What is a seepage pit?

A deep, vertical excavation that receives effluent from a septic tank and lets it disperse into the soil through the walls and floor of the pit. It does the same job as a leach field, but it does it downward through a small footprint rather than outward through a large shallow one. Other names for the same idea include a dry well, and in some places a cesspool, though that word is also used for something different.

Is a seepage pit the same as a cesspool?

Not quite, and the words get used loosely. A seepage pit normally receives effluent that has already passed through a septic tank, so the solids have settled out first. A cesspool in the strict sense receives raw waste with no tank ahead of it. If you have an older Arizona property, which one you actually have is worth establishing rather than assuming, and an inspection is how you find out.

Why were they used so much in Arizona?

Because they suit the terrain. A pit reaches deeper soil layers through a small surface footprint, which is useful where a shallow restrictive layer such as caliche would defeat a conventional field, where the ground is rocky, or where there is not much level area to work with. Those conditions are common across the Sonoran Desert.

I am selling a house with a seepage pit. What do I need to know?

The same rule applies as to any other on-site system. Under A.A.C. R18-9-A316 the seller retains a qualified inspector within the six months before transfer, the inspector documents condition and deficiencies, and the seller hands the buyer that report plus the permitting and maintenance documents in their possession. On an older pit those documents are often thin or missing, so start looking early.

Seepage pits in Arizona

If you own an older property in Maricopa or Pinal County, there is a reasonable chance the septic system is not the tank-and-leach-field arrangement most people picture. Arizona has a long history of seepage pits, and they work on a different principle. This page explains what they are, why the desert favoured them, and what to establish if you have one.

What a seepage pit actually is

A seepage pit is a deep vertical excavation that receives effluent from a septic tank and lets it disperse into the surrounding soil through the pit's walls and floor.

Compare that with a conventional disposal field, which spreads effluent horizontally through shallow trenches over a large area. Both are doing the same job, dispersing treated effluent into soil that finishes the treatment. They just do it in different directions:

A leach field goes wide and shallow. It needs a large area of suitable soil close to the surface.
A seepage pit goes narrow and deep. It needs suitable soil at depth, and very little surface area.

Why the desert favoured the vertical answer

Once you see it as a geometry problem, the Arizona preference makes sense. The conditions that make a wide shallow field difficult are exactly the ones this region has plenty of.

Caliche near the surface. A cemented hardpan layer stops water moving downward through a shallow trench. A pit can pass through it to soil below.
Rocky or broken ground. Excavating a large shallow field in rock is expensive and sometimes impractical. A single deep excavation is a smaller intervention.
Limited level area. On sloping or constrained lots there may simply not be room to lay out a conventional field within setbacks.

So a seepage pit is not a cheap shortcut, whatever their reputation. It is a design answer to a particular set of ground conditions, and those conditions are common here.

Seepage pit, dry well, cesspool: the words matter

These terms get used interchangeably and they should not be, because the difference changes what you own.

Seepage pit. Receives effluent that has already been through a septic tank, so the solids have settled out first.
Cesspool, used strictly. Receives raw waste with no tank ahead of it, so there is no settling stage at all. A different and more problematic arrangement.
Dry well. Used loosely for both of the above, and also for a stormwater structure that has nothing to do with sewage. Context decides which is meant.

On an older property, working out which of these you actually have is a real question rather than pedantry. An inspection settles it, and it is one of the things a transfer inspection will surface if you are selling.

What to establish if your property has one

Is there a septic tank ahead of it? This is the seepage pit versus cesspool question, and it is the first one.
Where is it, and how deep? On older properties this is frequently unknown, which is its own problem at sale time.
Is there any permit record? Under A.A.C. R18-9-A316(D) the permitting and maintenance documents in your possession go to a buyer, and older systems often have thin records.
How is it performing? Slow drains, backups that return quickly after pumping, or surfacing are the signals, same as with any other system.
What would replacing it involve? Worth knowing before you need the answer urgently, since the ground conditions that led to a pit are still there.

Can you still install one?

We are not going to give you a yes or no, because that would be exactly the kind of confident unverified answer this site tries not to publish. Design criteria for on-site wastewater systems sit in A.A.C. Title 18, Chapter 9, Article 3, and they are applied by the county that governs your property. Whether a pit is an available design for a specific site is a question for that county and for a designer, and the answer may depend on the site.

Status as of July 2026.

Which county permits your system

Arizona delegates on-site wastewater permitting to counties, and the two in this service area run separate programs. Find your county before you start any paperwork.

Maricopa County

Permitting and inspections run through Maricopa County Environmental Services. Covers the unsewered Phoenix metro fringe: Buckeye, Cave Creek, New River, Rio Verde, the Surprise fringe and the Queen Creek edges.

Source: maricopa.gov, Onsite Wastewater program.

Pinal County

Permitting and inspections run through Pinal County Environmental Health. Covers Casa Grande, Apache Junction, San Tan Valley and the city of Maricopa, which despite the name is in Pinal County, not Maricopa County.

Source: pinal.gov, Environmental Health.

If it fails

Treat it the way you would any failed system: establish what failed before accepting a quote. The ground conditions that made a pit the original answer have not gone anywhere, so a replacement design starts from a fresh soil evaluation rather than from an assumption that what worked before will work again. Depending on what that finds, the answer may be a conventional field, another pit if the county permits it, or an engineered system.

For the whole picture, see the Arizona Septic System Guide.

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Questions homeowners ask

What is a seepage pit?: A deep, vertical excavation that receives effluent from a septic tank and lets it disperse into the soil through the walls and floor of the pit. It does the same job as a leach field, but it does it downward through a small footprint rather than outward through a large shallow one. Other names for the same idea include a dry well, and in some places a cesspool, though that word is also used for something different.
Is a seepage pit the same as a cesspool?: Not quite, and the words get used loosely. A seepage pit normally receives effluent that has already passed through a septic tank, so the solids have settled out first. A cesspool in the strict sense receives raw waste with no tank ahead of it. If you have an older Arizona property, which one you actually have is worth establishing rather than assuming, and an inspection is how you find out.
Why were they used so much in Arizona?: Because they suit the terrain. A pit reaches deeper soil layers through a small surface footprint, which is useful where a shallow restrictive layer such as caliche would defeat a conventional field, where the ground is rocky, or where there is not much level area to work with. Those conditions are common across the Sonoran Desert.
Can I still install one?: That is a question for the county that governs your property rather than one with a single answer we can give you. Design criteria for on-site wastewater systems sit in A.A.C. Title 18, Chapter 9, Article 3 and are applied by Maricopa County Environmental Services or Pinal County Environmental Health. Ask them, and ask a designer, before assuming either way.
I am selling a house with a seepage pit. What do I need to know?: The same rule applies as to any other on-site system. Under A.A.C. R18-9-A316 the seller retains a qualified inspector within the six months before transfer, the inspector documents condition and deficiencies, and the seller hands the buyer that report plus the permitting and maintenance documents in their possession. On an older pit those documents are often thin or missing, so start looking early.

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