Well water that smells like rotten eggs

Several different problems produce this same symptom, and they need completely different fixes. Work down the list — each one names the check that rules it in or out.

1. 1

   The water heater's anode rod (hot water only)

   Every tank water heater contains a sacrificial magnesium anode rod that protects the steel liner. In water containing sulfates, that magnesium can drive a reaction that produces hydrogen sulfide inside the tank. The well is not involved at all.

   How to tell
   Cold water is odourless everywhere; only hot water smells. The smell is often strongest after the tank has sat, such as first thing in the morning or after a holiday.

2. 2

   Dissolved hydrogen sulfide in the aquifer

   The gas occurs naturally in groundwater, particularly in wells drilled into shale, sandstone or areas with peat or oil-bearing rock. It is dissolved in the water before it ever reaches your house.

   How to tell
   Both hot and cold smell at every tap, the intensity is fairly constant day to day, and it does not go away after shock chlorination — or returns within days.

3. 3

   Sulfur-reducing bacteria in the well

   Bacteria living in the well, the pressure tank or the plumbing that convert sulfate into hydrogen sulfide as they metabolise. Harmless to drink but self-sustaining, and they recolonise if you do not remove their habitat.

   How to tell
   The smell disappears completely after shock chlorination and then creeps back over weeks or months. Often accompanied by black slime or staining in the toilet tank.

4. 4

   A septic or drain problem being blamed on the water

   Sewer gas escaping a dry trap or a failed vent smells much the same and is easy to attribute to the water supply. It is a plumbing fault, not a water quality one.

   How to tell
   The smell is in the room rather than in the water. Fill a glass, take it to another room and smell it there — if it is odourless away from the sink, the water is fine and the drain is not.

Almost every page on this subject lists the same causes. Very few tell you how to sort between them, which is the only part that matters, because the fixes range from a fifty-dollar part to a several-thousand-dollar treatment system.

Run this in order. It takes about five minutes and it will land you on one of the four causes above.

1. Run the cold tap at the kitchen sink for two minutes, fill a glass, and smell it. Then take the glass to another room and smell it again.
2. If it is odourless in the other room, the smell is coming from the drain, not the water. Stop here.
3. Run the hot tap at the same sink and smell it. If only the hot smells, it is the water heater's anode rod.
4. If both smell, check an outside spigot or the closest tap to the pressure tank — something upstream of any treatment equipment.
5. If that smells too, the source is the well. How quickly the smell returns after shock chlorination then separates dissolved gas from bacteria.

This is the most common single cause and the cheapest to resolve, which is why it is worth checking first. The magnesium anode rod is doing its job — protecting the tank — but doing it in a way that generates hydrogen sulfide in water with sulfate in it.

The standard fix is to swap the magnesium rod for an aluminium or an aluminium-zinc alloy rod. Both still protect the tank; neither drives the same reaction. Removing the rod entirely is a bad idea and will shorten the heater's life considerably, whatever anyone on a forum says.

Flushing the tank and raising the thermostat to around 160°F for a few hours will kill bacteria living in the tank and clear the smell temporarily. It is a useful diagnostic, but if the anode rod is the cause the smell returns.

• Turn off power or gas and the cold supply before touching the anode rod.
• The rod is often seized — a breaker bar and a 1 1/16" socket is the usual requirement.
• Check whether the heater is still under warranty first: on some models, changing the rod yourself voids it.
• If you raise the thermostat to sanitise, turn it back down afterwards. Water above 120°F scalds quickly, and above 140°F it does so in seconds.

If the source is the well, shock chlorination is both the first treatment and the decisive diagnostic. Chlorinating the well kills sulfur bacteria and clears dissolved gas temporarily, so what happens next tells you which you were dealing with.

If the smell stays away for months, it was bacterial and a repeat shock — or continuous chlorination if it keeps returning — is the answer. If it comes back within days, the hydrogen sulfide is dissolved in the groundwater itself and no amount of disinfection will change that. You need permanent treatment.

For permanent treatment, the concentration decides the equipment. Light odour is handled by activated carbon, though cartridges saturate and need changing on a schedule. The middle range is the realm of air injection oxidation, which introduces a pocket of air that converts the gas to elemental sulfur for a filter bed to catch. Heavy concentrations need continuous chlorine injection with a retention tank, followed by filtration and a carbon stage to remove the chlorine again.

• Under about 0.5 mg/L — activated carbon is usually sufficient.
• 0.5 to 6 mg/L — air injection oxidation with a catalytic media bed.
• Above 6 mg/L — continuous chlorination, retention tank, filtration, then carbon.
• Any of the above — a sediment pre-filter first, because oxidised sulfur is a solid.

How to shock chlorinate a well properly→

If this turns out to be the cause, here is where the equipment sits in the treatment train. Order matters — each stage protects the one after it.

1. 1
   Shock chlorinationOne-off, at the wellheadDisinfects the well itself before any equipment is sized. A one-time procedure, not a permanent stage — if bacteria return, that is what UV is for.
2. 2
   Sediment pre-filterFirst, where the line enters the houseProtects everything downstream from grit and silt. Cheap to replace, and it is the stage that keeps the expensive stages alive.
3. 3
   Iron / manganese / sulfur filterBefore the softenerRemoves iron, manganese and hydrogen sulfide by oxidation. Sits ahead of the softener because iron fouls softener resin and voids most warranties.
4. 4
   Carbon filterAfter the softenerPolishes taste and odour and strips any chlorine used upstream. Placed after the softener so it is not fouled by hardness or iron.

Enter your test results for a train built around your water→