Sump pump guide
Why Does My Sump Pump Smell? Causes by Smell, and How to Get Rid of It
Most sump pump smells come from water sitting in the pit: bacteria in stagnant, low-oxygen water make a rotten-egg (hydrogen sulfide) or swampy odor. Cycling fresh water through, cleaning the pit and sealing the lid usually fixes it. A true sewage smell usually points elsewhere — a dry floor-drain trap, a sewage ejector pit, or the plumbing — and that is a job for a plumber.
Quick answer
What the smell usually means
- Rotten eggs with water in the pit: stagnant water. Flush, clean, seal.
- Sewer gas near a dry pit: often a dry floor-drain trap nearby.
- Sewage with a bolted lid and vent pipe: it's an ejector. Call a plumber.
- Burning smell: overheating pump. Don't touch; cut power safely.
Strong rotten-egg smell through the house: leave and call the gas utility.
First check
Is it actually a sump pump?
Basements can have two kinds of pit that look alike from a distance, and they smell for different reasons.
A sump pit collects groundwater from the drain tile around the foundation and pumps it outside. It should never receive sewage. It usually has a loose, snap-on or clear lid, a 1¼- or 1½-inch discharge pipe that leaves through the wall, and no vent pipe.
A sewage ejector pit collects waste from a basement toilet, shower or laundry that sits below the sewer line and pumps it up into the house drain. Because it holds sewage, the International Residential Code requires a gastight removable cover and a vent. If your pit has a lid bolted down with a gasket, a vent pipe coming out of the lid, and a larger discharge that ties into the house drain, it is almost certainly an ejector.
If it is an ejector and it smells, call a plumber. A sewage smell there usually means a failed lid seal, a vent problem or a pump issue, and opening a sewage basin exposes you to sewage and sewer gas. The rest of this page is about groundwater sumps.
Last reviewed: September 24, 2026
| Clue | Groundwater sump | Sewage ejector |
|---|---|---|
| Lid | Loose, snap-on, clear or missing | Bolted down with a rubber gasket, airtight |
| Vent pipe | None | A vent pipe rising from the lid |
| What enters | Perforated drain tile, sometimes a floor drain line in older homes | Pipes from a toilet, shower, sink or laundry |
| Discharge goes to | Outside: yard, drywell or storm system | The house sewer or main drain |
| Water looks like | Clear to cloudy groundwater | Dark, with solids or paper |
| Who fixes a smell | Often the homeowner (clean and seal) | A plumber |
The IRC's section on sumps and ejectors (P3007) calls for a gastight removable cover and a vent where a sump receives drainage from plumbing fixtures. Local codes vary.
Diagnose it
What your sump pump smell means
Find the smell, then do the first check before cleaning anything. Unplug the pump from a dry spot before touching anything in the pit.
| Smell | When it's worst | Likely cause | First check | Fix |
|---|---|---|---|---|
| Rotten eggs (sulfur), pit has water in it | After dry weeks when the pump rarely runs | Hydrogen sulfide from bacteria in stagnant, low-oxygen water and sludge | Is the water dark or is there black slime or sludge on the bottom? | Flush with clean water, clean the pit, seal the lid |
| Rotten eggs or sewer gas, pit is dry or nearly dry | Dry spells; smell strongest near a floor drain | Usually a dry trap in a nearby floor drain, not the sump itself | Sniff at the floor drain, laundry tub and any unused basement drain | Pour water into the floor drain; ask a plumber about a trap primer |
| Sewage (feces), with a sealed lid and a vent pipe | Any time; worse when fixtures drain | It is a sewage ejector pit, and its seal, vent or pump has a problem | Look for a bolted lid and a vent pipe (see the diagram) | Plumber; do not open the lid yourself |
| Sewage or soapy greywater smell in an open sump | When the washer, laundry tub or a sink drains | Greywater plumbed into the groundwater sump | Follow every pipe and hose entering the pit | Plumber; greywater usually needs its own sealed, vented system |
| Sewage after heavy rain | During or after storms | A leaking sewer line or septic field near the foundation, or a floor drain backing up | Is water coming up a floor drain? Is the pit water discolored? | Plumber with a camera inspection; avoid contact with the water |
| Musty or earthy | Humid weather; open pit | Mold and mildew in a damp basin, moisture from an open pit, soil gas through gaps | Is the lid missing, cracked or loose? Is the room humidity above 60%? | Clean and seal the pit; if the whole basement smells, see musty basement smell |
| Swampy, rotting or dead-animal | Any time, often suddenly | Leaves, algae or a drowned mouse, frog or insect in the pit or drain tile | Look into the pit with a flashlight (pump unplugged) | Remove debris with gloves, clean, fit a sealed lid |
| Burning, hot plastic or electrical | When the pump runs | Overheating motor: jammed impeller, running without moving water, failing motor or cord | Do not touch. Is the pump humming, hot or running nonstop? | Cut power from a dry spot if you safely can; call a pro. See sump pump not working |
| Gasoline, oil, solvent or chemical | Any time | Something spilled or poured nearby, a leaking oil tank or contaminated groundwater | Is there a sheen on the water? | Ventilate, no flames or sparks, do not run the pump into the yard; call the fire department or a pro |
| Rotten eggs through the house, not just at the pit | Any time | Possibly natural gas: utilities add a rotten-egg odorant so leaks can be smelled | Do not investigate | Leave, and call the gas utility or 911 from outside |
Why pits smell
What actually makes a sump pit stink
Stagnant water and sulfate-reducing bacteria
The water below the pump's off level never leaves the pit. In dry weather it can sit for weeks with leaves, silt and slime settling to the bottom. Oxygen in that water gets used up, and sulfate-reducing bacteria take over. Penn State Extension describes the same process in wells: these bacteria thrive in low-oxygen water and produce hydrogen sulfide, the rotten-egg gas, which most people can smell at very low concentrations. When the pump finally runs, it stirs the water and the gas comes out, which is why many people notice the smell right after a cycle.
That is also why shock-dosing a pit rarely works for long. Penn State notes that in wells, the odor often returns quickly after chlorination. The durable fixes are to remove the food (sludge and debris), replace the water more often, and keep the pit sealed.
The “dry sump lets sewer gas in” myth
Many pages say a dry sump pit lets sewer gas into the house, like a dry sink trap. For a groundwater sump that is usually not how it works: the pit connects to drain tile in the soil, not to the sewer. A dry pit can let soil gas in, including earthy smells and radon, which is one reason to seal the lid (see how common high radon is in your area on our radon levels by zip code map). But when a basement smells like a sewer during a dry spell, the most common culprit is a floor drain whose trap has dried out. Every floor drain has a water seal that blocks sewer gas, and it slowly evaporates in a drain that never gets used. Pour a gallon of water into each basement floor drain and see whether the smell goes.
There are exceptions. Some older homes have a floor drain or laundry line piped into the sump, and some sumps share piping with systems they should not. If you find any drain from a fixture entering your pit, have a plumber look at it. Sewer lines and backups are a plumbing job, not a waterproofing one.
Mold, mildew and an open pit
An open or loose-lidded pit is a small pond in the basement. It adds moisture to the air and gives mold a damp surface to grow on. The US Department of Energy's Building America Solution Center recommends a rigid, gasketed sump cover, noting that an open pit raises indoor humidity and that uncovered pits can let radon and other soil gases into the basement. If the musty smell is throughout the basement rather than at the pit, the pit is probably one contributor among several; see musty basement smell and basement humidity targets.
Fix it
How to get rid of sump pump smell, step by step
Work from the least effort up. Most smells go at step 1 or 2. Wear gloves and eye protection for anything beyond step 1, keep the basement ventilated, and unplug the pump from a dry spot before your hands go near the pit.
- Flush it with clean water. Pour a few buckets of clean water into the pit until the pump starts and runs a full cycle. This replaces the stagnant water with fresh, oxygenated water and also shows whether the pump, float and check valve work (the sump pump test uses the same method). Repeat every few weeks in dry weather if the smell returns.
- Pour water into nearby floor drains. If the smell is sewer-like and the pit is dry, refill the floor drain traps. If the smell stops, the sump was never the source.
- Clean the pit. Unplug the pump. Scoop out leaves, silt and sludge from the bottom, and remove anything that has fallen in. Lift the pump out only if it is on a union or quick-disconnect and you are comfortable doing it; otherwise have a plumber do it during a service visit. Clean the intake screen.
- Wash the basin with a mild bleach solution, if needed. The CDC's limit for household cleaning with bleach is no more than 1 cup of bleach per gallon of water, and a weaker mix is often enough for a pit. Scrub the walls, let it sit a few minutes, then rinse with clean water. Keep the room ventilated.
- Plug back in and cycle clean water through so that the pump sends out mostly rinse water, not bleach. The discharge goes onto your yard or into a storm system, so use as little as will do the job; some municipalities restrict what may enter storm drains.
- Seal the lid. Fit a rigid lid with a gasket and sealed openings for the pipe and cords. A replacement sump lid is commonly a $25–$80 part. Leave the pit accessible for testing.
Keep it away
How to keep the smell from coming back
- Keep the lid sealed. A gasketed lid holds odor, humidity and soil gas in the pit. If your house has a radon mitigation system, the sump lid is often part of it: do not remove or unseal it without checking with the installer.
- Cycle it in dry spells. A bucket or two of water every few weeks keeps the water fresh and doubles as a quick pump check.
- Clean the pit once a year, before the wet season, at the same time as the annual pump test.
- Fix what keeps water standing. A failed check valve sends the same water back to the pit every cycle, and a pump that runs constantly or short-cycles may be recycling water from the foundation.
- Keep the basement dry and ventilated. Humidity below 60% and some air movement help; see basement ventilation.
Who to call
When a smell is not a DIY job
| Situation | Call |
|---|---|
| Sewage smell from a sealed, vented pit (an ejector) | A plumber |
| Sewage smell after rain, or water coming up a floor drain | A plumber; sewer line inspection |
| A fixture or laundry line drains into the sump | A plumber |
| Burning or electrical smell, a hot pump, a tripped breaker | A plumber or electrician; never touch a pump in water |
| Rotten-egg smell through the house, not just at the pit | The gas utility or 911, from outside |
| Fuel or chemical smell, sheen on the water | The fire department or an environmental professional |
| Smell comes back within days after cleaning, with water always standing | A waterproofing or drainage contractor to look at the system |
A pump that smells hot or burnt has usually reached the end of its life. Replacing a pump in an existing pit is covered on the replacement cost page, and sump pump installation explains what a proper installation includes, a sealed lid among them.
Related pages
Where to go next
Musty Basement Smell
When the whole basement smells, not just the pit.
Read nextBasement Ventilation
Moving air without making humidity worse.
Read nextHow to Test a Sump Pump
The bucket test that also flushes the pit.
Read nextSump Pump Running Constantly
When the pump is busy but the water is going nowhere.
Read nextSump Pump Check Valve
Why water falls back into the pit.
Read nextBasement Humidity
Targets that keep mold and odor down.
Read nextFAQs
Sump pump smell FAQs
How do I fix a smelly sump pump?
Start by pouring a few buckets of clean water into the pit until the pump runs, which flushes the stagnant water out. If the smell comes back, unplug the pump, remove debris and sludge, scrub the basin, and use a mild bleach solution (no more than 1 cup of bleach per gallon of water, never mixed with ammonia or other cleaners), then rinse and cycle clean water through. Finish by fitting a sealed, gasketed lid.
Why does my sump pump smell like rotten eggs?
Usually hydrogen sulfide made by sulfate-reducing bacteria living in stagnant, low-oxygen water and sludge at the bottom of the pit. It is most common after long dry spells when the same water sits for weeks. Flushing, cleaning and sealing the pit usually fixes it. If the rotten-egg smell is strong and not coming from the pit, treat it as a possible natural gas leak: leave and call the gas utility from outside.
Why does my sump pump smell like sewage?
A groundwater sump is not normally connected to the sewer, so a real sewage smell usually has another source: a dry trap in a nearby floor drain, a laundry tub or sink draining into the pit, a sewage ejector pit that was mistaken for a sump, or a leaking sewer line near the foundation. Pour water into nearby floor drains first; if the smell persists, call a plumber.
Why does my sump pump smell like sewage after heavy rain?
Heavy rain can push water from a cracked or leaking sewer line, or a failing septic field nearby, into the soil around the drain tile, and it can also back up floor drains that share the building sewer. Either way the source is sewage plumbing, not the sump. A plumber can camera-inspect the line. Avoid contact with the water and wash your hands after touching anything in the pit.
Is sump pump smell toxic?
A faint musty or rotten-egg smell from a sump pit is usually an irritant and a nuisance rather than a danger. Hydrogen sulfide can be smelled at very low levels, far below workplace limits. Strong or worsening odors, headaches, nausea or eye and throat irritation are reasons to ventilate, leave the space and have a professional investigate. At high concentrations hydrogen sulfide can deaden your sense of smell, so fading odor is not proof it is gone.
What can I put in my sump pump for the smell?
Clean water is the first thing: cycling fresh water through replaces the stagnant water that bacteria live in. For cleaning, a diluted bleach solution (no more than 1 cup per gallon of water, CDC's limit for household cleaning) works on the basin walls. Do not mix bleach with ammonia, vinegar or drain cleaners. Check your pump manual before adding any chemical, and never pour oil, fuel or solvents into the pit.
What are the first signs of sewer gas exposure?
At the low levels found in homes, typical effects are irritation of the eyes, nose and throat, headache, nausea and dizziness. These should ease quickly in fresh air. If they do not, or someone has trouble breathing, get medical help. Any persistent sewage odor indoors is a reason to have a plumber find the source.
Why does my sump pump smell like it's burning?
A hot, electrical or burning-plastic smell usually means the motor is overheating, often because the impeller is jammed, the pump is running without moving water, or the motor or cord is failing. Do not touch the pump or reach into the pit. If you can reach the breaker or plug from a dry spot, cut the power, then call a plumber or electrician. If there is smoke, leave and call 911.
Sources
Sources
- ATSDR — Hydrogen Sulfide ToxFAQs (odor threshold, olfactory fatigue, health effects)
- Penn State Extension — Hydrogen sulfide (rotten egg odor) in water wells (sulfate-reducing bacteria)
- CDC — Mold clean-up (no more than 1 cup of bleach per gallon; never mix with ammonia or other cleansers)
- Washington State Department of Health — Dangers of mixing bleach with cleaners
- Building America Solution Center (PNNL) — Sump pump cover gasket (humidity, radon and soil gas)
- IRC Section P3007 — Sumps and ejectors (gastight cover, vent), via UpCodes
- Mr. Rooter — Trap primers and dry floor-drain traps (sewer gas)
- Liberty Pumps — Submersible Sump Pumps manual 7108000 (basin cover; warranty excludes pumping chemicals)
- US Department of Energy, AFDC — Natural gas safety (rotten-egg odorant)
This page is educational. It is not a gas-leak, mold or plumbing assessment; if in doubt about any odor, ventilate, leave and call a professional.