Title 124 - NEBRASKA DEPARTMENT OF ENVIRONMENTAL QUALITY
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 twelve 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. 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 in less than ten minutes, the test can proceed immediately. See 002.05C
002.05 Percolation rate measurements should be made on the day following the saturation process, except in sandy soils.
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 In sandy soils, or in other soils in which the first six inches of water seeps away in less than 30 minutes even after the overnight swelling period, the time interval between measurements can be taken as ten minutes, refilling the hole to a depth of 6 inches as necessary after each interval, six test measurements will be made at ten minute intervals. The drop that occurs during the final ten minutes is used to calculate the percolation rate.
002.05D Soils with moderately slow permeability and/or containing greater than 30% clay in the testing zone will require several days saturation when the soil is dry in the summer and other seasons of the year to obtain the required saturation prior to making measurements.
002.06 The percolation test data shall be recorded and maintained on the premises or submitted to the Department as required in Chapter 3.
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 To obtain the percolation rate of a test hole (the average time in minutes for water to fall one inch), divide the number of minutes elapsed by the drop in inches. An average percolation rate for the entire field should then be determined from the percolation rate of each of the test holes. If tests in the area vary more than 20 minutes/inch, variations in soil type are indicated.
Under these circumstances, percolation rates should not be averaged and design should be based on the slowest rate, or site relocation should be considered.
004 Site Acceptability Based on Soil Conditions.
004.01 Soil is unsuitable for a soil absorption system if the percolation rate is faster than 5 minutes per inch, or is slower than 60 minutes per inch.
004.01A Soils faster than 5 minutes per inch are acceptable if a twelve-inch thick loamy sand soil liner with a percolation rate of 15 to 20 minutes per inch is installed in the trench in accordance with Chapter 14. A trench is then sized on this soil liner’s percolation rate.
004.02 A soil absorption system will not be installed if the percolation rate is outside the range defined above unless designed by a professional engineer and a construction permit is issued in accordance with Chapter 3, Section 003.
004.03 Any alternative on-site wastewater treatment system which is proposed to accommodate unsuitable soil conditions will have to be approved by the Department in accordance with Chapter 3, Section 003.
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.
Nebraska Department of Environmental Quality
1200 "N" Street, Suite 400
P.O. Box 98922
Lincoln, Nebraska 68509
(402) 471-2186