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Drywell or seepage pit construction & uses: this document explains what a drywell (or seepage pit) is, describes how drywells are used, gives safety and maintenance advice for drywells, and defines the criteria for drywell failure.
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.
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This material is a chapter of our SEPTIC BOOK, FREE ONLINE. 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, and also see SINKHOLES, WARNING SIGNS. Additional septic system safety warnings are at SEPTIC & CESSPOOL SAFETY.
We use the terms "drywell" and "seepage pit" differently as follows:
DRYWELLS for GRAYWATER - Drywells as Graywater Disposal Facilities
A drywell or seepage pit, sometimes called a leaching pit, leaching pool, or incorrectly a cesspool, is a covered pit with an open-jointed or perforated lining through which septic tank effluent seeps into the surrounding soil.
In other words this is one of the oldest and simplest methods of disposing of gray water or in some cases septic effluent - a simple pit or hole in the ground, open to soil at its sides and bottoms, intended to receive and dispose of gray water (water from building non-sewage drains such as laundry, showers, sinks).
A drywell, or "seepage pit" is used at some building sites to receive "gray water" from a laundry, sink, or shower. The pit may be site-built of stone or dry-laid concrete block, rubble-filled, or constructed of (safer) pre-cast concrete.
Modern drywells may also be sold as pre-designed units constructed of fiberglass or plastic. A drywell design may be similar to that of a cesspool, but only gray-water and not sewage is discharged into a drywell. The hole or absorption pit is typically 6-8 feet deep and 4-10 feet square (or round) depending on the amount of drainage to be handled and the percolation rate of the soil.
Some drywells were filled with large rubble. A top of concrete or stone slab is used to keep surface water and children out of the opening, and is usually covered and graded to be invisible in the yard. Older covers of wood or steel may be very unsafe and present a child or adult hazard, possibly including the risk of fatality. Mark off, prevent access to, and investigate such systems immediately.
What's the difference between a seepage pit or drywell and a cesspool?
The term "drywell" or "seepage pit" might be used by some people to describe a simple pit for disposing of septic effluent. Note that I said effluent not sewage. By contrast, a cesspool holds both solid waste and septic effluent. I use the term "seepage pit" to refer to a pit used to dispose of septic effluent which originated as blackwater, the solids having been retained in the septic tank.
I use the term "drywell" to refer to a pit used to dispose of graywater (greywater) which originated in sinks, laundry facilities, or showers. While their uses and implications of their presence at a property are quite different, the actual construction details of a seepage pit or a drywell are about the same. Let's sum up the use of these terms being rather particular:
Drywell: a hole in the ground intended to receive graywater from sinks, showers, or even roof or surface runoff. It may be site-built using stone or concrete block, or it may be constructed of pre-cast concrete buried in a pit and surrounded by gravel to increase its capacity.
Seepage pit: a hole in the ground intended to receive septic effluent such as the outlet from a septic tank. Seepage pits may be permitted where site space or soil conditions do not permit a conventional leach field. However even if effluent is successfully "disposed-of" it is probably not being adequately treated if it's coming out of a conventional septic tank. A Seepage pit might be constructed just as the drywell above but it is likely to require different (larger) site clearance requirements.
Cesspool: a hole in the ground intended to receive sewage or blackwater from a building. The cesspool may also receive graywater, or in order to reduce the loading of the cesspool, some buildings may direct their graywater to a separate drywell. The cesspool might be constructed just as the two systems above but may have different site clearance requirements.
Why are drywells installed?
A drywell is used at a property typically for these reasons
To relieve the liquid load on an onsite waste disposal septic system leach field, particularly in an area where the leach field is in trouble, at or near the end of its life, or has very limited capacity to dispose of effluent
To receive gray water from a building plumbing fixture (laundry or sink) which has been installed at a location so remote from the main house drain (to the septic system) that it is more convenient to construct a separate, nearby outside facility to receive this graywater than to route an indoor or outdoor buried rain to the existing septic system.
(Readers are invited to send additional suggestions)
Tips for Extending Drywell Life
Some experts recommend installing a filter on water entering the drywell. Particularly for a drywell used to receive water from a clothes washing machine, installing a lint filter between the washing machine and the drywell can extend the life of the drywell by reducing the moment of soil-clogging particles of lint and debris into the system.
Intermittent dosing systems such as are used for some alternative septic system designs, can also be adapted to graywater systems. In simple terms, this means that multiple drywells are used, and graywater is routed intermittently among them, giving the unused drywell time to recover. A simple valve system on the graywater drain line can serve to route greywater (graywater) to alternating drywells.
Cesspools for more in-depth information about those systems.
Using Drywells & Catch Basins for Basement Waterproofing
Reader Question: The drywell at my new home stays full of water and my house floods: who is responsible for the repair?
The gutter downspouts and sump pump are being led to the drywell however it is overflowing at all times and is not taking on any new water.
Even the catch basin is filled with water and will never completely drain into the drywell. to temporarily try to resolve this issue I have removed all downspouts and sump pumps out of drywell and are now draining onto my property which inevitably is flooding but better outside my home than in my home.
It is new construction and I am basically looking for a little advice on whether this is a structural defect and builder should be responsible for this or is it going to fall on me and how can this be properly fixed. - M.N. Staten Island, NY
Reply: For your site the drywell may have never been a workable solution to handling runoff; real estate attorney should review your contract of sale and advise about the new home warranty law
Was the Drywell a Usable Solution for your Building Site?
At ARE DRYWELLS DRY? we express the view that in some areas of high water table and wet soils, a drywell is not likely to be a useful means of collecting and disposing of surface or roof runoff in wet weather. That's because in those areas the "drywell" is in fact wet, or flooded, or at least partially flooded seasonally if not all year long.
From your description it sounds as if one or more of these problems underlies the flooding problem in your new home
The site is one that has a high water table, keeping the drywell flooded - this seems most likely from your description.
The drywell was improperly constructed, too small, or lacking proper site preparation, for example by setting a pre-cast drywell into a pit without adequate pit size or adequate gravel
Groundwater, surface runoff, or roof runoff directed into the drywell are of much larger volume than anticipated.
Who is Responsible for Groundwater, Roof Runoff, or Flooding in a New Home
This question is an ugly morass that is unfortunately all too common in new construction projects, and it is one that needs assistance from an attorney expert in real estate law and familiar with local building codes in your neighborhood.
Our photo ( left) shows a combination that is a near guarantee of a flooded basement: the builder constructed the deck before final site grading, leaving in-slope grade and a virtual pool under the deck and draining towards the home. And gutter improprieties send water running down the house wall during rain.
In general one would expect that proper construction of a new home would include taking whatever means were necessary to protect the home interior from water entry.
And one would wish that initial site investigation (for questions such as where is the water table?) and building code approval would have reviewed any special water or drainage requirements that the builder needed to meet.
Some new home construction contracts excuse the builder from certain home waterproofing steps, explicitly or implicitly leaving those responsibilities up to the homeowner. And unfortunately some new homeowners are unaware that those additional steps are not optional if the home is to be protected from flooding.
When the home floods at the first rain, or when it floods three years later after footing drains have become clogged with silt, the owner and builder end up in a finger-pointing contest that might have originated in the these two basic questions, even if other site water problems are absent:
When a new home site combines both problems 1 and 2 above, there is a very high risk of basement or crawl space water entry and ultimately flooding. Just how quickly flooding appears depends further on other site conditions:
Some possible solutions to the Flooded Drywell Problem
Your question does not include information about your neighborhood and site nor specifics about the home. But it may be useful to start by asking:
Continue reading at DRYWELL SAFETY CONCERNS or select a topic from the More Reading links shown below.
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