in the well or plumbing	Shock chlorination
Hot and cold smell, doesn’t fade	Hydrogen sulfide in the groundwater	Test the level, then choose a treatment

Based on the Minnesota Department of Health's troubleshooting chart.

Is it safe to drink?

Usually the smell is a nuisance rather than a danger. Sulfur bacteria aren’t harmful, and the smell usually has nothing to do with whether the water is sanitary¹. To be sure, Minnesota recommends testing for coliform bacteria and nitrate, and testing for coliform whenever the taste, smell or look of the water changes¹. Our guide to well water testing explains how.

If the smell is more like sewage than rotten eggs, treat it as a contamination warning and have the water tested for bacteria².

The gas also makes water corrosive. It can darken silverware, discolor copper and brass fixtures, and even corrode stainless steel².

If it’s the water heater

Many water heaters contain a magnesium rod that protects the tank from corrosion. Sulfate in the water can react with it and produce hydrogen sulfide, which is why only the hot water smells²³.

• Replace the rod, don’t just remove it. A rod made of a different material, such as aluminum or zinc, usually solves the problem¹³. Removing the rod altogether may shorten the heater’s life¹. Check the heater’s warranty before changing anything², and consider having a plumber do it¹.
• Kill the bacteria in the tank. Flushing the heater with a chlorine bleach solution can work, but if any bacteria survive the smell may come back within a few weeks¹. Another method is raising the heater to 160°F for several hours¹³.

If it’s sulfur bacteria in the well or plumbing

The standard fix is to disinfect the well and the plumbing with a strong chlorine solution¹². Our shock chlorination guide explains the steps and includes a bleach calculator.

Sulfur bacteria can be stubborn. Minnesota notes they’re hard to remove once established and the well may need extra work before disinfection, ideally by a licensed well contractor¹. Georgia’s extension service goes further for sulfur and iron bacteria: it recommends a stronger dose than a standard shock treatment, applied by a licensed well driller or pump installer, and repeating it every spring and fall in wells with an early-stage problem³. Some sulfur bacteria also produce a dark slime that can clog plumbing³.

If the smell keeps coming back after several disinfections, a permanent treatment system is the next step.

If it’s in the groundwater: treatment by level

When hydrogen sulfide comes from the groundwater, the right treatment depends on how much there is. Get the level measured first; the sample has to be stabilized right away, so use the lab’s bottle and instructions².

Hydrogen sulfide level	Options
Trace amounts, up to a few tenths of a mg/L	Activated carbon filter. Filter life varies a lot, from years to days
Up to about 6 mg/L	Oxidizing filter with manganese greensand, regenerated with potassium permanganate
Above 6 mg/L	Feed chlorine or potassium permanganate into the water, then filter it
Any level	Aeration with filtration, or ozonation with filtration

Sources: Virginia Tech (442-658) and Minnesota Department of Health. mg/L = milligrams per liter.

Virginia Tech gives the carbon and greensand limits²; Minnesota adds aeration, continuous chlorination and ozonation (each followed by filtration), and notes that carbon filters work below 1 mg/L but last anywhere from years to days¹. In some cases, Minnesota says, drilling a new well into a different formation is an option too¹.

Check the sulfate too

Where there’s hydrogen sulfide, there’s often sulfate, a natural mineral that gives water a bitter taste and can have a laxative effect, especially on people who aren’t used to it². EPA’s guideline for sulfate is 250 mg/L⁴, and it’s easy to test at a lab². EPA’s guidance also lists a “rotten-egg” odor among the nuisance problems it tracks⁴.