Cold water that smells like rotten eggs almost always contains hydrogen sulfide gas, a compound produced when bacteria feed on sulfur compounds in groundwater, pipes, or plumbing fixtures. The human nose can detect it at concentrations as low as 0.05 milligrams per liter, so even a tiny amount creates a strong, unmistakable stench. The fact that the smell shows up in your cold water (not just hot) narrows down the possible sources considerably.
Why Cold Water Points to a Different Cause Than Hot
When only hot water smells like rotten eggs, the culprit is usually the water heater. Water heaters contain a magnesium rod designed to prevent corrosion, and that rod can chemically convert naturally occurring sulfates in water into hydrogen sulfide. But when the smell comes from your cold tap, the water heater is off the hook. The problem is upstream: in your well, your groundwater, your pipes, or in some cases your drain.
Before you go further, run a simple test. After being away from home for a few hours, fill a glass from the cold tap before letting any water run down the drain. Smell the water in the glass. If it smells fine, the odor is likely coming from bacteria growing inside your drain, not from your water supply. If the glass of water itself smells like sulfur, the problem is in the water.
Sulfur Bacteria in Wells and Groundwater
The most common biological source is sulfur-reducing bacteria living in your well, the surrounding groundwater, or your plumbing. These bacteria thrive in oxygen-poor environments and produce hydrogen sulfide as a byproduct of their metabolism. They also create a slime that can be white, grey, black, or reddish-brown (especially when iron bacteria are also present). That slime can clog wells, pipes, and irrigation systems over time, so the smell is often a sign of a larger maintenance issue.
Wells drilled into acidic bedrock like shale and sandstone are especially prone to hydrogen sulfide problems. The geology itself contains sulfur compounds that dissolve into groundwater, feeding the bacteria and sometimes producing hydrogen sulfide through purely chemical reactions as water filters through rock and organic material underground.
Municipal Water Can Smell Too
If you’re on city water, sulfur smells are less common but not impossible. Groundwater-fed municipal systems can carry hydrogen sulfide from the source. Decaying plant material in lakes and reservoirs also creates organic compounds that produce musty or sulfurous odors, particularly toward the end of summer when water temperatures are higher and algae blooms are more active. A sudden change in smell from city water is worth reporting to your utility, since it can sometimes indicate a disruption in treatment or distribution.
Water Softeners as a Hidden Source
If you have a water softener and the smell appears only in the softened water (not at an outdoor spigot or bypass tap), the softener’s resin bed may be harboring sulfur-reducing bacteria. The warm, low-oxygen environment inside the softener tank is ideal for these organisms. This is a commonly overlooked cause, especially when homeowners assume the problem must be in their well because the smell is in the cold water. Testing your water before and after it passes through the softener can confirm whether the unit is the source.
Is It Safe to Drink?
Despite the alarming smell, hydrogen sulfide in drinking water is not considered a health hazard at the concentrations typically found in household supplies. The World Health Organization reviewed the available evidence and concluded that it’s unlikely anyone could consume a harmful dose of hydrogen sulfide through drinking water, so no health-based guideline limit has been set. The EPA classifies the rotten-egg smell under its secondary drinking water standards for odor, which are non-mandatory guidelines for aesthetic quality rather than safety. In short, the water may be unpleasant, but it’s not poisonous.
That said, sulfur bacteria themselves can promote the growth of other organisms, including iron bacteria that stain fixtures and further degrade water quality. Addressing the problem improves more than just the taste and smell.
How to Fix the Problem
The right solution depends on where the hydrogen sulfide is coming from.
If the Source Is Your Well
Shock chlorination is the standard first step for well owners. This involves introducing a concentrated chlorine solution into the well and running it through the entire plumbing system at a concentration of 50 to 100 parts per million. You then let the chlorinated water sit in the system for 6 to 12 hours without using any taps. This kills sulfur bacteria in the well casing, pipes, and pressure tank. The process often needs to be repeated if bacteria recolonize, and some wells with persistent problems benefit from a continuous chlorination or filtration system.
If the Source Is Your Drain
If your cup test showed the water itself smells fine, bacteria in the drain are producing the gas. Cleaning the drain with a diluted bleach solution or a bacterial drain cleaner usually resolves it. This is especially common in sinks that aren’t used frequently, where standing water in the P-trap allows bacteria to thrive.
If the Source Is Your Water Softener
Sanitizing the resin bed with a manufacturer-approved cleaning solution can eliminate the bacteria colony. Some homeowners switch to a hydrogen peroxide or chlorine injection system upstream of the softener to prevent recolonization.
If You’re on City Water
Contact your water provider. They can test for hydrogen sulfide at your meter to determine whether the smell is entering from their system or developing inside your home’s plumbing. Running your cold water for a few minutes to flush standing water from the pipes sometimes eliminates the odor, which suggests bacteria are growing in the pipes themselves rather than in the supply.
Narrowing It Down
A systematic approach saves time. Start by checking whether the smell is in all cold taps or just one. A single fixture points to bacteria in that specific pipe or drain. If every cold tap smells, the issue is in the main supply, whether that’s a well, a water softener, or municipal water entering the home. If you have both a well and a softener, test water at the wellhead, after the pressure tank, and after the softener to isolate exactly where hydrogen sulfide levels spike. Local health departments and cooperative extension offices often provide low-cost or free water testing that includes hydrogen sulfide measurement.