Photo of a leak into a septic tank at the sewer line connection (C) Daniel Friedman Back-Flooded Septic Tanks
Diagnose & fix septic tanks with abnornal sewage levels or backflooded septic tanks

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Septic tank flooding or back flooding:

What causes flooded septic tanks or abnormally-high sewage levels in septic tanks and what do we do about it? A flooded septic tank can cause a sewage backup in a building or ejector pump flooding. Backflooding means that water or wastewater is flowing backwards into the septic tank from a soakbed or drainfield or from surface runoff.

To understand how to fix the problem of high waste levels in a septic tank we need to diagnose the cause, then outline, in order from least costly to more expensive, the approaches to fixing this problem.

Here we explain back-flooded septic tanks & an understanding of how to diagnose abnormal septic tank sewage levels can tell us what septic system repairs are needed. Do we just need to seal a pipe connection, clear a clogged sewer line, or do we need to control surface runoff, fix septic tank leaks, or replace a failed septic soak-bed?

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Septic Tank Flooding & Back-Flooding: Why it is important to diagnose & fix water leaking into the septic tank

Septic tank diagram shows normal sludge and scum thicknesses[Click to enlarge any image]

Reader question: my leach field floods in the yard and flows backwards into my septic tank

4/17/14 in the FAQs section of SEPTIC DRAINFIELD FAILURE DIAGNOSIS Joe Paciga said:

Hello, thanks for all the great information your site provides. I live in Southeaster VA, 60 feet above sea level. My house is 17 years old and I bought it 2 years ago. I have septic problems. I will try to make this short, but I will give you all the info too.

My leach field is flooding the surface of my yard. I have planned to replace the field, but I decided to talk to a professional about my problem before making a wrong decision.

Basically he said there is a "why" to the reason my field is failing. He asked do I have any leaking toilets or anything else, answer is no. I thought it may have been crushed by a heavy truck I had in the yard.

My own fault. I inspected the D box, all seems fine there. He said it sounds like you might be pumping extra water into field for some reason, and that's why I'm having problems. We have had the wettest winter in over 20 years. I have concrete 2 tank system. The 2nd tank to pump the effluent to back yard. Recently I had them pumped out the monitored the pump tank.

After a recent rainstorm of about 2 inches of rain, I checked the tank. The effluent level rose 2 feet overnight. So, there has to be a leak. Thus the extra pumping into my leach field and failure.

Ok, so is there a leak, but how and where. With the tanks relatively young, (I pour concrete for a living) I doubt they are broken. What about the gasket for tank tops or the pipe in between the 2 tanks. What would be the most likely ground water intrusion to the system?

No tree roots remotely close.

Reply: tips for diagnosing septic tank back-flooding problems

I've dealt with this leach field flood problem as have countless others for a long time now and appreciate the frustrations involved. The professional you cite names some reasons that the field could be flooded, but may be shooting in the dark in that unless we do some diagnosis we don't know what's actually going on and don't know what to fix.

I agree that a running toilet or water softener stuck in regeneration cycle can flood a drainfield.

So can water leaking into the tank at the tank top, at the tank inlet, tank outlet, or from a crack or damaged tank bottom, sides, or cover or from an improperly sealed septic tank access riser.

Water can also back-flood a septic tank (I'm more or less "inventing" this term but it will be obvious to most) by running backwards from a flooded septic drainfield into the septic tank.

The drainfield may be flooded by high ground water (which means the drainfield is not properly constructed and is too low), or by mishandling of surface or subsurface seasonal runoff that needs to be intercepted and directed away from the drainfield using a curtain drain or surface swale or both.

How to Diagnose a Flooded Septic Tank - a common cause of building sewage backups

While I agree that we would expect a relatively new septic tank to be un-damaged, it makes sense to actually inspect the tank so that we don't waste time with a lot of arm-waving speculation when a little bit of digging and looking can go a long way towards sorting out the problem.

Definition of Flooded Septic Tank: a septic tank in which the wastewater level is above the bottom of the septic tank outlet pipe's bottom-most surface is flooded, not working properly, and inviting a sewage backup into the building.

Note: here we are discussing septic tanks flooded by system construction, maintenance, site drainage control or other errors, or by a failed drainfield. Separately
at FLOODED SEPTIC SYSTEMS, REPAIR we discuss what to do when a septic system has been flooded due to area flooding from a hurricane or other storm causing high floodwaters.

Normal and abnormal septic tank sewage levels are explained
When the septic tank level is abnormally high we need to determine why, as knowing the cause defines the necessary repair.

In simple terms the septic tank wastewater level may be abnormally high because

  1. The septic tank outlet is blocked from one of several causes we'll detail below, such as a clogged pipe or a failed drainfield
  2. Water is leaking into the septic tank from surface or subsurface runoff at one of several possible leak points we will discuss below.
  3. The septic tank is back-flooding from one of several causes we'll detail below.

Definition of Septic Tank Back-Flooding: a septic tank that is suffering back-flooding is one into which wastewater can be observed entering the septic tank at its outlet end or outlet effluent pipe as the septic tank is being pumped or emptied.

A partially-full septic tank may still be considered flooded if it is filling too quickly

Watch out: a septic tank may also be technically flooded even if the total level of wastewater is below the septic tank outlet. How the heck could that be? We encountered just this condition at a home recently. The septic pumping company reported having completely emptied the septic tank during a period of wet soils and lots of surface runoff due to snow-melt.

A day later the building basement suffered a flood that owners correctly believed was due to surface runoff and snow melt leaking into the building. But in the course of diagnosing that condition the septic contractor returned to the home to inspect the septic tank liquid level again.

In a day after a (reportedly) 1,500 gallon septic tank had been pumped "empty" according to the contractor the tank was by his estimate 85% "full" again. The home, occupied by a single older resident, certainly had not produced 3/4 of a septic tank's worth of wastewater in a day.

As the owners believe there was no wastewater being produced by running toilets nor other water-dispensing equipment, it was evident that surface runoff and snow-melt that were flooding the home's basement were probably also the source of water entering the septic tank. Further diagnosis was needed to determine if the septic tank flooding was due to leaks into the tank or due to a failed drainfield that was also saturated by snow-melt

Relationship of Septic Tank Baffles to Septic Tank Flooding

Septic tank baffle (C) Daniel FriedmanThe tank baffle condition does not itself immediately cause a septic tank to flood or not.

Watch out: A broken or missing septic tank baffle on the tank inlet side invites sewer line clogs between building and tank and building sewage backups; a broken or missing septic tank baffle on the outlet end of the septic tank means we're pushing solids into the drainfield or soakaway bed, basically destroying that expensive component by clogging the soil around the drainfield trenches with solid waste, grease, scum, etc.

Details about septic tank baffle inspection and repair are at


There we explain that by inspecting the septic tank sewage level prior to pumping the septic contractor can see if the waste level is abnormally high (over the baffles - a flooded septic tank) or abnormally low (a leaky septic tank if the tank has been in use long enough that it should be full).

Open & Inspect Septic Tank Inlet & Outlet to Diagnose Tank Flooding

By opening the tank at its inlet and outlet ends, exposing the septic tank baffles and pipe connections, and inspecting with some thought, particularly if we make observations while having the septic tank pumped, we can see where water is coming in and thus know what needs to be done next.

  • If we see sewage marks over the top of the tank baffles we know the tank has been flooding.
  • If we see sewage wastewater that is above the bottom of the septic tank outlet pipe then the septic tank is flooded even if waste is not actually over the baffle tops.
PHOTO of repair method for a leak into a septic tank at the inlet pipe.
  • If we see waste water entering the septic tank at its inlet: With all water turned off at the building and nobody flushing toilets, we should not see water entering the septic tank at its inlet connection to the building sewer piping.

    • Groundwater leaks into the septic tank at its inlet pipe connection: If we do see water entering the tank when it's not coming from the building drain system, there may be groundwater leaks at the pipe-to-tank connection itself where the sewer pipe enters the septic tank or there is a different septic tank leak point (listed below).

      A poorly-sealed sewer line connecting the building sewer drain to the septic tank often causes leaks into a septic tank.

      The trench originally cut to run the sewer line between building and septic tank forms a natural surface or subsurface water catchments or intercept that, because it slopes towards the septic tank, also delivers all of that water to the septic tank connection.

      We repair this by sealing the connection and/or by excavating a diverter trench to send trench water away from the septic tank to a downslope location (not onto the drainfield).

      In our photo at above left we had a sewer line making a steep downhill plunge towards the septic tank inlet end. The trench ran at a slight diagonal across the hillside, making a very effective water catchment that sent all of that surface runoff and ground water right to the septic tank inlet where the pipe was not sealed at all. Our contractor poured a concrete seal around the tank at this location - not a repair that will tolerate future settlement of the tank or pipe, but ... well, it worked this time. (The bottom of this septic tank was by coincidence on bedrock).
  • If we see wastewater entering the septic tank at its outlet pipe connection or baffle: we need to figure out which of several unacceptable conditions exist, since this clue means that the drainfield is flooded, clogged, blocked, or basically at the end of its life, or that there is a groundwater leak into the system. In short we should not see water backflowing into the septic tank during pump-out nor at any other time.

    This condition is easier to detect if the septic tank outlet baffle cover is opened and watched during septic tank pumping and almost impossible to see otherwise, but an astute, experienced septic pumping contractor knows to listen during septic tank pumping. The step by step septic tank opening, pumping, and inspection procedure are documented

    As the waste level in the tank falls the pumper listens for the sound of splashing - of water entering the tank. By watching and listening at both inlet and outlet ends of the tank s/he can observe this important clue. Possible causes of back-flow into the septic tank at its outlet include:
    • Failed septic effluent pump (or check valve) in a pump-up system intended to pump sewage effluent to a mound or raised bed septic system that is located uphill from the septic tank itself. If your septic system works entirely by gravity you won't have this component.
    • Broken, clogged, damaged piping between the septic tank and Distribution box or D-box. This condition is not most-common but might be traced to vehicles driving over the drainfield area, tree roots, or settlement of piping due to poor construction
    • Broken, tipped, clogged D-box that is not sending wastewater to the individual drainfield lines. Not very likely since usually at least one of the drain lines will still be taking effluent, or flooding occurs first at the D-box itself since in a gravity system it's downslope from the septic tank.
    • Groundwater leakage into the septic tank at its outlet end pipe connection. A poorly-sealed sewer line at the septic tank outlet may permit local surface runoff or groundwater to leak into the septic tank.
    • Drainfield / soak-bed failure: a sluggish drainfield will accept wastewater out of the septic tank but at a rate slower than wastewater enters the tank. Depending on the extent of drainfield soil blockage (or partial drainfield piping blockage) the septic tank wastewater level may rise to flooded condition only when the system is under heavy use: multiple visitors, multiple loads of laundry.

      Watch out: running toilets or a water softener stuck in back-wash or regeneration cycle mode will also flood a drainfield and may show up as a flooded septic tank.
  • If we do not see water entering the septic tank at its inlet or outlet pipe connections, nor from the drainfield itself, there maybe other leaks into the septic tank. Check these locations:
    • Leaky septic tank cover: groundwater or surface runoff may enter at a septic tank cover itself (the cover of the entire septic tank)
    • Leaky septic tank cleanout or inspection covers: leaks at the septic tank top openings (there may be three, one at either end and one in the center used for cleaning) can flood a septic tank if there is surface or subsurface runoff in the area.
    • Leaky septic tank riser base: when a septic tank access riser is installed, the base of the riser needs to be sealed to the septic tank cover (typically using an epoxy putty) to prevent water entry at that point.
    • Septic tank bottom or side cracks or damage: the body of a septic tank may be cracked or have developed a hole due to vehicle traffic, tank settlement, tank age, or for steel septic tanks most commonly by rust. These openings may leak ground water into the septic tank in wet weather, flooding the septic tank, while in dry weather the inspector might find the septic tank wastewater level abnormally low as sewage effluent leaks out of the tank.

      Leaks in plastic, HDPE, fiberglass and concrete tanks can often be repaired. A rusted steel septic tank usually needs replacement.
    • More about septic tank leaks can be read at SEPTIC TANK LEAKS

Watch out: at FLOODED SEPTIC SYSTEMS, REPAIR [live link is given at Continue Reading just below] we explain how to fix a problem with septic tank or soakbed or drainfield flooding. But be warned, that while in an emergency we might pump the septic tank, simply pumping the septic tank won't fix this problem and worse, if the back-flow rate into the septic tank is rapid (as it often is) you are simply wasting your money as back-flooding from a drainfield or surface runoff will quickly fill the septic tank again in hours to days. Also

See SEPTIC BACKUP PREVENTION for tips to avoid a septic problem during times of anticipated heavy septic system usage.


Continue reading at FLOODED SEPTIC SYSTEMS, REPAIR to read suggestions for repairing a flooding septic tank or soakbed system, or select a topic from the More Reading links or topic ARTICLE INDEX shown below.

Or see PUMPING the SEPTIC TANK to continue with the septic tank cleaning & inspection procedure

Or seePUMPER TRUCK OPERATION PROCEDURE where we explain how to spot backflooding during septic tank pumping


Suggested citation for this web page

SEPTIC TANK BACK FLOODING at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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