NEBRASKA ADMINISTRATIVE CODE
NEBRASKA DEPARTMENT OF ENVIRONMENTAL QUALITY

Title 124 - Rules And Regulations For The Design, Operation And Maintenance of Onsite Wastewater Treatment Systems
Chapter 6 - Soil Percolation

001 Soil percolation tests shall be conducted in the area where the soil absorption system will be located. Such tests shall not be made on disturbed ground or frozen ground. Where fissured or creviced formations are encountered below the ground surface, the Department shall be consulted for assistance. Soil percolation tests shall be conducted by a professional engineer, registered environmental health specialist, or a certified professional holding a certificate in the category of Inspector, Soil Evaluator, Master Installer, or Journeyman Installer.

002 Percolation tests shall be performed as follows:

002.01 At least three test holes shall be dug and spaced uniformly over the proposed absorption field site. If the difference between the fastest and the slowest measured percolation rate is greater than 20 minutes per inch, or there are other indications that soil conditions are highly variable, a minimum of four test holes and two test holes per lateral is required.

002.02 These holes shall be dug or bored with horizontal dimensions of from four to twelve inches and vertical sides to the depth of the bottom of the proposed distribution trench. Holes can be bored with a posthole type auger.

002.03 Roughen or scratch the bottom and sides of the holes to provide a natural surface. Remove all loose material from the hole. Place about two inches of 1/4 to 3/4 inch gravel in the hole to prevent bottom scouring.

002.04 Fill the hole with clear water to a minimum depth of 12 inches over the gravel. By refilling, if necessary, or by supplying a surplus reservoir of water (automatic siphon), keep water in the hole for at least four hours and preferably overnight.

002.04A Soils with moderately slow permeability or that contain greater than 30 percent clay will require several days soaking to reach saturation, especially when the soil is dry, in order to obtain the required saturation prior to making measurements.

002.04B In sandy soils containing little or no clay, soaking is not necessary. If after filling the hole twice with 12 inches of water the water seeps completely away each time in less than 10 minutes then the test can proceed immediately and described in 002.05C below.

002.05 Percolation rate measurements should be made on the day following the saturation process, except in highly permeable sandy soils with fast percolation rates as noted below or in less permeable soils with high clay content and slow percolation rates, as note above. For all soils, the percolation rate of the planned last test measurement for any one test hole should approach a uniform rate and not vary more than 10 percent from the previous measurement for that test hole.

002.05A If the water remains in the test hole after overnight saturation, adjust the water depth to a minimum of six inches over the gravel. From a fixed reference point, measure the drop in water level during an approximate 30 minute period.

002.05B If no water remains in the hole after overnight saturation, add clear water to a depth of six inches over the gravel. From a fixed referenced point, measure the drop in water level at approximate 30 minute intervals over a four hour period, refilling the hole to a depth of six inches as necessary after each 30 minute period. The drop which occurs during the final 30 minute period is used to calculate the percolation rate.

002.05C A shorter measurement time interval of 10 minutes may be used for sandy or coarse grained soils with fast permeability where the first six inches of water seeps away in less than 30 minutes even after the overnight saturation or swelling period. Six test measurements shall be taken, one at the end of each 10 minute interval, refilling the hole to a depth of six inches as necessary after each interval. The drop that occurs during the final 10 minutes is used to calculate the percolation rate.

002.06 The percolation test data shall be recorded and maintained on the premises, and made available to the Department by the owner or installer upon request.

002.07 Other methods of determining the percolation rate may be approved by the Department if the method is recognized as providing accurate and consistent results.

003 The percolation rate of a test hole (the time in minutes for the water level in the test hole to drop one inch) is determined by dividing the number of minutes elapsed by the water level drop in inches during the final measurement period. The design percolation rate for the soil absorption system shall be determined by averaging the percolation rate of all the test holes unless the difference between the fastest and slowest measured rates in an area is more than 20 minutes per inch, in which case the slowest percolation rate shall be used. If any percolation test is faster then five minutes per inch or slower than 60 minutes per inch, then see 004 below.

004 Site Acceptability Based on Soil Conditions
soil absorption system in fill is prohibited except as provided for in Chapter 14.

004.01 Soil is unsuitable for a soil absorption system if the percolation rate is faster than five minutes per inch or is slower than 60 minutes per inch, except as provided for below.

004.02 Soils with a percolation rate faster than five minutes per inch are acceptable if a 12-inch thick loamy sand soil liner with a percolation rate of 15 to 20 minutes per inch is installed in the trench or bed in accordance with Chapter 14. The trench or bed is then sized based on this soil liner percolation rate.

004.03 A soil absorption system shall not be installed if the percolation rate is slower than 60 minutes per inch unless designed by a professional engineer and a construction permit is issued in accordance with Chapter 3.

004.04 Installation of an alternative onsite wastewater treatment system to accommodate unsuitable soil conditions requires a construction permit approved by the Department in accordance with Chapter 3.

004.05 Construction of a soil absorption system to fill is prohibited except as provided for in Chapter 14.

005 The Department may require verification of percolation rates when submitted results are inconsistent with other known data.


Enabling Legislation: Neb. Rev. Stat. §81-1505(8), §81-15,248.

Legal Citation: Title 124, Ch. 6, Nebraska Department of Environmental Quality.