Residential Onsite Wastewater Treatment: Site Evaluation

This NebGuide, a cooperative effort by the Nebraska Onsite Wastewater Treatment Task Force, whose members are listed on the last page, discusses evaluation of a site to gather information that will help determine the type and size of residential onsite wastewater treatment system best suited to that location. It is recommended to select the location of the wastewater treatment system before finalizing house and well placement, since site characteristics may limit system options.

Jan Hygnstrom, Extension Project Manager
Sharon Skipton, Extension Educator
Wayne Woldt, Extension Specialist

Site Evaluation — Before Visiting the Site
Site Evaluation — Initial Evaluation at the Site
Site Evaluation — Detailed Evaluation at the Drainfield Site
Summary

The most common onsite wastewater treatment sys tem for rural homes consists of a septic tank and drainfield, with more than 25 percent of all American homes having such systems. The success of an onsite wastewater treat ment system depends largely upon soil characteristics, as well as system design, installation and maintenance. This NebGuide will explain how to evaluate a site for a system and how to narrow the choices between different types of systems. For general information on how a septic tank/effluent treatment system works, see the NebGuides Residential Onsite Wastewater Treatment: Septic Tank Design and Installation (G1473), Traditional Drainfield Systems for Septic Tank Effluent Treatment (G1479), and Gravelless Drainfield Systems for Septic Tank Effluent Treatment (G1480).

Site Evaluation — Before Visiting the Site

A great deal of information is required to conduct a site evaluation. Much of it can be collected and organized before the onsite evaluation. Nebraska law states that Diggers Hotline of Nebraska (1-800-331-5666) must be called at least two business days before excavation. They will contact local utilities to mark registered electrical, gas, phone, fiber optics, cable, public water supply and public sewer lines on the property. Following is a list of information needed to plan the site evaluation and where to find it.

Figure 1. Drainfield trenches on contour.

Topography (slope, surface drainage) from 7.5 minute USGS topographic maps: UNL Conservation & Survey
Local setbacks (wells, property line, surface water, roads): County Planning and Zoning
Soils from U.S. Department of Agriculture county soil surveys: UNL Conservation & Survey, Natural Resources Conservation Service (NRCS)
Legal description: Deed, County Planning & Zoning
Easements, utilities: County Planning & Zoning, local utilities
Legal requirements of design/installation: County Planning & Zoning, Nebraska Department of Environmental Quality (NDEQ)
Flooding information: NRCS
Well location and depth: Well driller, Nebraska Department of Natural Resources Groundwater Section

Table I. State minimum setback distances (local codes may be more stringent)

Item	Tanks	Effluent Treatment Systems (Drainfields, etc.)	Lagoons

Surface water Private drinking water wells	50 ft 50 ft	50 ft 100 ft	50 ft 100 ft
Public drinking water wells: Non-community system Community system	50 ft 500 ft	100 ft 500 ft	100 ft 500 ft
Water lines: Pressure-main Pressure service connection Suction lines	10 ft 10 ft 50 ft	25 ft 25 ft 100 ft	25 ft 25 ft 100 ft
Trees	No setback	No setback	50 ft
Property Lines	5 ft	5 ft	50 ft
Building foundations Class 1 foundations Class 2 foundations Class 3 foundations	15 ft 10 ft 7 ft	30 ft 20 ft 10 ft	100 ft 100 ft 50 ft
Neighbors’ building foundations Class 1 foundations Class 2 foundations Class 3 foundations	25 ft 20 ft 15 ft	40 ft 30 ft 20 ft	200 ft 200 ft 100 ft

Class 1 foundations: Full basements or non-basement footing foundations and slab on grade for living quarters that are lower in elevation from the onsite wastewater treatment system. Class 2 foundations: Non-basement footing foundations, trailer houses and slab on grade living quarters that are higher in elevation than the onsite wastewater treatment system. Class 3 foundations: Structures using slab on grade construction and are not used as living quarters.

Slope

Sites with a slope of less than 15 percent (15-foot drop in 100 feet of run) usually do not create a serious problem for constructing a traditional septic tank and drainfield system. Construction is more difficult and costly on steeper slopes. Place drainfield trenches on the contour (Figure 1) to keep trenches from being too deep for oxygen to reach the wastewater treatment area; bacteria that treat wastewater need air. Slope can be estimated using a topographic map, available at UNL Conservation and Survey. Verify slope at the site using a transit, laser level or 10-foot board and carpenter’s level.

Setback distances

Nebraska Department of Environmental Quality (NDEQ) Title 124, Rules and Regulations for the Design, Operation, and Maintenance of Onsite Wastewater Treatment Systems lists setbacks, or minimum distances, from septic tanks, drainfields and lagoons to features listed in Table I. Check local codes as they may be more stringent. Maintaining setback distances will reduce the possibility of contaminating ground and surface water. In addition, following setback distances between a drainfield and buildings will help prevent rainwater from a roof from inundating and overtaxing the drainfield.

Consider the placement of the house, water supply well, onsite wastewater treatment system and reserve area, as well as where neighbors have, or might place, these features. Thought should be given to potential areas for a driveway, garage, swimming pool and other improvements.

Soil characteristics

Soil plays a very important role in determining the type of wastewater treatment system best suited for a site. The county soil survey report is a valuable starting point for evaluating soil at a site. The survey report shows soil characteristics such as soil type, soil permeability, depth to bedrock or seasonal high groundwater table; slope; and limitation ratings for drainfields. Soil survey reports may be available at local University of Nebraska–Lincoln Extension or Natural Resources Conservation Service (NRCS) offices, or at Conservation and Survey within the UNL School of Natural Resources. See the NebGuide Residential Onsite Wastewater Treatment Systems: The Role of Soil (G1468) for more detailed information on soils and the soil survey report.

Figure 2. Sketch of site.

Site Evaluation — Initial Evaluation at the Site

When preliminary information on slope, soils and existing improvements has been collected, an onsite evaluation can be done. Begin with a scaled sketch of the site showing property lines and other features (Figure 2). The property owner should be present to discuss options and issues. Following is information on two important aspects of the site evaluation — looking at vegetation and the soil.

Vegetation

Vegetation can provide clues to soil drainage and soil type. For example, cattails, reeds and willows indicate wet soil areas. Record these on your sketch or map of the site.

Soil

An initial soil evaluation should provide information on soil characteristics, including seasonal high groundwater level. Alternate saturation and drying of soil results in discoloration or staining that is not part of the dominant soil color. These various shades of gray, brown, red or yellow are called mottling. They show that groundwater has, at some point in time, been at that elevation, and remained long enough to cause a chemical reaction, changing the soil color. The highest groundwater elevation is assumed to be the highest point at which mottling is observed, regardless of whether water is present at the time of observation.

One, or a combination, of several methods can be used for an initial soil evaluation at the site. These include probing, augering, or digging a pit.

Probing — A soil probe is a hollow tube that, when pushed into the soil and extracted, gives an undisturbed column of soil for viewing. Probes vary in length and diameter. Usually extensions can be added to probe deeper into the soil. It is the quickest method of looking at soil, and also allows you to detect faint soil mottling or cemented layers. Disadvantages are the relatively small diameter of the sample and inability to penetrate soil in rocky areas or under very dry conditions.
Augering — Like a probe, an auger provides a column of soil for viewing when extracted. However, unlike a probe, an auger will disturb the soil sample. Auger diameters are typically larger than probes, and extensions can be added to access deeper into the soil. An auger produces larger samples and is more effective in rocky areas than a probe, although it still may be difficult to use due to rocks. Some disadvantages are that the auger is slower and more labor intensive, and the disturbed nature of the sample may not reveal faint mottles, cemented layers, or structure.
Soil Pit — A pit is a dug-out area near the perimeter of an expected drainfield area. Do not dig a pit within the proposed drainfield area, as soil can settle after the system has been installed, disrupting the distribution lines. Sunshine should reach the sidewall during the observation period so subtle differences in soil color will be most visible. Soil pits provide the best method for viewing both undisturbed soil and how soil varies over the depth of the pit. Pits may be the only reliable method to determine depth to bedrock. However, digging a pit requires considerable manual labor or a backhoe, which can be costly. Safety is another consideration. Pits are especially hazardous in sandy soil or if excavated below the current water table depth. When digging, construct walls with a step-type configuration for safe entry and exit. Take appropriate safety precautions when working around the hole to prevent a cave-in, especially if sidewall soil is unstable or sandy. Heavy equipment or large objects such as boulders should not be allowed to rest on the surface immediately next to the pit sidewalls. Pits should be fenced to avoid falls or unauthorized entry and backfilled as quickly as possible after use.

Site Evaluation — Detailed Evaluation at the Drainfield Site

The owner probably will have a potential building site selected. Select two or three appropriate areas for the onsite wastewater treatment system, keeping setback distances and slope in mind. Measure soil permeability with a soil percolation test. Note that in Nebraska, only a certified inspector, installer, soil evaluator, registered environmental health specialist or professional engineer may conduct a perfolation test for onsite wastewater treatment application. Check depth to bedrock and seasonal high groundwater at the area considered to be best suited for the wastewater treatment system. If conditions are not favorable, try the next appropriate area for the treatment system. If that area is not suitable, either select a different building site and site for the wastewater treatment system, or be prepared to consider an alternative system.

Soil Percolation Test

Soil percolation or perc tests are used to determine the ability of a soil to transmit wastewater effluent through the soil profile. The soil percolation rate is the amount of time it takes water to move through soil, measured in minutes per inch. Finer textured soils have slower percolation rates; it takes longer for water to drain from a test hole. These soil types need larger drainfields than soils with faster percolation rates, such as sandy soils, to handle a given amount of wastewater. Soils with very slow percolation rates may not be suitable for drainfields. In Nebraska, if soils perc at a rate slower than 60 minutes per inch, consider installing a lagoon system if the lot is at least 3 acres. Otherwise, an engineer must design a specialized system. Soils with very fast percolation rates, less than 5 minutes per inch, must be modified by adding a loamy sand liner to the drainfield, so that proper treatment can occur.

The Nebraska Department of Environmental Quality Title 124 requires that a percolation test be conducted to help determine the feasibility, type and size of an onsite waste?water system. Title 124 specifies who may conduct a percolation test. In addition, some counties have a list of approved individuals. Contact your local health or planning department for requirements in your area. Specifics for conducting a perc test for onsite wastewater treatment application can be found in the NebGuide Residential Onsite Wastewater Treatment: Conducting a Percolation Test, (G1472).

Soil Borings

Borings help determine soil texture and properties, as well as depth to bedrock or seasonal high groundwater at the drainfield site. To do a soil boring:

Locate the center of the proposed wastewater treatment system area.
Make one soil boring, at least five to six feet deep, and preferably 10-20 feet deep, noting changes in the soil.
Estimate the maximum depth of the system (typically 20 to 66 inches of vertical depth) and make a written note of soil texture (proportion of sand, silt, or clay) at that depth.
Make four additional borings at the system corners, four feet below the proposed depth of the system and make a written note of soil texture, and vertical depth to bedrock and seasonal high groundwater if encountered at those locations.

Summary

Collecting and understanding site information is critical to selecting and installing the most appropriate onsite wastewater treatment system, and siting it in a location for best performance and longest life expectancy. Slope, setbacks and soil characteristics must be evaluated. Soil permeability is an important soil characteristic to consider. Therefore, while determining where to construct any improvements, including buildings, a well and an onsite wastewater treatment system, a professional must measure soil permeability with a soil percolation test and check depth to bedrock and seasonal high groundwater. The professional will use information gathered from the site evaluation, including the percolation rate, to select the most appropriate system, and to help determine the proper size and location for septic tank/drainfield systems.

Partial funding for materials development was provided by the U.S. Environmental Protection Agency, Region VII, and the Nebraska Department of Environmental Quality under Section 319 of the Clean Water Act (Nonpoint Source Programs).

Nebraska Onsite Wastewater Task Force members included: Tom Baker, Douglas County Health Department; Francis Belohlavy, UNL School of Natural Resources; John Chess, Lincoln-Lancaster County Health Department; Doug Clark, Douglas County Health Department; Duane Eversoll, UNL School of Natural Resources; Steven Goans, Nebraska Department of Environmental Quality; DeLynn Hay, UNL Extension; Rick Hoopes, Nebraska Department of Environmental Quality; Troy Huffman, Nebraska Health and Human Services System; Jan Hygnstrom, UNL Extension; Mark Kuzila, UNL School of Natural Resources; Randy Nickeson, Douglas County Health Department; Terry Philippi, Nebraska Health and Human Services System; Randy Pryor, UNL Extension; George Reese, Sarpy County Planning and Building Department; Sharon Skipton, UNL Extension; Wayne Woldt, UNL Extension; and Gary Zoubek, UNL Extension.

Technical reviews were provided by Steve Goans and Rick Hoopes, Nebraska Department of Environmental Quality; Paul Jasa and Jim Peterson, UNL Extension; and Francis Belohlavy, UNL School of Natural Resources.

Visit the University of Nebraska–Lincoln Extension Publications Web site for more publications.
Index: Waste Management
Home Waste Systems
2002, Revised September 2006

Extension is a Division of the Institute of Agriculture and Natural Resources at the University of Nebraska�Lincoln cooperating with the Counties and the United States Department of Agriculture.

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