Site Evaluation for On-lot Sewage Systems

Proper design and operation of an on-lot sewage system hinges on the ability of the soil to absorb and renovate the wastewater flowing from the treatment tank.
Site Evaluation for On-lot Sewage Systems - Articles


Therefore, before any on-lot sewage system can be selected and designed, the soil properties at the site must be evaluated. For the soil to properly renovate the treatment tank's effluent, two soil-related events must occur;

  • the wastewater must be absorbed into the soil profile, and
  • the wastewater must percolate through the suitable soil, thus coming into contact with an aerobic (oxygenated) soil environment.

To maximize the likelihood that your on-lot sewage system will perform properly for many years, several soil evaluations are required before the type of system can be chosen and the system can be designed and built.

The first soil evaluation is known as the Deep Probe or Probe. The Probe consists of digging a minimum of two deep pits near the intended absorption area. The purpose of the Probe is so the Sewage Enforcement Officer (SEO) or a Certified Professional Soil Scientist can evaluate the soil profile and locate the Limiting Zone.

The second soil evaluation is designed to determine how rapidly the septic-tank effluent will percolate into and through the soil (the Water Movement Rate). Depending on the depth to the Limiting Zone and the type of disposal system chosen, this evaluation may require either (a) the Percolation Test or "Perc Test" or (b) a soil scientist's soil morphologic evaluation. Note that the Probe and Water Movement Rate evaluations should be completed successfully before you agree to purchase a lot. Failure of either of these evaluations may make it very expensive to properly dispose of the wastewater generated on your property. The on-lot treatment system is outlined in Box 1. The purpose of this fact sheet is to explain why these two soil evaluation procedures are important to the success of your on-lot sewage system and to help you understand what soil characteristics are necessary for the soil to renovate the wastewater.

Box 1. On-Lot Wastewater Treatment System

If you cannot connect to a public sewer system, all of the wastewater from your home must be delivered to an on-lot sewage system, see Figure 1. An on-lot sewage system consists of a treatment tank (most frequently a septic tank) that is designed to remove the solids, and an absorption area containing a number of pipes to distribute the treatment tank's effluent to the absorption area where the wastewater is distributed so it will enter the soil and receive final treatment by the soil. This two-stage, solids-liquid treatment system, is an effective, low maintenance system designed to properly dispose of your wastewater and protect the local surface and ground water from contamination.

Figure 1. On-lot sewage system with a septic tank and trenches absorption area. The insert illustrates wastewater movement through the soil.

The Probe

The first soil evaluation procedure, following a general site survey to determine where the house and absorption area should be located, is the Probe. The Probe consists of bringing in a backhoe and digging two or more soil observation pits within 10 feet of where the absorption area is expected to be located. The pit must be dug to the limiting zone or seven feet deep. The purpose of this soil pit is to permit the Sewage Enforcement Officer (SEO) or a Certified Professional Soil Scientist to examine the soil profile for, what is referred to in the regulations, as a Limiting Zone. The limiting zone is defined as the upper limit of any zone or soil layer that is expected to limit the soil's ability to treat the wastewater. There are three types of limiting zones;

  1. a soil layer that contains a watertable or redoximorphic features (mottling) providing evidence of a seasonally high watertable,
  2. a soil layer or rock that has such slow permeability that the effluent will not be able to penetrate this layer at a rate that will permit the proper treatment of the wastewater, or
  3. a soil layer that does not contain sufficient fines (such as a gravel layer or a layer of shattered rock) to provide contact between the effluent and the soil particles to provide proper treatment of the effluent.

Research and experience have shown that effluent from a treatment tank needs to percolate through at least four feet of an aerobic soil profile for the soil to properly treat the effluent. In most systems, the 4-foot minimum distance is measured vertically upward from the limiting zone to the bottom of the absorption area. In some alternate systems, the 4-foot separation requirement has been relaxed, but these special cases require an additional pre-treatment unit, such as a sand or peat filter, or an aerated geotextile unit between the treatment tank and where the effluent enters the soil. In a few alternate systems, disinfection may also be required. The 4 feet of suitable soil is where the treatment tank effluent undergoes the necessary physical, chemical and biological transformations needed to convert the complex organic waste compounds to relatively harmless simple organics, carbon dioxide and water. These treatment processes are enhanced by contact with the soil particles, especially when there is sufficient oxygen present in the soil to support the growth of aerobic microorganisms.

In Pennsylvania, as the distance between the soil surface and the limiting zone decreases, the number of on-lot sewage disposal options available to the homeowner decreases and the systems become more costly. The minimum depth-to-limiting-zone requirements for various types of soil absorption areas are shown in Table 1.

Table 1. How the depth from soil surface to limiting zone affects the type of absorption system that maybe used.
Depth between
the Soil Surface
and the
Limiting Zone
Absorption Area Required
*These systems require a sand or peat advanced-treatment filter between the treatment tank and the absorption area.
≥ 60In Ground Seepage Bed or Trenches
≥ 58Shallow Placement with Pressure Dosing*
≥ 48At-Grade System
≥ 20Elevated Sand Mound
≥ 16 to Rock LZAt-Grade System; Drip System*; GFS System
≥ 10 to Rock LZIndividual Residential Spray Irrigation System; At-Grade system*; GFS System

Water Movement Rate

The second soil evaluation is designed to determine how rapidly the septic-tank effluent will percolate into and through the soil (the Water Movement Rate). Depending on the depth to the Limiting Zone and the type of disposal system chosen, this evaluation may require either

  1. the Percolation Test or "Perc Test" or
  2. a soil scientist's soil morphologic evaluation.

Percolation Test (Perc Test)

The Perc Test is a relatively complex procedure designed to determine how rapidly effluent will enter the soil on your property during the wettest, worst-case conditions. Briefly, the Perc Test consists of measuring the rate of water drop (or infiltration) in at least six vertical holes dug in the intended absorption area, see Figure 2. The Perc Test must be conducted by the SEO.

Figure 2. Typical cross-section of a perc test hole.

Evaluating the Perc Rate

Once the site's Perc Rate has been determined, this information along with the depth to limiting zone is used to determine the suitability of the site for one or several absorption areas. It should be noted that Individual Residential Spray Irrigation, Drip Irrigation and Greenhouse Systems do not require a Perc Test. In Pennsylvania, the Perc Rate must fall between 6 and 90 minutes per inch to use most conventional absorption areas. For elevated sand mounds, a Perc Rate between 3-180 minutes per inch is required. For absorption areas that require a Perc Test, the absorption area is sized by taking into account the site's Perc Rate and the home's maximum expected wastewater flow, which is based on the number of bedrooms. For example, a three-bedroom home may require an absorption area as small as 476 square feet if the Perc Rate is 6 min/inch. The same three-bedroom home will require an absorption area as large as 1,064 square feet if the Perc Rate is 60 minutes per inch. If the Perc Rate is faster than 3 minutes per inch or slower than 180 minutes per inch, the site is unsuitable for any absorption area that requires a Perc Test.

Soil Morphological Evaluation

For Drip Irrigation systems and other soil-based disposal sites with Limiting Zones at less than 20 inches, the Perc Test is not an approved method of measuring how rapidly effluent will enter and flow through the soil. On these sites the absorption area size is determined based the soils morphological characteristics. This evaluation must be conducted by a Certified Professional Soil Scientist and is conducted in conjunction with the Probe evaluation. Three soil characteristics must be determined for the site (taken as the most restrictive conditions found in several Probes);

  1. the soil's texture,
  2. the soil's structure shape, and
  3. the degree or grade of the structural development.

Soil scientists have correlated the rate at which water will move through a soil to these three soil morphological characteristics. Table 2 summarizes the water flow parameters that are to be used in sizing these shallow systems. Note from Table 2 that by knowing the soil's texture (left column) and the shape and grade of the soil's structure, two design related parameters are given for each soil. These design parameters are (a) the Infiltration Loading Rate (ILR) in units of gallons per square foot per day (gal/ft2/d), and (b) the Hydraulic Linear Loading Rate (HLLR) in units of gallons per feet per day (gal/ft/d). The HLLR is also dependent on the slope of the site and the depth to which the effluent can penetrate before reaching the Limiting Zone (the Infiltration Distance).

On those sites where the Limiting Zone is closer to the soil surface than 20 inches, a single linear absorption system is required. In other words, the effluent is uniformly distributed along a single pressurized, perforated pipe laid along the contour adding effluent to the soil so the soil can accept and treat the effluent. The length of the distribution unit is determined by dividing the design daily flow from the household by the HLLR. The width of the distribution unit is determined by dividing the HLLR by the ILR. The following example will demonstrate these parameters.

Example: A site is found to have a silt loam (SIL) soil with friable, weak (1) blocky (BK) structure. The site has a 12% slope and a Limiting Zone at 15 inches. The home has a design daily discharge rate of 400 gallons per day.

Solution: From Table 2 the Infiltration Loading Rate (ILR) = 0.6 gal/ft2/day. Also from Table 2, the Hydraulic Linear Loading Rate (HLLR) = 3.5 gal/ft/day.

The length of the distribution area should be the design daily flow divided by the HLLR, or

L = 400 gpd/3.5 gpd/ft = 115 ft.
Table 2. Hydraulic Linear Loading Rates (HLLR) (gal/ft/d)
TextureStructure ShapeStructure GradeInfiltration
Loading Rate (ILR)
slope = 0-5%
Infilt. 10-12"
slope = 0-5%
Infilt. 12-20"
slope = 5-10%
Infilt. 10-12"
slope = 5-10%
Infilt. 12-20"
slope = >10%
Infilt. 10-12"
slope = >10%
Infilt. 12-20"

Adapted from Tyler, 2000.

Width of Infiltration Field = Hydraulic Linear Loading Rate divided by Infiltration Loading Rate.
Length of Infiltration Field = Wastewater Volume divided by Hydraulic Linear Loading Rate.

The abbreviations used in the first three columns of Table 2 are:
Soil Texture: COS = coarse sand, S = sand, LCOS = loamy coarse sand, LS = loamy sand, FS = fine sand, VFS = very fine sand, LFS = loamy fine sand, VLFS = loamy very fine sand, COSL = coarse sandy loam, SL = sandy loam, FSL = fine sandy loam, VFSL = very fine sandy loam, L = loam, SIL = silt loam, SI = silt, SCL = sandy clay loam, CL = clay loam, SICL = silty clay loam, SC = sandy clay, C = clay, SIC = silty clay
Structure, Shape: PL = platy, PR = prismatic, BK = blocky, GR = granular, SG = single grain, M = massive
Structure, Grade: 0 = structureless, 1 = weak, 2 = moderate, 3 = strong

The width of the distribution area should be the HLLR divided by the ILR, or

W = 3.5 gpd/ft/0.6 gpd/ft2 = 5.8 ft (use 6 ft).

Therefore, the distribution area for this property should be 6 feet wide and 115 feet long and placed along the contour.

For additional assistance contact our local Sewage Enforcement Officer and/or your county Extension Educator.

Pennsylvania Association of Sewage Enforcement Officers (PASEO)
4902 Carlisle Pike, #268
Mechanicsburg, PA 17050

Pennsylvania Septage Management Association (PSMA)
P.O. Box 144
Bethlehem, PA 18016

Prepared by Albert R. Jarrett, Professor Emeritus of Agricultural Engineering and Joseph (Jake) Eckenrode, Certified Professional Soil Scientist