Prairie Hills Soils Consulting
Interpreting soils information from the USDA Soils Database to meet customer's planning needs for urban and farming uses since 1980

Assessing soils for on-site wastewater dispersal

On-site waste water dispersal is used in more than 28% of the homes in America but increases to 80%  in many rural areas and non urban communities.  The system consists of a septic tank with effluent that flows into a septic absorption field for wastewater dispersal.  

Soils must be assessed to determine whether the soils are suitable for systems or to determine what soil properties must be overcome to make a site available.  

The most common method of soil dispersal (or 'leach') fields is the PVC perforated pipe, however the chamber system is becoming very common.  The national NRCS Septic tank absorption fields interpretation is created based on  EPA criteria .  The interpretation evaluates soils for subsurface systems of tile or perforated pipe that distribute effluent from a septic tank into the natural soil. The centerline depth of the tile (or floor of the chamber) is assumed to be 24 inches or deeper. The natural soil material between depths of 24 and 60 inches is evaluated in making the ratings. Soil properties and site features considered are those that affect the absorption of the effluent, those that affect the construction and maintenance of the system, and those that may affect public health.

Proper site selection provides the septage to be confined within the soil absorption field

On site failures are indicatied by
  • persistent pooling water or muddy soil around your septic system.
  • persistent toilet or sink backs up when you flush or do laundry.
  • persistent strips of bright green grass during all seasons over the drain field.
  • wastewater comes into contact with groundwater or surface waters.

The first step for analysis should be to check the soils within the area using Web Soil Survey  .  Web Soil Survey will allow you to get an idea of the types of soils found on the property and what may be some of the issues that may appear when placing a onsite wate dispersal system.  

Tools for Preliminary Assessment

  • Web Soil Survey
    • Office/PC environment
    • More robust
    • Useful for customer documentation
  • Soil Web
    • Smartphone/PC application
    • Useful field application for soil identification
    • Lacks thematic maps and soil reports
  • Munsell Color book
    • Soil color identification
  • Older Soil Survey manuscripts
    • Handy truck reference

Web Soil Survey Maps

Web Soil Survey allows users to view a map presenting the map unit interpretive rating for Septic Tank Absorption Fields.  The basic mode displays a map of the Dominant Condition of the many soils contained within the map unit.  The map displays a 3 color stoplight with Red being 'Very Limited', Yellow being 'Somewhat Limited' and ​Green being 'Not Limited'.  The map is suitable for broad land use planning.  The reports provide more detailed information.

Web Soil Survey Reports

Web Soil Survey displays maps and reports.  Each thematic map has a compatible report providing more detailed information.  The report displays all soils within the map unit and its percent of the map unit composition.  It also displays the soil rating and the soil property restrictions, if they exist.  In the example above, the Tobin soil identifies restrictions of 'susceptible to flooding' and 'slow water movement'.  In comparison, the Lancaster soil has 'Depth to bedrock' meaning bedrock is underlying the soil and 'slow water movement' meaning the soil has slow permeability.  These rare site and soil issues that much be addressed in the design of the onsite waste dispersal system.  These site and soil issues make the most common gravity trench and pipe system an unsuitable choice.

Understanding WSS ratings

  • Very limited – properties within the soil must be overcome before proper function of the septic system
  • Somewhat limited - properties within the soil must be addressed before proper function of the septic system
  • Not limited – properties within the soil make it suitable for use
  • Not rated – soil is not evaluated for use

Soil Web

Soil Web is a newer app/web browser tool that is well suited to field applications.  Using the ‘Zoom to Location’ tool, the client can zoom to the current location using the phone GPS or enter a variety of address methods. 
There are different features on the Soil Web application that differentiates if from the WSS tool.  The Soil Web tool has a soil profile feature designed to provide the expected horizons that can be found and the basic matrix color scheme.  This profile description comes from official series description horizon depths and colors.   
 ?What makes a suitable leach field site
"Septic" systems have been the primary method of suburban and rural wastewater dispersal more many decades.  The primary design issues have not changed much over the years even though technology has improved the ability to overcome many site and soil issues.  Regardless of the system, the primary design issues that make a suitable site are:
  • Site not prone to flooding or ponding
  • Slopes less than 8 percent
  • Bedrock or Limiting Layer 4 feet below the pipe depth
  • Water table no closer than 4 feet below the pipe depth
  • No trees in the leach field
  • Sufficient area for the leach field
  • Down slope from the drinking water well
  • Soil permeability 0.6 to 6.0 in/hr (10 to 100 minutes per inch) in the 2 to 6 feet depth

Soil Properties Consideration

Soil properties and qualities that affect the absorption of the effluent are:
  • permeability,
  • fragments,
  • depth to a seasonal high water table,
  • depth to bedrock,
  • depth to a cemented pan,
  • susceptibility to flooding,
  • susceptibility to ponding,
  • subsidence,
  • slope.

The soil is assessed for a variety of properties. Those properties that do not meet the suitable standard are flagged as a 'limitation'. The limitation must be considered during the design of the wastewater dispersal system.

Soil Composition

Soils are made up of
  • Minerals,
  • Water,
  • Air, and
  • Organic Matter. 

The various combinations of these four elements influence the suitability for onsite wastewater dispersal.

Soil Permeability

  •  “the property of the soil to transmit water and air and is one of the most important soil quality. A onsite system built in impermeable soil will cause seepage to find a route outside the soil. The more permeable the soil, the greater the seepage.”
  • Soil Texture and Soil Structure directly influence soil permeability.
  • Percolation tests should be used with caution when assessing soil permeability

Permeability that is too fast is unsuitable with the potential to inadequately treat septage.  Permeability that is too slow is unsuitable with the potential of septage backup into the house of surfacing within the leach field.  

Soil texture and structure are used as a proxy to identify suitability of the soil.  Permeability can also be analyzed using percolation or permeameter tests.  

Soil Texture

Soil Texture (Mineral portion)
  • refers to the weight proportion of the separates for particles less than 2 mm
  • Sand particles feel gritty and can be seen individually with the naked eye.
  • Silt particles cannot be seen individually without magnification; they have a smooth feel to the fingers when dry or wet.
  • Clay particles are microscopic and have a sticky feel to the fingers.

The NRCS web tool allowing the pictorial assignment of mineral percentages.

Soils are typically hand textured to identify the texture.  A texture guide is found here .

You tube videos provide detailed instructions on hand texturing, the UC Davis video is a good start.

Soil Structure

  • The arrangement of soil particles into clusters or aggregates of various sizes and shapes
  • Formed by the combination of Sand, Silt, Clay, and Organic Matter.
  • Influenced by wet/dry, freeze/thaw, root growth, microorganism activity

YouTube videos are available to view the various soil structure:





Sanitarians use 'Loading Rates' to design leach fields.  There is a correlation between Permability, Percolation, and Loading Rates that can be used in the design of a system.  Analysis of the soil horizons below 24 inches is necessary to assign the proper loading rates.

Soil Textures              Permeability             Percolation                   Loading Rates                          Permeability Class
  USDA                             in/hr                            min/in                                gpd/ft2                                                              

Silty Clay, Clay                <0.06                                   >1000                                           0.0                                         Very Slow

Silty Clay,                       0.06 - 0.2                            1000-300                                       0.2                                          Slow
Silty Clay Loam  

SiCL, Silt Loam                0.2 - 0.6                             300-100                                      0.4-0.6                                 Moderately Slow

Silt loam, Clay Loam
Sandy Clay Loam          0.6 - 2.0                              100 - 30                                       0.2 - 0.4                               Moderate               
Fine Sandy Loam    

Fine Sandy Loam,
Sandy Loam, Loam,       2.0 - 6.0                              30 - 10                                        0.4 - 0.6                               Moderately Rapid
Sandy Clay Loam, 
Loamy Fine Sand

Sand, Loamy Sand,
Loamy Fine Sand            6.0 - 20.0                       10 - 3                                              0.6 - 0.9                                Rapid

Sands                              > 20.0                                   < 3                                                    > 1                                   Very Rapid

Permeability Assessment:  Too Slow

Slow Water Movement
For the national WSS interpretation, the soil horizon with the minimum permeability governs the rate of water movement through the whole soil. When this rate is low, transmission of fluids into and through the soil is impeded and runoff, infiltration, and percolation of pollutants may become environmental, health, and performance concern. Soil feature considered is the layer deeper than 24 to 60 inches and above a restrictive layer with the minimum saturated hydraulic conductivity.
  • Limiting                                              < 0.6 inches/hour
  • Somewhat Limiting    => 0.6 and <= 2.0 inches/hour
  • Not Limiting                 => 2.0 and <= 6.0 inches/hour

Permeability Assessment:  Too Fast

  • Limiting > 6.0 inches/hour
  • Not limiting <6.0 inches per hour
 Filtering Capacity
The soil horizon with the maximum permeability governs the leaching and seepage potential of the soil. When this rate is high, transmission of fluids through the soil is unimpeded and leaching and seepage may become environmental, health, and performance concern. Soil feature considered is the layer with the maximum permeability between a depth of 24 to 60 inches or to the depth of a cemented restrictive feature

Content of Large Stones

Large stone impede the workability of the soil and restrict trafficability of heavy machinery and site reclamation. Soil feature considered is the weighted average percentage of rock fragments of size greater than 3 inches in the horizons above a restrictive feature or from 0 to 40 inches deep

  • Limiting                     > 50%
  • Somewhat limiting  >= 25% to <= 50%
  • Not limiting               < 25%

Depth to High Water Table

Soils with shallow depth to a water table may become waterlogged during periods of heavy precipitation and are slow to drain. These soils have the potential to contamination the ground water which may create health and environmental hazards. Soil feature considered is the top depth of the first layer where soil moisture layer status is wet or saturated during any month. The vertical separation between the pipe depth and water table depth is the important consideration to make sure sufficient nonsaturated soil material is available for proper dispersion.  
  • Limiting                                < 48 inches
  • Somewhat limiting              => 48 to < 70 inches
  • Not limiting                          => 70 inches

Depth to Bedrock

The depth to bedrock restricts the construction, installation, and functioning of septic tank adsorption fields and other site applications. Shallow soils have limited adsorptive capacity and biologically active zones through with waste materials can percolate. These soils may pose environmental and health risks when used as filter fields.The vertical separation between the pipe depth and depth to bedrock is the important consideration to make sure sufficient  soil material is available for proper dispersion.  
  • Limiting                                      =< 40 inches
  • Somewhat limiting       > 40 to < 60 inches
  • Not limiting                              => 60 inches

Depth to Cemented Pan

Depth to cemented pan restricts the construction, installation, and functioning of septic tank adsorption fields and other site applications. Shallow soils have limited adsorptive capacity and biologically active zones through with waste materials can percolate. These soils may pose environmental and health risks when used as filter fields. Soil feature considered is the top depth of the first restrictive layer where restrictive kind is "fragipan", "duripan", "petrocalcic", "ortstein", or "petrogypsic" and restrictive feature hardness is not noncemented. The vertical separation between the pipe depth and depth to restriction is the important consideration to make sure sufficient  soil material is available for proper dispersion.  
  • Limiting                                      =< 40 inches
  • Somewhat limiting       > 40 to < 60 inches
  • Not limiting                              => 60 inches

Susceptibility to Flooding

 Flooding has the potential to transport agricultural waste off site and pollute surface waters. Flooding also limits building, recreational, and sanitary facility use and management of these soils. Soil feature considered is maximum flooding frequency classes over 12 months.

  •  Limiting                   = "occasional" or "very frequent" or "frequent“
  • Somewhat limiting = "very rare" or "rare“
  • Not limiting             = "none"

Susceptibility to Ponding

Ponding is the condition where standing water is on the soil surface for a given period of time. Soil that pond has restrictions that limit the installation and function of most landuse applications. Soil features considered are ponding duration and frequency classes for all months

  • Limiting              = ponded
  • Not limiting = not ponded


Soils with high subsidence can cause septic tanks and septic tank adsorption fields to shift or collapse. Soil feature considered is total subsidence.

  • Limiting              > 24 inches
  • Not limiting       < 24 inches


Steep slopes impede trafficability of heavy machinery and reclamation of borrow sites. Soil feature considered is the component slope.

The “onsite waste dispersal must be sloped between 1/8" per foot and 1/4" per foot. The pipe must drop at least one inch for every 8 feet of sewer line, and not more than one inch for every 4 feet all the way to the septic tank. Less slope and the flow is too slow to clear the pipe.

Limiting                            > 15%
Somewhat limiting         >= 8 and <= 15%
Not limiting                      < 8%
What are the most common soil limitations?  Here are two analyses:

The Left is limitation by acres on a national scale.  

The Right is a count of the number of times a limitation appears for a soil.

As you can see, the 'slow water movement' is the most common limitation necessary to overcome in the septic design.

Soil Color

Next to texture and structure, soil color is the most useful indicator  identifying water movement within the soil.

Bright colors indicate oxygen saturation and proper water drainage.

Gray coloring indicates oxygen reduction due to stagnant water movement. 
Soil Horizons
How to separate soil horizons
Visualizing and separating the soil horizons is important to understand the soil properties.  The 3 to 4 major soil horizons are usually parallel to the soil surface.  The surface horizon is typically denoted as 'A' if mineral material or 'O' if an organic surface.  The subsoil is labeled as 'B'.  The substratum is labeled as 'C'.  And, the bedrock materials are labeled as 'R'.  Combinations of these master horizons allows the soil scientist to explain the soil profile.  
Horizon subscripts are used to provide further explanation of soil properties.  For additional information, view this web page.

Overview of Septic System Design

Alternative or Non-conventional systems

The conventional system is the least expensive method of septic dispersal.  Alternative wastewater systems are available and designed based on the specfic soil limitations.  Knowledge of the soil limitation is necessary for septic system design.  The most common alternative systems are mound, lagoons, controlled dispersal, and aerobic wastewater treatment systems.
Mound Systems

Typically used in instances where the soil has a water table within minimum vertical separation distance or in soils containing a high clay content.
Lagoon Systems

Lagoon (and wetland) systems are an aerobic system typically available in rural areas. 

Controlled Dispersal:​ 
Controlled dispersal allows for dosing the expanded septic dispersal field. 

Aerobic Wastewater Treatment  Units (ATU):

These mini wastewater systems treat the septage with oxygen prior to release.  These systems require electricity and maintenance.
Holding Tanks:

The most drastic method typically used as a last resort.  The holding tank is designed to hold 1,000 to 4,000 gallons and pumped regularly.  A management agreemnt with a spetage hauler is required for regular pumping of the tank.   

Additional details on alternative systems can be found on the InspectAPedia web page