When well water smells like rotten eggs, the gas is hydrogen sulfide, and the first job is finding where it is being produced. The single most useful test costs nothing: run the cold tap and then the hot, in different rooms, and note which ones smell. The answer divides the problem into three quite different repairs.
Hot taps only means the water heater
If cold water is fine everywhere and only hot water smells, the reaction is happening inside the water heater. The magnesium sacrificial anode rod reacts with sulfate-reducing bacteria in the tank and generates hydrogen sulfide. The usual fix is to replace the magnesium anode with an aluminum or zinc alloy rod, or a powered anode, and to disinfect and flush the tank. Do not simply remove the anode: it is what stops the tank corroding, and pulling it trades a smell for a leak in a few years.
One tap only means that fixture or drain
If the smell appears at a single sink and follows the drain rather than the glass, the culprit is often bacteria in the drain or a dry trap rather than the water at all. Fill a glass from the tap, take it to another room and smell it there. If the glass is clean and the sink smells, clean and disinfect the drain, and check whether the trap has dried out on a rarely used fixture. This is a common false alarm that sends people shopping for treatment they do not need.
Cold water everywhere means the well
If cold water smells at every tap, hydrogen sulfide is arriving from the source, either dissolved from the aquifer or produced by sulfate-reducing bacteria in the well and casing. Test the water, and include coliform and iron, because sulfur problems commonly travel with iron and iron bacteria. Because private wells are not federally regulated under the Safe Drinking Water Act, nobody is monitoring this for you and no results exist until you order them. Our well water testing guide covers what to ask the lab for.
Treatment: oxidize, then filter
Every real fix works the same way: convert the dissolved gas into something filterable, then filter it out. Aeration systems bubble or cascade air through the water in a tank and then pass it through a catalytic carbon or manganese-based media filter, which is a chemical-free approach suited to moderate sulfide. Chemical injection doses chlorine or hydrogen peroxide into the line, holds it in a retention tank for contact time, and follows with backwashing carbon; that handles heavy sulfide and simultaneous iron and bacteria. Catalytic carbon alone can cope with light sulfide on its own.
Choosing between the approaches
Aeration suits households that want no chemical handling and have moderate sulfide with modest iron. Chlorine injection suits heavy sulfide, high iron, or a well with a bacteria history, at the cost of managing a solution tank and a metering pump. Peroxide injection oxidizes sulfide strongly without leaving a chlorine taste, and also needs contact time and carbon after it. In all three, a softener belongs last in the train, because oxidized iron fouls softener resin. Shock chlorinate the well before deciding, since a well that stops smelling after shocking and then smells again in weeks is telling you the bacteria are living in the casing.
Questions people ask about well water smells like rotten eggs
Is water that smells of rotten eggs dangerous to drink?
Hydrogen sulfide is mainly an aesthetic problem at household levels, but it often accompanies bacteria and iron, so a smell is a reason to test rather than a reason to panic.
Will shock chlorination fix the smell permanently?
Sometimes, if the source was a one-off bacterial colonization. If the sulfide is coming from the aquifer or the bacteria recolonize, the smell returns within weeks and continuous treatment is needed.
Does a carbon filter remove the rotten egg smell?
Catalytic carbon handles light hydrogen sulfide. Heavier concentrations exhaust it quickly, and those need oxidation by aeration or chemical injection before filtration.
Why does the smell get worse after a vacation?
Water standing in the well, the pressure tank and the plumbing gives sulfate-reducing bacteria time to work undisturbed, so the first draws after an absence are the strongest.