Septic System Perc Tests and Perc Test Pits for Septic System Design

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Septic soil percolation tests or soil absorption test procedures:

The ability of a drain field, also called leach field, or drain field, to absorb septic effluent determines the size, location, and type of effluent absorption system which can be installed at a property.

Septic drainfield percolation test procedures: this article describes the need for and process of “soil testing” or the preparation and use of soil test pits for septic system absorption system or drainfield design or repair.

Page top sketch of a sloped-side soil perc test pit is adapted from Oregon DEQ cited in this article series.

We also provide an ARTICLE INDEX for this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Perc tests may also be performed in order to evaluate soils when a septic system is believed to have failed,
and when repair or septic field replacement are being considered.
Readers should also see our example of state-regulated soil percolation tests at the New York State Septic System Design Regulations 75-A.4 – Soil and site evaluation for septic system design page.

[Click to enlarge any image]

In specifying the size and type of absorption field (leach field, seepage pits, galleys, other) a septic engineer or health department official will require that a soil percolation test or “soil perc test” be performed. You may hear it described as a “deep hole test.”

In brief, one or more holes are dug in the soil of the property where (or near where) a septic leach field is to be installed. Water is placed in the hole, and the engineer observes the amount of time it takes for the soil to absorb the water, or for the water to “percolate” through the soil. The engineer will also examine the exposed soil layers to obtain additional site design information. (Details follow).

Steps in a Perc Test / Soil Percolation Test

More precise “perc tests” may involve using a specific quantity or volume of water or a perc test hole of specific dimensions to make these observations. Some perc tests may specify the inclusion of a couple of inches of gravel in the hole bottom – depending on the nature of the soil itself.

The first time I participated in a soil perc test procedure I found myself smiling with surprise at how low-tech the procedure actually was (in New York State.)

After identifying the most-likely location on the lot for placement of a septic drainfield, the excavator used a backhoe to dig a very rough hole about 5 ft. deep.

Happily no groundwater immediately filled in the hole (which would have been bad news). Perhaps this is why builders try to have this test done in July which is the period of most-dry weather and lowest groundwater table levels.

After digging this rough hole, the septic engineer poured a 5-gallon (joint compound) bucket of water into the hole. In some cases a few buckets might be dumped therein.

After that sophisticated move, the observers simply watched the rate at which the water disappeared. a one-inch drop in water level in this hole in three minutes was considered very good.

If the water was found still in the hole at no drop in level the next morning, this was considered seriously bad and probably requiring some soil exchange or other special design measures.

There are two different questions here:

Why not do our perc tests during the dry season when a site is most-likely to “pass” local soil perc test requirements? I’ve absolutely seen builders use this trick to pass a marginal site for locating a septic soakbed.

Unfortunately designing a septic effluent disposal system based on “dry season” perc testing results means that the septic system design is likely to be inadequate: that is, during the wet season when soil water tables are higher and perc rates are slower, the septic system is going to discharge un-treated effluent into the environment: basically your are peeing and pooping into the water supply.

2016/08/29 Anonymous said:
what s the cost to get a perc test

Perc test costs vary depending on where you live and also on just what local regulation require. For example, a deep hole perc test in some jurisdictions will require the use of a backhoe – you’ll be paying the hourly rate for the backhoe operator that might be $100. or more, plus the cost for the consulting engineer or septic designer to introduce water and watch and measure the percolation rate.

The number of perc test holes, hole depth, and ease of site access all affect the total cost for a soil perc test for septic system design and approval.

Actual costs range from a low of about $100. (presuming you’re in the U.S.) for a shallow hand-dug perc test hole to $1000. for several perc test holes dug deeper (typically using a backhoe) plus the septic designer’s measurements and report of soil percolation rates on the proposed septic field site. In some municipalities, the backhoe rate alone can be higher: $300. to $500./hour.

In VERY general terms, I’d figure $500. for the backhoe for 2-3 deep hole excavations and another $500. to pay the septic engineer.

We discuss perc tests in detail at inspectapedia.com beginning at SEPTIC SOIL & PERC TESTS but you’ll also want to see the next articles SOIL PERC TEST HOLE SPECIFICTAIONS and SOIL PERC TEST STANDARDS in this series given at Continue Reading below, as the specifics of exactly what sort of soil perc test your local health or building department will require for septic system design approval will determine the amount of trouble, work, time and cost of your particular soil perc test

Our sister site the Building Advisor (Steve Bliss) also discusses SOIL AND PERC TESTING at https://buildingadvisor.com/buying-land/septic-systems/soil-and-perc-testing/

(Nov 30, 2015) Charlotte said:
We have s septic and leachfield. It is 20 years old. We are careful not to put any kitchen grease, and NO paper is flushed. This system has never given any problem, and has never been pumped. It is a concrete tank. Do you think it will pass inspection after being pumped? Am selling the property.
Thanks.
[email protected].

Charlotte:

“Pass inspection” depends on where you live, what local laws require of your system, and who performs the inspection (and what she knows). The level of usage, daily wastewater flow, soil conditions and other factors also affect septic system life – search InspectApedia.com for “SEPTIC SYSTEM LIFE” to see what factors are involved and how one can maximize the life of the system.

In my OPINION, however, there is no conventional septic tank and drainfield that should go 20 years without being pumped; even if you have not observed a problem that does not tell us the remaining life of the system nor the condition of the drainfield.

If I were inspecting the home for a buyer I would recommend a septic loading and dye test, then finding and pumping the septic tank to inspect the sludge and scum levels and the condition of the tank baffles.

If the tank is un-damaged and sludge/scum levels are so low that we have not been pushing solids into the leach field, I’d improve my guesstimate of remaining drainfield life from poor to unknown.

If I see that baffles have been lost then I know we’ve been sending solids into the fields and the remaining life is very poor.

(May 22, 2016) Richard W said:

We are putting in a studio apt. over the garage. About 500 square feet, with a 3 piece bath ( toilet, sink, and shower) and a small kitchenette ( sink and dish washer). In addition a laundry room with a washing machine.

There is a existing septic system, generally can you add to the current system to increase capacity or is it likely we would need to start from scratch? As it is currently, the system serves a small 1000 SF manufactured home with 1 bath, a kitchen and laundry. Thank you

You may be able to extend the drain fields but tank size is also a concern

Watch out: while it’s often possible to extend leach field size and to dig up and install a bigger septic tank, running heavy equipment over the existing drainfield could damage or destroy it.

2016/08/29 Anonymous said:
what s the cost to get a perc test

We include soil percolation test cost estimates at SEPTIC SOIL & PERC TESTS

(June 12, 2015) Delores Lyon said:

Thanks for sharing this advice on buying and maintaining a septic system. It’s nice to know what to do when you are first starting to have septic tank issues– it makes sense to first try DIY options. However, when it isn’t being fixed, diagnosing the issue is also an important party to getting it fixed. With this guide, I don’t think it would be too hard to maintain a septic system at all!

(Sept 11, 2015) Carter Michaelson said:

Our neighbors recently had to deal with some issues with their septic tank and had to have some repairs done. It got me thinking about our septic tank and when the last time we got it looked at was. Getting your septic tank pumped and checked on every few years, if not once a year, is a great idea.

You never know when something might not be right and the next think you know, you are digging up your entire yard.

(Feb 19, 2016) Jackie Oliver said:

Wow, this was a huge help! My husband and I are building a cabin, and it has a septic tank. It’s good to know that we will have to get on a schedule to have it pumped. I have never had a system like this before, so it’s all totally new to me. I will be sure to monitor how much it’s in use, and then get on a consistent schedule to have it pumped!

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Continue reading at PERC HOLE SPECIFICATIONS – how should the soil perc test hole be constructed, or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

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