Your septic system in heavy rain or a flood: what to do

A septic system depends on dry soil. The drain field sits in ground that normally has air in it, and wastewater from the tank filters down through that ground to be treated⁴. When rain or floodwater fills the soil, there is nowhere for the wastewater to go. Penn State Extension puts it simply: too much water in the drain field can overload it and slow or stop treatment, and wastewater can be pushed back into the home².

So the rule for the whole event is the same, from a week of rain to a flood: the less water you send down the drains, the better. EPA says that when the water table is high, the only way to prevent a backup is to use the system less¹.

Heavy rain, but no flood

You may notice drains that are slow, a gurgle in the plumbing or wet ground over the field. NC State Extension lists drains that slow down after rain among the signs of possible trouble⁵.

• Cut water use until the ground dries. Shorter showers, no laundry, and fix that running toilet.
• Check where the rain is going. Roof and driveway water should drain away from the tank and the field, and downspouts shouldn’t point at the field².
• Stay off the field. Wet soil compacts easily, and compaction can lead to failure¹.

If it happens after every storm, the field is struggling. See the signs of septic system failure.

Before a flood

If flooding is forecast and you have time²³:

1. Reduce water use now. Limit showers and laundry.
2. Know where everything is. Washington State’s health department advises keeping accurate records of where the tank and drain field are.
3. Line up alternatives, such as a laundromat and public restrooms.
4. Ask about a backflow preventer. In flood-prone areas, Penn State says a licensed plumber can install one on the building sewer so sewage can’t back up into the house.

During a flood

Do	Don’t
Stop using the system if the field is under water	Don’t use toilets, showers, sinks, the washing machine or the dishwasher
Plug basement drains that connect to the septic system	Don’t pump or bail floodwater into sinks or toilets
Turn off electricity to any septic pump	Don’t pump the septic tank
Turn off automatic water softener regeneration	Don’t drive, dig or run equipment on the drain field

Sources: EPA, Penn State Extension, Washington State Department of Health.

EPA calls plugging the basement drains and drastically cutting water use “the best solution” while the ground is flooded¹. Washington State’s list is the bluntest: stop using toilets, showers, sinks, the washing machine and the dishwasher³. Penn State describes the tank as a holding tank until the water goes down².

Why you shouldn’t pump the tank yet

It feels like the obvious thing to do, and it is the one thing all the sources warn against.

• An empty tank can float. EPA says that pumping a tank in saturated ground could, in the worst case, make it float out of the ground and damage the inlet and outlet pipes¹. It adds that newly installed systems are more likely to pop out than older ones¹.
• It doesn’t solve the problem. EPA calls pumping at best a temporary fix¹.
• It can pull dirt in. Penn State warns that pumping during a flood risks drawing solids and sediment into the system², and EPA says not to open the tank while the soil is saturated, because mud and silt can get in and reach the drain field¹.

After the water goes down

1. Wait before using it. Don’t use the system until the water in the drain field has dropped below the level of the water around the house¹. Penn State notes this may take several weeks².
2. Tell your local health department if the system or the field was affected³. It can advise you and give you a list of septic contractors¹.
3. Have it inspected by a licensed septic professional once the soil has dried³, especially if you see settling or the system won’t accept water¹.
4. Pump out the silt. EPA says to pump both the tank and any lift station as soon as possible after the flood, once the ground is no longer saturated¹. If the tank has to be emptied while the ground is still unusually wet, Penn State suggests refilling it with clean water so it doesn’t float².
5. Check the filters, covers and ports. Make sure filters aren’t clogged with sediment³, the manhole cover is secure and the inspection ports aren’t blocked or damaged¹.
6. Repair the surface. Fix erosion and reseed so the tank and field have grass cover again¹.
7. Keep machinery off the field until the soil is dry¹.

Systems with extra treatment units need more work. EPA notes that aerobic units and media filters clog easily with mud and sediment and have to be washed and raked¹.

If sewage came into the house

EPA’s instructions¹:

• Clean the area and disinfect the floor with a chlorine solution of half a cup of bleach to each gallon of water.
• Remove floodwater from the house without dumping it into sinks or toilets. Sending it through the tank can push solids out into the drain field.
• If the system backs up after the flood, check the tank for a blocked outlet first. Flooding can lift the scum layer and partly plug the outlet.

Keep people and pets away from the sewage, wear gloves and wash thoroughly afterwards. Never mix bleach with other cleaners.

If you have a well

A flood that reaches the septic system may have reached the well too. EPA’s advice is short: don’t drink well water until it has been tested, and contact your local health department¹. Penn State suggests testing for E. coli and nitrate².

Use bottled water in the meantime. Our guides explain what a positive bacteria test means and how to shock chlorinate a well after it has been flooded.

Before the next storm

Most of the prevention is ordinary drain field care. Penn State’s list for flood-prone properties²:

• Route roof and driveway runoff away from the tank and field, and consider berms to redirect surface water.
• Keep sump pumps and foundation drains disconnected from the septic system.
• Have the system inspected every year and look over the field for odors, wet spots or surfacing sewage.
• Pump the tank on schedule.
• Ask a licensed plumber about a backflow preventer, and about waterproofing the electrical connections on any pump.