Why your sump pit smells like rotten eggs in late summer.

A sump pit that smells like rotten eggs in August has usually dried out. The water that normally sits in the bottom of the pit acts as a seal, and when a dry stretch evaporates it, hydrogen sulfide gas from the soil comes straight up into the basement. Pour a few gallons of water into the pit and the smell usually clears within an hour. If it doesn't, something else is going on.

A sump pump that ran every other hour through the spring can sit idle for weeks by the middle of August. That's good news for the pump, but a quiet pit is also when a basement that's never smelled like anything starts to smell like rotten eggs, and it catches people off guard because nothing seems to have changed.

Why a dry pit smells

Your sump pit is connected to the drain tile that runs around the foundation, and through it, to the soil under and around your house. Normally there's a few inches of water sitting in the bottom of the pit, because the pump shuts off before it empties completely. That standing water works like the trap under a sink: gases in the soil can't get past it.

The rotten egg smell is hydrogen sulfide, a gas that occurs naturally in groundwater and soil. When a long dry spell evaporates the water in the pit, there's nothing holding it back, and it drifts up into the basement. The pump isn't broken and nothing is leaking. The seal just dried up.

The five minute fix

Pour two or three gallons of water into the pit. If the smell was from a dry pit, it should fade within an hour or so.

While you're there, keep pouring until the float rises and the pump kicks on. Watch it empty the pit and shut off on its own. That's the same test in our sump pump maintenance checklist, and doing it now, in the quiet month, means you'll know the pump works before the fall rain shows up. If the dry weather continues, top the pit off with a bucket every week or two.

Seal the lid

A lot of pits around here have a loose cover, a cracked one, or no cover at all. A sealed lid with a gasket keeps soil gases and humidity in the pit instead of your basement, keeps debris out of the pump, and keeps kids and pets from getting hands or paws near it. If your home has a radon mitigation system, the sump lid should already be sealed as part of it, and a gap there is worth fixing for that reason alone.

When water doesn't fix it

• The pit needs cleaning. Silt, sludge, and bacteria build up in the bottom of a pit over the years, and stagnant water on top of that smells no matter how much you add. Unplug the pump first, then scoop out the debris and rinse the pit. If the pump sits in muck, that's a good time to check its intake screen too.
• A household drain runs into the pit. In some older homes, the washing machine or a floor drain was piped into the sump. That sends soapy, linty water into a pit that sits warm all summer, and it smells like it. The sump is meant for groundwater, which is why it discharges outside, so this is a setup we recommend correcting.
• It's not a sump pit. If you have a basement bathroom, there may be a second, sealed pit with a sewage pump in it. A sewage smell from that one means a seal or vent problem, and it needs a plumber, not a bucket of water.

Use the quiet month

August is the easiest time of year to look at your sump pump, because it's not busy. Beyond the bucket test, listen for a check valve that thunks or lets water fall back into the pit, and check the pump's age. Most last 7 to 10 years, and our list of signs a sump pump is failing covers what to look for. If you have a battery backup, check that its battery isn't showing a warning. Fall rain around Milford, Novi, and Farmington Hills is a few weeks away, and a pump problem is a lot easier to deal with before it starts.

Our sump pump page covers installation, repair, and backups if anything turns up.

Quick answers

Why does my sump pump smell like rotten eggs?

Usually because the pit dried out. The standing water in the bottom of the pit normally blocks hydrogen sulfide gas from the soil, and when a dry spell evaporates it, the gas comes into the basement. Pouring a few gallons of water into the pit usually clears the smell within an hour.

Is a sump pit smell dangerous?

At the levels a dry pit usually releases, it's mostly a nuisance, but you shouldn't have to live with it. A strong or persistent sewage smell, especially from a sealed pit near a basement bathroom, is worth having a plumber look at.

Should there be water in my sump pit?

Yes, a few inches at the bottom is normal. The pump shuts off before the pit is completely empty, and that standing water helps keep soil gases out of the basement.

Countryside Plumbing is a licensed master plumber serving Milford, Novi, Farmington Hills, and southeast Michigan since 1979. Sump pump installation, repair, and battery backups are a big part of what we do. Free quotes at 248-705-7543.