This document explains the signs and causes of seepage pit or drywell failure and discusses how to avoid early failure or "fill-up" of the seepage pit.

This article series discusses What are Septic Drywells, What Drywell Maintenance is Needed? How do Drywells Fail? We warn readers that while installation of a drywell to receive graywater at a property may be a good idea, the use of drywells for this purpose may be a warning about the capacity and remaining life of the septic system.

How Seepage Pits or Drywells Work, Fill-up, and Eventually Stop Working

Seepage pits, drywells, or cesspools can collapse and thus be dangerous. The photo shows a contractor's truck after it drove over and collapsed a seepage pit being used to receive effluent from a septic tank.

This case is discussed later in this section. The general stages of pit life and failure are discussed first.

Stages in the Life of a Drywell

In a perfectly functioning, new seepage pit or drywell, wastewater enters the pit and seeps at first out of the pit bottom into the surrounding soils. The wastewater may be septic effluent if it's coming from a septic tank, or it may be graywater from other building drains if the pit is being used just for graywater disposal.

Regardless of the effluent source, as wastewater enters the seepage pit or drywell, it contains some non-dissolved solid particles. This debris settles to the bottom of the pit. The bottom of a seepage pit, drywell becomes clogged with scum and debris fairly quickly (and in the case of cesspools, very very quickly.

Watch out: Readers trying to diagnose and deal with sudden soil subsidence or yard collapses should

see CESSPOOL SAFETY WARNINGS as those hazards can also apply to drywells and septic tanks

Watch out:  falling into a collapsing drywell, cesspool, or seepage it can be a quick death.

As the pit bottom becomes sealed with settled debris, effluent entering the pit can no longer drain out immediately through the pit bottom surface. So as the seepage pit is used more and more, the level of liquid in the pit/drywell/cesspool rises. The rising wastewater then seeps out through the pit sides into the surrounding soil.

As the seepage pit ages, the soil around the seepage-out area of the pit bottom and lower sides becomes clogged and stops accepting wastewater. This causes wastewater inside the pit to rise still further, where it can exit the higher sides of the pit which are in contact with soil.

At the end of its life, the soil under the bottom and around the sides of a seepage pit has become clogged all the way up to near the very top of the pit. Then it's time to dig a new one.

Damaged or Collapsing Drywells, Cesspools, or Seepage Pits

When a contractor accidentally damages a seepage pit, such as when the dump truck shown in the photo above collapsed a seepage pit cover, the best repair would be to excavate the pit and repair any damaged components. If only the top were damaged it could be replaced. If the sides of the drywell were damaged on a site-built pit (such as one constructed of large stones or of concrete blocks), it may need to be rebuilt.

If a damaged drywell or seepage pit is also already near the end of its life (static liquid level near the pit top) it would make more sense to fill it in and relocate a new pit.

One correspondent wrote that after the truck (in the photo) collapsed her seepage pit cover, the contractor "repaired" the system by partially filling it in with gravel, saying that this would make no difference or would even improve the system.

Now I've seen a few drywells that were filled with large rubble as insurance against having the sides collapse inwards. But I'm not sure I agree with the contractor.

If instead of a rebuild the contractor dumps gravel into the pit s/he may be simply covering up debris which fell into the pit when it was damaged - the broken cover or other components.

By filling the pit and reducing its liquid volume capacity, we are forcing effluent to exit the pit higher on the pit's sidewalls and thus moving it more quickly to the end of its life.

So filling in a seepage pit with gravel both reduces its capacity to receive, buffer, and then dispose of effluent (you have less septic system capacity) and probably shortens its remaining life.

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2017-11-26 - by (mod) - foul odor might be from old cesspool

M Beecher,

Please try using the search box just about find our article series using these keywords

ODORS GASES SMELLS, DIAGNOSIS & CURE

Or see

ODOR DIAGNOSIS SIX STEPS - taking an orderly approach to odor source tracking

And you'll see a series of diagnostic steps that should help track down the odor source

On 2017-11-11 by M.Beecher

I have An old cesspool and a 20 yr old dry well.

I came home and smelled a foul order in the house but can’t tell where it is coming from .

Nothing is backing up and there are no soft spongy areas on my lawn. What is the cause of the odor?

On 2017-10-18 - by (mod) - how long is the life of a seepage pit?

Luigi

I can but speculate since I have no information except property age and number of pits: typically people add on a new seepage pit downstream from and daisy-chained from the old seepage pit when it stops working

- so your home could have three pits of which at most the third might be working -somewhat. An inspection of the seepage pit to find the sewage level will tell you if it's nearly full and thus at end of life. That inspection would be more valuable than a wild guess about seepage pit life, since life of a seepage pit depends on many variables including size, construction, soil properties, and level of use.

Let's clear up drywell (accepts only graywater) differences from cesspools (seepage pits that accept sewage) - see CESSPOOLS at https://inspectapedia.com/septic/Cesspool_Guide.php for details of what sounds as if it's your situation.

On 2017-10-18 by Luigi crispini

How long is the life expectancy of a seepage pit. Looking at a house that is 47 year old with a 3 seepage pit system, in place of a septic system due to property size restrictions.

Thank you, Luigi Crispini, loucrispini@gmail.com

On 2016-04-03 by Patrick

Thank you.

On 2016-04-02 - by (mod) - sinkhole with an open PVC pipe - was it a drywell?

Good question, Patrick. I can only speculate as we don't know where you live, soil conditions, home connections to the -whatever- that had the PVC pipe sticking (up?) nor much else.

IF the pipe was marking a drywell that was intended to catch and dispose of runoff around your home or from your roof, then perhaps it was collapsing and filling in the hole just continued to finish it off. That could contribute to a water backup into your basement.

BUT flash rainstorms dump a lot of water over a very short time. It's easy for that water volume to overwhelm roof or surface drainage systems around a home unless they were 1. designed for that water volume and 2. have been kept in good working order.

Finally: in my experience, "drywells" are ... well, sort of a mis-nomer.

Drywells are rarely dry, certainly they don't stay dry, and they're limited in the water volume that they can accept and limited in the rate at which they can dispose of it.

They make good sense in some situations - handling graywater for example, but you'd need a pretty enormous drywall to handle flash flood water volumes from heavy rainstorms or flood conditions.

On 2016-04-02 by Patrick

Last summer I noticed a sinkhole that had an open PVC pipe in it. I filled the sinkhole with gravels and dirt, and during February my basement flooded after a flash rainstorm. Could I have disturbed a dry well by covering in the sinkhole?

On 2016-03-05 - by (mod) - get rid of roots invading the drywell

You'd need to completely remove nearby plants.

On 2016-03-05 by Anonymous

I have a dry well that's brick lined, the bottom of the well at 15' is full of roots. Is there any thing that l can do to eliminate these roots?

On 2015-07-01 by Lance - had a seepage pit collapse after less then 5 years of being dug

In California, I had a seepage pit collapse after less then 5 years of being dug. Should I expect warranty repair from the contractor?

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Continue reading  at SEPTIC & CESSPOOL SAFETY or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• CESSPOOLS
• CESSPOOL FAILURE CRITERIA
• CESSPOOL AGE ESTIMATES
• CESSPOOL SAFETY WARNINGS
• CISTERNS, WATER STORAGE
• DRYWELL DESIGN & USES
  • DRYWELL SAFETY CONCERNS
  • DRYWELLS ARE THEY ACTUALLY DRY?
  • DRYWELL / SEEPAGE PIT SPECIFICATIONS
  • DRYWELL / SEEPAGE PIT CONSTRUCTION DETAILS
  • DRYWELL / SEEPAGE PIT COLLAPSE or FAILURE
• GREYWATER SYSTEMS
• SEEPAGE PITS
• SEPTIC & CESSPOOL SAFETY
• SEPTIC TANKS - home

 

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