A rotten egg smell from a cold tap is one of the more alarming things a Missouri homeowner can notice at the sink. The odor is often hydrogen sulfide gas dissolved in the water, but the cause is not always the well itself. This guide describes how to sort the likely sources, what to check first, and when to stop and get the water tested before using it for drinking or cooking.
What the Smell Usually Is
Hydrogen sulfide is a naturally occurring gas that can dissolve in groundwater, and it smells like rotten eggs even when only a little is present. Some Missouri wells draw from bedrock or aquifers where natural sulfur-bearing minerals or local conditions can contribute to the gas. Bacteria that live without oxygen can also produce sulfur odors in a well, a pressure tank, a water heater, or a long stretch of pipe that has sat unused. A smell that comes and goes points to a different cause than one that is always present.
Sort the Smell by Where It Shows Up
Start with a simple survey before touching any equipment. Write down the answers to these questions.
- Is the smell in cold water only, or in both hot and cold?
- Does it appear at every tap, or only at one fixture?
- Did it begin after a long dry spell, a power outage, a pump repair, or a new treatment device?
- Is it strongest on the first draw in the morning, or after the water has run for a while?
Hot-water-only smells are a separate issue, and a water heater's anode rod is a common source. Cold-water smells point more toward the well, the pressure tank, or the plumbing between them.
Check the Home Side First
Run each cold tap for several minutes, then check whether the smell fades. Look at the drains too, since a sewer or drain odor can be mistaken for a water odor. Check any water softener, carbon filter, or other treatment equipment, because a filter holding trapped organic material can support bacteria and release odor. Do not open or change equipment without following the maker's written instructions, and keep notes of anything you do.
Check the Well Side
If the smell is at every cold tap, the well and pressure system are the likely area. Look at the well cap and vent for insects, debris, or damage. Look at the pressure tank and its connections for leaks or moisture. Note whether the pump has been running more or less than usual. A well that has been idle for a long period can develop odor when it starts again, which is one reason a careful restart routine matters.
Test Before You Decide
A smell is a clue, not a measurement. Missouri homeowners can have water tested by a state-certified laboratory that reports hydrogen sulfide and bacteria results. Ask the lab which sample container, handling steps, and holding time it requires, and follow those instructions exactly. Collect the sample from a cold tap that represents the well, after following the lab's flushing instructions. Field test kits can offer a quick look, but they do not replace a laboratory report when a result matters. Keep a copy of every report with your well records.
When to Stop and Get Help
- Do not drink or cook with water that smells strongly until you have a result.
- Call a licensed well contractor if the smell started after pump, tank, or cap work.
- Contact your county or state environmental office if you suspect contamination near your property.
- Recheck after any change, since odor can come and go over weeks.
Keep a Simple Record
Note the date, the tap, the time of day, the weather, any recent work, and what you did about it. A short log helps a tester or contractor see the pattern faster, and it helps you tell whether a change actually worked. Over time, the record also shows whether a smell belongs to the well, the home plumbing, or a device that needs attention.