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Stepped side soil percolation test pit for wet soils areas adapted from Oregon DEQ at InspectApedia.comPerc Test Standards
Soil Percolation Soil Percolation Rate Standards for Septic Drainfields

  • POST a QUESTION or COMMENT about the standards for septic system test hole tests, deep hole tests, and percolation tests for drainfield qualification and soil testing

This article defines & explains septic system soil percolation rate specifications - what are the required soil perc rates when testing a eptic system absorption system or drainfield?

How do we perform a soil perc test when designing or testing a septic system?

The page top soil perc test hole sketch is adapated from Oregon DEQ cited in this article. - Daniel Friedman

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

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Soil percolation rate standards for septic systems

Backhoe in operation doing excavation (C) Daniel FriedmanThe Massachusetts Title 5 Septic Inspection criteria does a great job of defining a (at least possibly) what is required to assure a functional septic drainfield.

The text explains the role of the biomass below the absorption bed, sets soil depth requirements, and recognizes the importance of keeping the bottom of the working biomass area in well drained soil sufficiently above the seasonal high water table.

Photo: After its elderly and sole occupant of this home had passed away, the author's investigation and excavation at the septic system found that there was no drainfield at all, simply an 8 foot perforated pipe out of the septic tank into surrounding rocky soil. That systsem's capacity would not support a family with children moving in to this Poughkeepsie, New York home.

[Click to enlarge any image]

Example of soil requirements for a functional drainfield.

This version is particularly clearly written and is for residents of Ohio but the principles apply anywhere. 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.

In Ohio, soil absorption systems can be used in areas where the percolation rate of the soil is between 3 and 60 minutes per inch (soil permeability between 1 and 20 inches per hour).

At least 4 feet of suitable soil 

is required under the soil absorption system to provide adequate treatment of the septic tank effluent. To accommodate the construction of the system and provide adequate soil cover to grade, a minimum of 5 1/2 to 6 1/2 feet of suitable soil is needed above the limiting layer.

A limiting layer may be bedrock, an impervious soil layer (hardpan, fragipan) or a seasonally high water table (gray soil or mottles). The soil absorption system must be at least 8 feet from any drain line on the lot, 50 feet from a water supply, and 10 feet from the property line, right-of-ways and the house.

Septic systems cannot be placed on the flood plain and are limited to areas with less than a 15 percent slope. Reference: ohioline.osu.edu/aex-fact/0743.html Ohio State University Fact Sheet Septic Tank - Soil Absorption Systems

Our separate article by Lockwood includes a description of the calculations to answer the question: How Big Should the Leach Field Be? our

article SEPTIC DRAINFIELD SIZE

includes a practical example using sample calculations as well as a table of soil percolation rate vs. the necessary leach field size.

Theoretical Soil Perc Test Hole Dimensions, Soakaway Rate, & Percolation Rate in MPI or Minutes per Inch
Percolation Hole
Water Area
Dimensions
in Inches 		 Water Test Volume
in Gallons		 Water Absorption
Time		 Percolation Rate
in MPI
(Minutes Per
Inch of water) 		 Comments
1 x 1 x 231 inches 1 1.2 days 1732 minutes This is a 0.83 gallon per square foot per day Perc Test result
1 x 1 x 231 inches 1 1 day 1440 minutes This is a 1-gallon per square foot per day Perc Test result
 
5 x 1 x 231 inches 5 gallons 1 day 288 minutes This is a 5 gallon pe square foot per day Perc Test Result
 
1 x 1 x 231 inches 1 gallon 2 minutes 2 minutes This is a 2 MPI perc test result
IF the soil perc test hole specification is as described in this table and its notes.
 
12 x 12 x 1 inches 0.62 gallons 60 minutes 108 minutes This is 0.83 gpd/sqft
12 x 12 x 8.02 inches 5 gallons 24 hours 180 minutes This is 5.0 gpd/sqft
1154 cu.in. = 5 gal

15.2 x 15.2 x 5 inches

5 gallons 24 hours 180 minutes This is 5.0 gpd/sqft
5" deep x 231 sq.in.

Notes to the Table Above

This data has not been reviewed by a septic design engineer. I may have missed something. - Ed.

The column on perc hole water dimensions emphasizes that we're only discussing the shape of the water volume in the test hole in order to permit calculation of a soil percolation rate that is standardized. Without specifying the dimensions of a soil perc test hole, simply pouring water into an arbitrarily sized hole at an arbitrary depth gives only a very crude guess at the soil's ability to absorb effluent.

In the real world, digging a soil pecolation test hole with a backhoe does not produce a perfectly rectangular opening with a perfecly-flat bottom of un-disturbed soil.

The actual depth of the soil perc test hole (typically set at 5 feet but varying significantly by local code and procedure) has to reflect the anticipated depth of the soil soakbed trenches and must consider the soil properties below that point as well as the seasonal high water table level.

Useful constants for soil absorption rate calculations

A 231 square inch flat bottomed (theoretical) perc test hole that absorbs one gallon of water in 24 hours

Here is a typical standard percolation test hole specification when soil testing for leachlines:

Test holes shall be augered or excavated to within 13 inches of the actual test depth which corresponds to the anticipated depth of the leachline or the bed trench bottom. Vary depths to include testing of side wall if the disposal system will be more than three feet below the ground surface.

In addition, perform one test in the least permeable soil stratum found during the deep excavation if the soil type changes within 5 feet of the proposed trench bottom. - San Bernadino soil perc test specifications, cited below.

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Reader Comments, Questions & Answers About The Article Above

Below you will find questions and answers previously posted on this page at its page bottom reader comment box.

Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

Question: California MPI soil perc rates vs. "Codes"

How does perc test minutes per inch compare with gallons per square foot per day for a drainfield capacity?

2016/11/24 Puzzled said:

Perc Rates for leach fields are commonly expressed in Minutes per Inch. The inspector said my perc rate of 2 MPI requires advanced treatment in California.

When I check the Plumbing Code it says perc rates should be between 0.83 and 5.0 gallons per sq. ft. per day. How does this relate to MPI?

Reply: How to convert soil perc in minutes per inch to soil percolation rate in gallons per square foot per day

Your MPI number means "minutes per inch" rate at which water in your septic perc test hole soaks into the ground.

The gallons per square foot per day is the allowable application rate for septic effluent and is based on the design assumption that over 24 hours that volume of wastewater will be successfully absorbed into the surrounding soil.

A perc test is simply a hole in the ground, too often not dug to a standard dimension or depth. Water is poured into the hole and the soil perc test expert tracks in minutes or hours how long it takes for the water level to fall one-inch.

Watch out: We can't accurately state how many gallons seeped into the soil unless we know the dimensions of the perc test hole and more, the hole needs to have a flat bottom and roughly vertical sides.

So here I'll make some assumptions and choose a test hole size that makes the arithmetic easy.

How to confirm that septic drainfield soil meets the plumbing code spec of 5.0 gallons per day per sq.ft.

1 U.S. gallon = 231 cubic inches of volume.

So if I had a 231 square-inch flat bottomed perc test hole and I filled it with five inches of water, I'd have 5 gallons in my soil percolation rate test hole.

Then according to the (presumably California) plumbing code you're citing, if

all five inches soaked away into the soil over a day (24 hours) that would meet the upper end of the

5.0 gpd/sqft soil percolation rate test specification.

Just as a sanity check, if your local building inspector allowed a gallon per square foot per day perc rate septic field, then you would need something like 2000 square feet for a functional field + recovery field area in a new septic design.

(Average daily wastewater production for the number of occupants or number of bedrooms x 2)

Convert perc rate of 0.83 gallons per square foot per day into the minutes for water to fall one inch in a perc test

Lets convert the model plumbing code required perc rate of 0.83 gallons per square foot per day into MPI - the number of minutes it should take for water to fall one inch in our perc test 12"x12" hole.

How much water is in a 12"x12"x12" test hole?

The volume of a cubic foot or 12" x 12" x 12" hole is 7.48 gallons or 28.32 liters. Or re-stating,

a cubic foot of water = 7.48 gallons.

The volume of 1-inch of depth that 12"x12"x12" square soil perc test hole is

0.62 gallons per inch in a 12" x 12" square hole, which we got simply by dividing 7.48 by 12.

Convert 0.83 gpd/sqft to Minutes per Inch of Absorption

How long should it take for water in a 12" x 12" perc test hole to fall 1 inch if the soil perc rate is 0.83 gallons per foot per day?

First convert the gallons per day per square foot per day (gpd/sqft) to gallons per square foot per hour (gph/sqft)

(0.83 gpd/sqft/day) / (24 hrs/day) =

0.034 gallons per hour per square foot (gph/sqft) perc rate for a 12" x 12" perc test hole.

Convert 0.034 gph/sqft in a perc test hole to minutes per inch (mpi)

If 1 inch of water in our 12"x12" hole = 0.62 gallons, then

.62 gal / 0.34 gph = 1.8 hours per inch or

108 minutes per inch

to absorb the water in the perc test hole.

That's the perc rate in minutes per inch for an absorption rate of 0.84 gallons per day per squre foot

and it's about 50 times longer than your 2 minutes per inch IF your soil perc test was performed properly and the dimensions of the perc test hole were known and its sides were vertical.

See details and soil perc test tables above on this page.


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