Turning on your kitchen faucet to fill a glass of water, only to be hit with an offensive “rotten egg” smell, is incredibly unsettling. Take a deep breath: while the odor is terrible, it is almost always caused by hydrogen sulfide ($H_2S$) gas or sulfur-reducing bacteria. This is primarily an aesthetic issue rather than a health emergency. Try the quick, systematic diagnosis below to figure out exactly where the smell is coming from and how to eliminate it.
The 2-Step Scent Test
To find the right fix, you first need to isolate where the gas is hiding.
- Step 1: Turn on a cold water tap and smell the water, then turn on a hot water tap and smell it again.
- Step 2: Isolate the source.
- If it’s only in the hot water: The issue is almost certainly a chemical reaction happening inside your water heater. Your water heater contains a magnesium anode rod that naturally reacts with sulfates in the water to create hydrogen sulfide gas.
- If it’s in both hot and cold water: The gas is originating directly from your groundwater aquifer, or it is being produced by bacteria hiding within your private well system.
- If the smell is only near the sink: The odor may actually be coming from the drain itself, not your water supply. Water flowing into a dirty drain can disturb built-up grime and release trapped odors. Because the drain is directly beneath the faucet, it creates a convincing illusion that the tap water is the culprit. (Pro tip: Fill a glass of water, walk into another room, and smell it to confirm).
The Fixes: How to Eliminate the Odor
Once you know where the smell is coming from, you can take direct action to resolve it.
- For Hot Water Only (Water Heater Issues): You will need to swap out the anode rod in your water heater. Replacing the standard magnesium anode rod with an aluminum/zinc rod or an electrically powered anode rod will stop the chemical reaction and usually solves the problem entirely.
- For Private Wells (Bacterial Issues): If the smell is isolated to your well system, the CDC and EPA recommend performing a “shock-chlorination” of the well. This heavy dose of chlorine will eliminate the sulfur-reducing bacteria causing the stench.
- For Ongoing High Sulfur Levels: If you have severe, persistent hydrogen sulfide gas coming from your groundwater, you will need a dedicated filtration setup. The Ten State Standards recommend using physical aeration or chemical oxidation (such as chlorination or ozone injection) followed by a media filter to oxidize and mechanically trap the sulfur gas before it reaches your taps.
- For a Smelly Drain: If your glass test confirmed the water is fine but the sink still stinks, simply clean and flush your drain lines and P-trap to clear out the decaying organic matter.