SKETCH of a typical aerobic treatment unit tank, aerator, chamberFlooding Drywells & Seepage Pits

  • DRYWELLS ARE THEY ACTUALLY DRY? - CONTENTS: Seasonal flooding of seepage pits means they stop working. What is the failure criteria for a drywell? How do seepage pits flood or fail? How can we prevent drywell flooding? Prevention of seepage pit flooding.
  • POST a QUESTION or READ FAQs about flooded drywells and seepage pits or cesspools

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This document explains why drywells or seepage pits in many areas simply do not work, or do not work reliably all year round due to seasonal high water tables and flooding.

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We include suggestions on protecting seepage pits and drywells from ground water and flooding in order to extend their life. This material is a chapter of our Septic Systems Online Book: This document explains septic system inspection procedures, defects in onsite waste disposal systems, septic tank problems, septic drainfield problems, checklists of system components and things to ask. Septic system maintenance and pumping schedules.

Are "Drywells" really "Dry"? - Flooding Seepage Pits Stop Working

No. In many areas "drywells" are a misnomer since during wet weather as water tables rise, the drywell "pit" is not very dry and in fact may fill up with water and simply stop working.

In wet areas of the Northeastern U.S., for example, we disagree with the practice of "solving" a roof drainage disposal problem at a flat site by building a "drywell" since in our experience these fail rather soon and in some cases even fill up and work backwards, sending water back to a building footing drain or roof drainage system where water then leaks into the building!

Worse, if the "drywell" was buried close to the building foundation wall it may actually serve as a conduit, sending water against the building foundation and actually into the building.

If soil and site conditions are adequate for absorption trenches, seepage pits shall not be used.

A minimum three foot vertical separation must exist between the bottom of any pit and the high groundwater level, bedrock, or other impervious layer.

Below we suggest keeping surface runoff and subsurface groundwater away from your seepage pit or drywell to keep it working.

More Reading:

Tips for Extending a Drywell or Seepage Pit Life & Protecting them from Flooding

Direct surface runoff away from the seepage pit or drywell, including both on-surface water that can be controlled by a swale or intercept drain, and also ground-water that may require a deeper intercept drain conducting surrounding groundwater to a safe destination at least 20' away from the pit.

Also see Clearances for Septic Components Required - Well distance to Septic, Septic clearance from well, trees, lakes, streams, property boundary, etc.

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.


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