A rotten egg smell from the tap is alarming, but it is also one of the easier problems to narrow down. In St. Louis homes the odor usually comes from one of three places: sulfur gas dissolved in the water, a bacterial reaction inside the water heater, or sewer gas rising through a drain that has lost its water seal. This guide explains how to tell them apart without special equipment.

What the smell is

The rotten egg odor most people describe comes from hydrogen sulfide, a gas that can dissolve into water. Water carrying it often smells strongest when it first comes out of the tap and fades after it has sat open in a glass for a few minutes, because the gas escapes into the air. Sulfur can show up in the same water as iron, so a homeowner who already deals with orange staining or a metallic taste may notice this smell as well.

Start with a hot and cold comparison

Run the cold tap in the kitchen first, then the hot tap. Smell the water from each one, ideally in a clean glass.

  • If both hot and cold water smell, the source is most likely the supply itself, whether that is a private well or water that already carries the gas when it reaches your home.
  • If only the hot water smells, the water heater is the most likely source. Bacteria that feed on sulfate can grow in a tank, and they produce the same gas.
  • If the smell is strongest near a sink, floor drain, or basement drain when no water is running, the cause is more likely a dry trap or a vent problem in the drain system.

Check the drains first

Sewer gas is a common source of this odor, and it is easy to mistake for water. Pour a full jug of water down each drain you suspect, especially floor drains, basement drains, and sinks that are rarely used. A dry trap lets gas come up from the sewer line. If the smell fades once the trap refills, the problem is in the drain and not in the water. If it returns, have a licensed plumber inspect the drain and its vent.

Look closer at the water heater

If only the hot side smells, the water heater deserves a closer look. Read the owner's manual before you work on the unit, and shut off power or fuel as the manual directs. Many homeowners flush the tank to clear loose sediment. If the sulfur smell returns soon after flushing, bacteria in the tank are a likely cause. The anode rod, a sacrificial metal rod inside the tank, can also contribute to the reaction, and a plumber or water heater technician can inspect it. Keep in mind that a flush by itself may not solve a recurring problem.

Test the cold water supply

If the cold water smells, fill a clean glass from the cold tap and smell it right away. Then let the glass sit uncovered for a few minutes and smell it again. A smell that fades quickly suggests dissolved gas. A smell that stays, or a dark or slimy film in the toilet tank or on fixtures, may point to iron bacteria, which can grow in wells and pipes. Iron bacteria are a separate issue from dissolved sulfur, so a lab test that includes sulfides, iron, and bacteria will give you more useful answers than your nose alone.

When to call for help

Write down what you found before you call anyone. Note which taps smell, whether the smell is strongest hot or cold, whether it fades after sitting, and whether it appears near drains. For municipal water in St. Louis, contact your water provider with those details so they can check the supply on their side. For a private well, a well contractor or water treatment specialist can test the source and recommend next steps. If the smell is strong and you have not yet found the cause, consider using bottled water for drinking and cooking until you know more.

Keep a simple record

Note the date, which taps smelled, and what you tried, including any flush or drain test. A short log helps a technician work faster, and it shows whether a fix actually held over the following weeks.