A rotten-egg or sulphur smell in well water is most often caused by hydrogen sulphide (H2S), a gas that bacteria in the well can produce or that a water heater can generate. The right fix depends on where the gas comes from and how much is present. Start by checking whether the smell is in the cold water as well as the hot, then decide whether bacteria need to be controlled at the well. Only after measuring the concentration should you choose a filter or oxidation system, because each method works within a specific range.
Why does my well water smell like rotten eggs?
Health Canada (2024) describes hydrogen sulphide as a colourless gas with a characteristic rotten-egg smell, commonly found in soil, air and water. In private wells, the most common source is biological. Alberta Agriculture and Irrigation (2018) identifies sulphate-reducing bacteria as the usual cause, and recommends treating them before trying other measures.
The odour threshold is very low. Alberta Agriculture and Irrigation (2018) reports that H2S can cause odour problems at concentrations as low as 0.05 mg/L in well water. Penn State Extension (2024) notes that most people can detect it at levels well below 0.5 mg/L. Health Canada’s operational parameters guideline (2025) sets an aesthetic objective of 0.05 mg/L or less for sulphide, expressed as hydrogen sulphide, because it causes unpleasant taste and odour. That is a guideline about taste and smell, not a health-based limit.
The health concern is a separate issue. Health Canada (2024) notes that at high levels of exposure, hydrogen sulphide can damage the nervous system, and its operational parameters document states that the gas is acutely toxic when inhaled. The strength of the smell is therefore not a reliable guide to the concentration, since sensitivity varies from person to person. Measurement is the only way to know the level.
Is the smell only in the hot water?
Compare the two taps. If both cold and hot water smell, the source is most likely the well or the plumbing that carries its water. If only the hot water smells, the magnesium anode rod inside the water heater is a likely cause. Alberta Agriculture and Irrigation (2018) explains that this rod can react with sulphates in the water and produce H2S.
Alberta’s guidance suggests a sequence. First, disinfect the tank with chlorine bleach or hydrogen peroxide. If the smell persists, replace the magnesium rod with a zinc or aluminum rod, which can reduce the odour. Removing the rod often fixes the smell, but it voids the warranty and may shorten the life of the tank. Before changing the anode, read the water heater manufacturer’s documentation for approved rod types, warranty terms and disinfection steps. Turn off the power or gas supply, shut off the water, and let the tank cool before draining it to avoid scalding. Never mix disinfectants or cleaning products in the tank. If you are not comfortable doing this work, a licensed plumber can do it.
How do I test for hydrogen sulphide safely?
Measuring the concentration is the step most often skipped, yet it determines which treatment will work. Penn State Extension (2024) notes that H2S testing must be done on-site, or the sample must be chemically stabilized before it goes to a laboratory, and that not all labs test for it. Health Canada’s operational parameters guideline (2025) describes standardized methods, online analyzers and test kits. It also notes that zinc acetate is commonly used to preserve samples at collection, and that samples should be collected with minimal agitation.
Call the laboratory before ordering, ask whether it tests for sulphide, and follow its sampling and preservation instructions exactly. Results from an accredited laboratory or your local public health unit are the most reliable.
Test for bacteria separately. Health Canada (2024) recommends testing well water at least once every six months for E. coli and total coliforms, and sooner when colour, taste or smell changes. If E. coli is found, the water is not safe to drink. Boil it for drinking, cooking and brushing teeth, or use another safe source until retesting shows it is safe. Chemical testing frequency varies by contaminant, so check with your local public health authority.
When is shock chlorination the right first step?
Shock chlorination is the standard first treatment for sulphate-reducing bacteria in a well, according to Alberta Agriculture and Irrigation (2018). That source also notes that badly biofouled wells need professional cleaning by a well driller. Shock chlorination is a temporary fix, however. Penn State Extension (2024) reports that in most cases the bacteria and the odour return quickly afterward. If the smell comes back, the bacteria are regrowing and need ongoing control, or a permanent treatment system.
Dosing, contact time and flushing depend on well depth, diameter and the product used, so follow the disinfectant manufacturer’s documentation. Concentrated chlorine is hazardous to handle. Wear eye protection, gloves and suitable clothing, work in a ventilated area, and never mix products. If you are unsure how to calculate the dose or flush the system safely, a licensed well contractor should do the work.
Which treatment removes the sulphur smell?
Choose the method by the measured H2S level, then confirm the design with the equipment manufacturer. Penn State Extension (2024) gives the typical ranges below. These are design guidelines from a United States university, not regulations that apply in Canada.
| Method | Typical H2S range | Notes from the sources |
|---|---|---|
| Activated carbon filtration | Below about 1.0 mg/L | Penn State Extension (2024) |
| Chlorination with a feeder | Any level, most often above 6.0 mg/L | Penn State Extension (2024); about 2 mg/L of chlorine per 1 mg/L of H2S, per Alberta Agriculture and Irrigation (2018) |
| Oxidizing filters | Up to about 2 to 3 mg/L | Penn State Extension (2024) |
| Aeration | Usually not above about 2 mg/L | Removes most H2S, but chlorination may still be needed (Alberta, 2018) |
| Manganese greensand filter | Typically under 2 mg/L | Needs more frequent regeneration; a chlorine feed is usually the better choice (Alberta, 2018) |
| Potassium permanganate | Above 6.0 mg/L | Penn State Extension (2024) |
| Catalytic carbon | Needs at least 4.0 mg/L dissolved oxygen in the source water | Penn State Extension (2024) |
Alberta Agriculture and Irrigation (2018) also lists hydrogen peroxide and ozone as oxidizers. After any oxidation step, a filter is still needed to remove the resulting iron sulphide and iron oxide particles. Aeration results depend on pH: Alberta notes that the high pH of most Alberta waters can leave odour behind, and that lower pH improves results.
Can a water softener help or make it worse?
None of the sources consulted presents a softener as a reliable hydrogen sulphide treatment for a private well. Penn State Extension (2024) warns that in rare cases, adding water treatment equipment such as a softener may cause H2S production, because it can create conditions that let the bacteria grow. Health Canada’s operational parameters document lists softening among general options for hydrogen sulphide removal in municipal systems, but that does not show it is effective on a private well.
If the smell started after a softener was installed or serviced, check the softener’s manufacturer documentation for maintenance and disinfection, and ask the installer to confirm the work. Do not assume the softener is the solution, and do not treat it as a substitute for measuring H2S.
When should I call a certified water professional?
Call a licensed well contractor when shock chlorination does not stop the smell, when the well needs cleaning, or when the well or pump needs inspection before disinfection. A water treatment professional can size a filter or oxidation system from a laboratory result and explain the maintenance it requires. Contact your local public health unit to learn which chemical tests your well needs, since recommended schedules differ by province and region.
If you are worried that you have been exposed to hydrogen sulphide gas, Health Canada (2024) advises talking to a health care provider. Do not enter a well pit, well house or other enclosed space where the smell of rotten eggs is strong. Ventilate the area first and call a professional.
Frequently asked questions
Can I shower or bathe in water that smells of sulphur?
The official sources consulted do not address bathing directly. The odour is an aesthetic concern, but a positive E. coli result means the water should not be used for drinking until it is cleared, and you should ask your public health unit about other uses.
How long should I wait after shock chlorination before drinking the water?
The official sources consulted do not give a waiting time. Follow the disinfectant manufacturer’s documentation, flush the system as instructed, and retest before drinking the water.
Can the sulphur smell harm my pets?
The official sources consulted do not address pets. If an animal drinks the water and shows signs of illness, contact a veterinarian promptly.
