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Soil Tests

The three basic types of tests used to determine whether or not soil is suitable for a septic tank system are: the percolation test (perc test), soil core analysis, and backhoe cuts to a maximum depth of 12 feet. The percolation test consists of digging a series of holes in the area of the effluent distribution field, prewetting the inside of the holes, adding a measured amount of new water to the holes, and then measuring how fast the water flows out. (See endnote 1 for total details of an acceptable procedure.) Core sample analysis consists of digging a hole to remove a core sample of the various soil layers and taking it back to a laboratory for analysis or digging a deep hole with a machine typically 7-10 feet deep or more and looking at the sides of the hole for variations in color and types of soils. This will indicate the drainage characteristics of that particular piece of land at the hole. The backhoe cut is made into good holes 25 feet apart. It is necessary in order to get a permit to demonstrate at least 4 feet of sand, sandy loam, or loam which is 4 feet above the seasonally high water table as determined by soil conditions. Where clay is encountered first and then sand still 4 feet above the seasonally high water table, 50% of the clay could be removed and replaced with clean sand which creates a conduit to the virgin sand below. In this type of test especially it is very important for the individuals conducting the tests, whether it be the local environmental health practitioner or others who are certified, to wear hard hats and special boots, and stay away from the perimeter of the hole to prevent a cave in and significant damage or death to the individual.

The problems associated with percolation tests are numerous. There is a huge variation in the qualifications of individuals who perform these tests depending on local and/or state regulations. Tests that are conducted during dry weather give false readings. There are times when the test holes are dug under the house and under the driveway according to the plans instead of in the distribution field. Where fill dirt has been used in a distribution field, the tests will be inaccurate. The number of test holes depends on the average percolation rate of the soil and where that is greater than 60 minutes per inch, there will probably be future problems of liquid dispersal and this will restrict the use of the land. Since soil conditions may vary considerably just a few feet away from the test hole, the results of the percolation test may not be accurate for the entire area being represented. The slope of the land may affect the test if it is very steep.

The problems associated with core samples and core sample analyses are numerous. Once again, the qualifications of the individuals making a decision on permeability by looking at the type of soil in a core analysis or a hole dug by a machine, lead to different interpretations of color and soil type. Thus there may be improper decisions made on soil usability for a septic tank system. A single core sample will not give a complete picture of the soil percolation capabilities of the entire potential system. Multiple soil core samples must be taken within the future distribution field site.

Best Practices for Soil Analysis for On-Site Sewage Distribution Systems

  • • Only use highly qualified certified individuals with local experience who are closely supervised to conduct percolation tests and core soil analysis tests.
  • • Follow all local and state rules and regulations concerning the tests and how to conduct them properly.
  • • Mark on the house plans and sewer plans the places where the percolation tests and soil core analysis tests are conducted and make sure that the places chosen are actually used.
  • • Use both a soil core analysis test and a percolation test to determine if a septic tank system can be installed, the size of the system, and the type of system.
  • • Document the seasonal high water table depth, soil types and depth of each different type of soil stratum and soil texture going down into the hole, and depth of bedrock or hardpan (a layer of tight clay or tightly packed soil).
  • • Presoak all holes used for percolation tests and maintain 12-14 inches of clear water for at least 4 hours. Begin the test 15-30 hours after the presoak. Adjust the water to 6 inches above gravel in the hole and take two readings at 30-minute intervals. Use the slower of the two readings as your percolation rate.
  • • Determine the size of the effluent distribution field by the percolation rate, the type of soil, and the number of bedrooms within the house, which gives a rough estimate of the number of people living there.
  • • If percolation rates are greater than 60 minutes per inch, then a much larger lot size will be needed for a house providing the slope of the land is not too great and the depth below the drain field to an impervious formation is at least 10 feet of usable soil for disposal purposes. The extra lot size will provide for reserve areas if the effluent distribution field fails and a new one has to be built.
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