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New York State Septic System Design Regulations 75-A.4 - Soil and site evaluation for septic system design
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• Specification for soil testing to evaluate land for septic system installation
• Soil percolation tests perc tests perk tests for septic systems

Soil Percolation Tests or Perc Tests: Section 4 of the septic design regulations discusses the procedure for evaluating site and soil conditions in preparation for the design of a private septic sytsem 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 toother site and building features is provided.

This document uses the New York State wastewater treatment standard 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. Readers needing to know more about soil percolation tests or who want to see other examples of soil perc test specifications should also see Perc Tests: Soil Percolation & Soil Depth Requirements for Septic Absorption Systems / Septic Drainfields

Citation of this article by reference to this website and brief quotation for the sole purpose of review are permitted. Use of this information at other websites, in books or pamphlets for sale is reserved to the author. Technical review by industry experts has been performed and is ongoing - reviewers welcomed and are listed at "References." © Copyright 2008 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use the links at page left to navigate this document or to go to Other Website Topics. Green links at left show where you are in our document & website.

Effective Date: 12/01/90 Title: Appendix 75-A.4 - Soil and site appraisal

(a) Site Investigation.

(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:

FIGURE 1 - ABSORPTION FIELD SEPARATION REQUIREMENTS

FIGURE 2 - SEEPAGE PIT SEPARATION REQUIREMENTS

TABLE 2 -  SEPARATION DISTANCES FROM WASTEWATER SYSTEM COMPONENTS

                                     TO STREAM, LAKE
                     WELL OR         WATERCOURSE (b)        PROPERTY
SYSTEM COMPONENTS    SUCTION LINE    OR WETLAND     DWELLING  LINE

House sewer           25'if cast iron    25'           10'     10'
(watertight joints)      pipe
                      50'otherwise

Septic tank           50'                50'           10'     10'

Effluent line to
distribution box      50'                50'           10'     10'

Distribution box     100'               100'           20'     10'

Absorption field     100' (b)           100'           20'     10'

Seepage pit          150' (b)           100'           20'     10'

Dry well
(roof and footing)    50'                25'           20'     10'

Raised or
Mound system         100'               100'           20'     10'

Evapotranspiration-
  absorption system  100'                50'           20'     10'

Composter             50'                50'           20'     10'

NOTES to the table

(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.

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

© 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.

More expert information on this topic

More Information on Septic System Diagnosis, Maintenance, & Repair

• The Septic System Information Website home page for this topic
• Septic Systems Inspection, Testing, & Maintenance- online textbook. Detailed how to inspect, maintain, repair information
• The Home Buyer's Guide to Septic Systems
• Septic Tank Pumping Guide: When, Why, How to pump the septic tank
• Home & Outdoor Living Water Requirements
• Septic Tank Capacity vs Usage in Daily Gallons of Wastewater Flow, calculating required septic tank size, calculating septic tank volume from size measurements
• How Big Should the Leach Field Be? - table of soil percolation rate vs. field size
• Septic System Drainfield Absorption System Biomat Formation - what leads to drain field clogging and expensive drainfield repairs
• Table of Required Septic & Well Clearances: Distances Between Septic System & Wells, Streams, Trees, etc.

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