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Perc 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 discusses soil percolation rate specifications - what are the required soil perc rates when testing for septic system absorption system or drainfield design or repair.
How do we perform a soil perc test when designing or testing a septic system? Specifications and guidelines for soil percolation testing.
The page top soil perc test hole sketch is adapated from Oregon DEQ cited in this article.
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PERC TEST STANDARDS - What are the soil percolation rate standards and other soil requirements for septic systems?
The 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.
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
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
includes a practical example using sample calculations as well as a table of soil percolation rate vs. the necessary
leach field size.
Question: California MPI soil perc rates vs. "Codes"
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 gallons per sq.ft. per day and 5.0 gallons per sq. ft. per day.
We cannot directly relate soil percolation rate numbers to gallons per square foot per day without knowing the dimensions of the test hole that was dug.
I'm assuming your MPI number means "minutes per inch" of soaking of water out of a test hole into the ground.
1 U.S. gallon = 231 cubic inches.
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'd meet the uppe end of the 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)
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.
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
144 square inches = 1 square foot
231 square inches = 1.6 square feet
231 cubic inches = 1 U.S. Gallon
1440 minutes = 1 day or 24 hours
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.
PERC HOLE SPECIFICATIONS standard soil percolation rate test hole excavation dimensions & procedures
Effluent entering a soil absorption system may contain varying combinations and amounts of potential contaminants.
A vertical separation distance of 24 inches between the bottom of a soil absorption system and the seasonally high water table has been suggested as a minimum soil depth fdor proper treatment of effluent and protection of groundwater.
Depth to the wet season water table can be monitored with observation wells or can be estimated from soil morphological characteristics.
Caution is advised when evaluating artificial drainage as a method to improve performacne of on-site sewage disposal systems.
We recommend this excellent document that offers detail about soil perc tests, deep hole tests, safety, and septic design. The Oregon DEQ onsite water quality program can be contacted at 811 South Ave, Portland OR 97204, 800-452-4011 or Oregon DEQ website: http://www.oregon.gov/DEQ/
Oregon's State DEQ has some nice sketches of a soil test pit from which we excerpt in the article above. These
notes paraphrase and expand on the Oregon DEQ's text.
SOIL PERCOLATION TESTING MANUAL, [PDF] CNMI Division of Environmental Quality, Gualo Rai, Saipan provides an excellent English Language manual guide for soil percolation testing. Original source: www.deq.gov.mp/artdoc/Sec6art108ID255.pdf CNMI Division of Environmental Quality, PO Box 501304, Saipan, MP 96950
"SOIL PERCOLATION (PERC) TEST REPORT STANDARDS" Suitability Of Lots And Soils For
Use Of Leachlines Or Seepage Pits", [PDF], San Bernardino County
Division Of Environmental Health Services
385 North Arrowhead Avenue
San Bernardino, Ca 92415-0160
Telephone: (909) 387-4666
Fax Number: (909) 387-4323
Http://www.sbcounty.gov/dehs retrieved 2016/11/24, original source: http://www.sbcounty.gov/uploads/dph/dehs/Depts/ EnvironmentalHealth/FormsPublications/ 550034_on_site_waste_water_disposal_system.pdf
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In addition to any citations in the article above, a full list is available on request.
Soil Test Pit Preparation, [PDF] fact sheet, Oregon DEQ Department of Environmental Quality, original source www.deq.state.or.us/wq/pubs/factsheets/onsite/testpitprep.pdf The Oregon DEQ onsite water quality program can be contacted at 811 South Ave, Portland OR 97204, 800-452-4011 or see http://www.oregon.gov/DEQ/
Percolation Testing Manual, [PDF] CNMI Division of Environmental Quality, Gualo Rai, Saipan provides an excellent English Language manual guide for soil percolation testing. Original source: www.deq.gov.mp/artdoc/Sec6art108ID255.pdf CNMI Division of Environmental Quality, PO Box 501304, Saipan, MP 96950
Test Pit Preparation for Onsite Sewage Evaluations, [PDF] State of Oregon Department of Environmental Quality, Portland OR, 800 452-4011. PDF document. We recommend this excellent document that offers detail about soil perc tests, deep hole tests, safety, and septic design.
Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
Advanced Onsite Wastewater Systems Technologies, Anish R. Jantrania, Mark A. Gross. Anish Jantrania, Ph.D., P.E., M.B.A., is a Consulting Engineer, in Mechanicsville VA, 804-550-0389 (2006). Outstanding technical reference especially on alternative septic system design alternatives. Written for designers and engineers, this book is not at all easy going for homeowners but is a text I recommend for professionals--DF.
US EPA ONSITE WASTEWATER TREATMENT SYSTEMS MANUAL [online copy, free] Top Reference: US EPA's Design Manual for Onsite Wastewater Treatment and Disposal, 1980, available from the US EPA, the US GPO Superintendent of Documents (Pueblo CO), and from the National Small Flows Clearinghouse. Original source http://www.epa.gov/ORD/NRMRL/Pubs/625R00008/625R00008.htm Onsite wastewater treatment and disposal systems,
Richard J Otis, published by the US EPA. Although it's more than 20 years old, this book remains a useful reference for septic system designers.
U.S. Environmental Protection Agency, Office of Water Program Operations; Office of Research and Development, Municipal Environmental Research Laboratory; (1980)
"International Private Sewage Disposal Code," 1995, BOCA-708-799-2300, ICBO-310-699-0541, SBCCI 205-591-1853, available from those code associations.
"Manual of Policy, Procedures, and Guidelines for Onsite Sewage Systems," Ontario Reg. 374/81, Part VII of the Environmental
Protection Act (Canada), ISBN 0-7743-7303-2, Ministry of the Environment,135 St. Clair Ave. West, Toronto Ontario M4V 1P5 Canada $24. CDN.
Septic Tank/Soil-Absorption Systems: How to Operate & Maintain, Equipment Tips, U.S. Department of Agriculture, 8271 1302, 7100 Engineering, 2300 Recreation, September 1982, web search 08/28/2010, original source: http://www.fs.fed.us/t-d/pubs/pdfimage/82711302.pdf
Septic System Owner's Manual, Lloyd Kahn, Blair Allen, Julie Jones, Shelter Publications, 2000 $14.95 U.S. - easy to understand, well illustrated, one of the best practical references around on septic design basics including some advanced systems; a little short on safety and maintenance. Both new and used (low priced copies are available, and we think the authors are working on an updated edition--DF.
Quoting from one of several Amazon reviews: The basics of septic systems, from underground systems and failures to what the owner can do to promote and maintain a healthy system, is revealed in an excellent guide essential for any who reside on a septic system. Rural residents receive a primer on not only the basics; but how to conduct period inspections and what to do when things go wrong. History also figures into the fine coverage.
US EPA ONSITE WASTEWATER TREATMENT SYSTEMS MANUAL Top Reference: US EPA's Design Manual for Onsite Wastewater Treatment and Disposal, 1980, available from the US EPA, the US GPO Superintendent of Documents (Pueblo CO), and from the National Small Flows Clearinghouse. Original source http://www.epa.gov/ORD/NRMRL/Pubs/625R00008/625R00008.htm
In addition to citations & references found in this article, see the research citations given at the end of the related articles found at our suggested
Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. Tel: (416) 964-9415 1-800-268-7070 Email: info@carsondunlop.com. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.
Carson Dunlop Associates provides extensive home inspection education and report writing material. In gratitude we provide links to tsome Carson Dunlop Associates products and services.