Soil & Site Evaluation Procedures for Septic System Design

  • SOIL & SITE EVALUATION for SEPTIC SYSTEMS - CONTENTS: Model specification for soil testing to evaluate land for septic system installation. Soil percolation tests perc tests perk test specifications for septic systems. Septic seepage pit or cesspool distance requirements. Septic drainfield or absorption field clearance distance specifications
  • POST a QUESTION or READ FAQs aboutsoil percolation test regulations & procedures - testing soil suitability for installation of a septic effluent disposal system, drainfield, leach field, soakaway bed
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Soil Percolation Tests or Perc Test Specifications:

Section 4 of these model septic design regulations discusses the procedure for evaluating site and soil conditions in preparation for the design of a private septic system to handle onsite wastewater disposal.

For the installation of a conventional septic tank and leach field, minimum useable soil depth, percolation rates, leach field clearance distances, and the requirement for a future repair or expansion area are reviewed, and a table of separation distances from septic components to other site and building features is provided.

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Soil and site appraisal for off-grid or onsite wastewater treatment systems: septic drainfields, soakaway beds, percolation beds

As we explain at SEPTIC SOIL & PERC TESTS, a standard soil percolation test to evaluate a site's suitability for onsite wastwater disposal is basically the excavation of one or more holes in the disposal area into which water is poured. The observation of the length of time required for that water to seep into the ground below is measured as the soil's "percolation rate".

But other factors such as weather, seasonal groundwater level variations, slope, and variations in soil properties mean that even where the septic design and approval rely on a soil perc test, additional site inspection and evaluation steps are necessary.

This document uses the Maine, New York State, and other wastewater treatment standards for individual household septic systems (Appendix 75-A) to provide an example of state regulated design and installation of both conventional tank and leach field septic systems and alternative septic system designs, including raised septic systems, septic mound systems, intermittent sand filter septic systems, and evaportion-transpiration septic systems. Effective Date: 12/01/90 Title: Appendix 75-A.4.

Article Contents

Numerical Classification Systems for Site Evaluation for Septic System Designs

Rather than rely on a compartively simple soil percolation test, some U.S. states, Canadian Provinces, and different jurisdictions in the U.K., Australia, New Zealand, India and other countries may specify different procedures for soil and site evaluation that in turn guides the design of an acceptable and functional onsite wastewater disposal system.

Where soil characteristics make it difficult to use a simple soil percolation test to design a suitable onsite septic system that disposes of wastewater in the soil, some jursidictions such as the State of Maine in the U.S. use a numerical classification system that combines multiple site evaluation factors.

The site inspection and evaluation must be performed by a qualified, licensed professional. Using Maine's 2009 DEH standards as a model, here are examples of factors that are considered in designing an onsite wastewater disposal system:

Note: The Subsurface Wastewater Team, within the MECDC's Division of Environmental Health, Drinking Water Program is charged under 42 MRS § 42 (3, 3-A, 3-B) to adopt rules which regulate subsurface sewage disposal systems, licensing of persons to evaluate soils for subsurface wastewater disposal systems, and inspection of plumbing and subsurface waste water disposal systems. - Maine DEHS, retrieved 8 June 2015, original source:

Maine provides several forms used to guide site inspections, variance requests, pluming permit applications, and septic system product registrations as well as a Voluntary septic inspection form [PDF], all available from the site link given just above.

Reader Question: Standards for a Soil Percolatin Test in Maine?

8 June 2015 Anonymous said:
What are the standards for a Percolation test in Biddeford Maine?

Reply: Maine's Numerical Classification System To Determine Overall Site Suitability For Subsurface Wastewater Disposal


Our reply to your question draws on several different documents from the Maine Subsurface Wastewater Unit.

According to the Maine Subsurface Wastewater Unit of Maine's Division of Environmental Health, since 1974 soil perc tests must be performed by a Licensed Site Evaluator. Our reply draws on several different documents from that organization.

The Subsurface Wastewater Team, within the MECDC's Division of Environmental Health, Drinking Water Program is charged under 42 MRS § 42 (3, 3-A, 3-B) to adopt rules which regulate subsurface sewage disposal systems, licensing of persons to evaluate soils for subsurface wastewater disposal systems, and inspection of plumbing and subsurface waste water disposal systems."

"The general rule of thumb is that a soil test (site evaluation) is needed any time sewage, wastewater, or human waste is being placed beneath the ground in a location where there was none before. This includes all first time development, all replacement systems, and expansions of existing systems. This also includes pit privies, greywater disposal areas, and full septic systems."

Key is this quote from a 1987 Main document:

The State of Maine abandoned the traditional percolation test in 1974 and replaced it with a system of site evaluation to determine suitability for subsurface wastewater disposal. These evaluations are performed by individuals licensed by the Department of Human Services.

Because of the diminishing number of sites with “suitable soils” and the belief that other site characteristics should be taken into consideration the Department developed the New System Variance procedure. This procedure assigns points to various site and system design characteristics and sets a minimum passing score of 50 points, with 65 points required for properties in Shoreland Zoning areas, and 75 points for lots in proposed subdivisions.

Properties not meeting the requirement of original soil over limiting factor are judged by this system. - Maine DEH, "Numerical Classification System To Determine Overall Site Suitability For Subsurface Wastewater Disposal, October, 1987 ", retrieved 8 June 2015, original source: - 2011-11-10

Since Maine's numerical classification system used to determine the suitability of a site for subsurface wastewater disposal can be a bit difficult to find (we give the original link above) we've included a link to that document as The Maine Numerical Classification System To Determine Overall Site Suitability For Subsurface Wastewater Disposal, October, 1987 [PDF].

To learn details about traditional soil percolation tests and exactly how they are performed, or to see other examples of soil perc test specifications read Perc Tests: Soil Percolation & Soil Depth Requirements for Septic Absorption Systems / Septic Drainfields

See SEPTIC SYSTEM DESIGN BASICS for a description of the specifications and properties of most common septic systems such as tank and drainfield, soil & perc tests, septic tank pumping table, septic system treatment chemicals, and steep slope system designs.

See SEPTIC SYSTEM DESIGN ALTERNATIVES for a description of alternative septic system designers, products, and design specifications such as cesspools, disinfection systems, evaporation/transpiration, filters, fixed film gravelless, greywater, holding tanks, lagoons, media filters, mound septic designs, outhouses, peat filters, pressure dosing, raised beds, sequencing batch, steep slope, toilet alternatives, vegetated submerged beds, and wetland septic designs.

(a) Site Investigation for Onsite Wastewater Displsal System Design

(1) Areas lower than the 10 year flood level are unacceptable for on-site systems. Slopes greater than 15% are also unacceptable.

(2) There must be at least four feet of useable soil available above rock, unsuitable soil, and high seasonal groundwater for the installation of a conventional absorption field system (75-A.8(b)).

(3) Soils with very rapid percolation rates (faster than one minute per inch) are not suitable for subsurface absorption systems unless the site is modified by blending with a less permeable soil to reduce the infiltration rate throughout the area to be used.

(4) Subsurface treatment systems and components of the sewage system shall be separated from buildings, property lines, utilities and wells, to maintain system performance, permit repairs and reduce undesirable effects of underground sewage flow and dispersion.

The minimum separation distances are shown in Figures 1 and 2. Reduced separation distances may be approved upon request when the site evaluation by a design professional or soil scientist clearly establishes there will be no adverse environmental impact and will not interfere with the satisfactory operation and maintenance of the system.

Table 2 shows the acceptable minimum separation distances from the various components of treatment systems.

(5) Once the required infiltration area is determined by daily flow, percolation tests and soil evaluation, the required useable area of the property for subsurface treatment can be found. An additional useable area of 50 percent shall be set aside for future expansion or replacement whenever possible.

(b) Separation Requirements for septic system components from buildings and other site features:

[Click to enlarge any image]



TABLE 2 - Separation Distances (in feet) From Wastewater System Component to Well, Stream, Lake, Watercourse or Other Components

System Component Well or Well Water Suction Line  Distance to Lake, Watercourse (b) or Wetland Distance to Dwelling Distance to
Property Line
House Sewer Piping (water-tight joints) 25 if cast iron sewer pipe
50 otherwise
25 3 10
Septic tank or watertight ETU 50 50 10 10
Effluent line to distribution box 50 50 10 10
Distribution Box 100 100 20 10
Absorption Field (c)(d) 100 (a) 100 20 10
Seepage Pit (d) 100 (a) 100 20 10
Raised or Mound Septic Absorption Bed (c)(d) 100 (a) 100 20 10
Intermittent Sand Filter (d) 100 (a)(f) 100 (f) 20 10
Non-Waterborne Systems with offsite residual disposal 50 50 20 10
Non-Waterborne Systems with onsite discharge 100 50 20 10

NOTES to the table on Septic Separation Distances

See SEPTIC CLEARANCE DISTANCES for a more extensive table of recommended clearances and distances between septic system or well components and other site features


(a) When sewage treatment systems are located in coarse gravel or upgrade and in the general path of drainage to a well, the closest part of the treatment system shall be at least 200 feet away from the well.

(b) Mean high water mark.

(c) For all systems involving the placement of fill material, separation distances are measured from the toe of the slope of the fill.

(d) Separation distances shall also be measured from the edge of the designated additional usable area as described in Section 75-A.4 (a)(5).

(e) The closest part of the wastewater treatment system shall be located at least 10 feet from any water service line (e.g. public water supply main, public water service line or residential well water service line).

(f) When sand filters are designed to be watertight and collect all effluent, the separation distance can be reduced to 50 feet.

(g) The listed water well separation distances from contaminant sources shall be increased by 50% whenever aquifer water enters the water well at less than 50-feet below grade. If a 50% increase can not be achieved, then the greatest possible increase in separation distance shall be provided with such additional measures as needed to prevent contamination.


(c) Soil Investigation for Septic System Design & Installation Approval


(1) The highest groundwater level shall be determined and shall include the depth to the seasonal high groundwater level and the type of water table -- perched, apparent, or artesian.

(2) If a subsurface treatment unit such as an absorption field is planned, at least four feet of useable soil shall be available over impermeable deposits (i.e., clay or bedrock). Highest groundwater level shall be at least two feet below the proposed trench bottom. Where systems are to be installed above drinking water aquifers, a greater separation distance to bedrock may be required by the local health department having jurisdiction.

At least one test hole at least six feet deep shall be dug within or immediately adjacent to the proposed leaching area to insure that uniform soil and site conditions prevail. If observations reveal differing soil profiles, additional holes shall be dug and tested. These additional holes shall be spaced to indicate whether there is a sufficient area of useable soil to install the system.

Treatment systems shall be designed to reflect the most severe conditions encountered. If the percolation tests results are inconsistent with field determined soil conditions, additional percolation tests must be conducted and the more restrictive tests must be the factor used for the system design.

(3) Specifications for soil percolation septic test holes: Test holes for seepage pits shall extend to at least mid-depth and full depth of the proposed pit bottom. At least three feet of useable soil shall exist between the pit bottom and rock or other impermeable soil layer and the highest groundwater level.

This shall be confirmed by extending at least one deep test hole three feet below the deepest proposed pit.

(4) Alternatives to soil percolation tests: A local health department may accept or require other soil tests in lieu of the percolation test when such tests are conducted or observed by local health department personnel.

(d) Soil Percolation Test Detailed Specifications

(1) At least two percolation tests shall be made at the site of each proposed sewage treatment system.

(2) For seepage pits, one test shall be conducted at the bottom depth, and the other at half the pit depth.

 If different soil layers are encountered when digging the test pit, a percolation test shall be performed in each layer with the overall percolation rate being the weighted average of each test based upon the depth of each layer.

The local health department having jurisdiction may adopt an alternative procedure for determining the permeability of soil for the installation of seepage pits.

(3) A percolation test is only an indicator of soil permeability and must be consistent with the soil classification of the site as determined from the test holes.


Continue reading at SEPTIC SOIL & PERC TESTS for details about how soils are tested when designing or specifying a septic system or soakaway bed, or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.


Or see SEWER LINE SPECIFICATIONS to continue in the NYS model sanitary code for septic systems.

Or see these

Septic Soil Percolation Test or Wastewater Disposal Field Testing Articles


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