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Soil Percolation Tests

The percolation test is designed to determine the suitability of a site for a subsurface private sewage disposal system (i.e. septic system). More specifically, a percolation test measures the ability of the soil to absorb liquid. Septic system designers use the results of percolation tests to properly construct septic systems. In DuPage County, the witnessed percolation test is the primary means by which the suitability of a lot for a septic system is determined.

The percolation tests are designed to simulate conditions in a septic system. Most septic fields consist of a series of trenches about two feet deep. Therefore, the percolation test holes are dug to a depth of two feet. In addition, since the soil in a typical septic field is wet all the time, it is necessary to duplicate that condition by prolonged soaking of the test holes. This process is called the "presoak".

"Seasonal" or "perched" water tables within 48 inches of the surface will interfere with the proper operation of a septic system. Thus a 48-inch test hole must be dug in the lowest area of the proposed septic field area. The presence of water and/or certain types of soils in the 48-inch test hole determine the seasonal high water table. Therefore, the 48-inch test hole should be wide enough (Minimum 8") to allow a complete visual inspection by the percolation test technician.

Percolation Test Procedures

  1. Water & Sewage - Soil Percolation ADig at least six test holes. The holes should be:
    1. Evenly spaced, approximately 30-40 feet apart, but not less than 30 feet, in the area of the proposed septic field (Diagram A).
    2. At least six inches in diameter. Larger holes are acceptable, but will require more water.
    3. Dug to a depth of 24 inches.
    4. No closer than 3 feet to the 48-inch test hole.
    5. No closer than 75 feet to the nearest water well or proposed water well.
    6. No closer than 5 feet to any lot line or easement.
    7. No closer than 20 feet to any building.
    8. Not located in any easement or flood plain area.
    9. Not located in any area that has previously failed a percolation test.

      (Note: The DuPage County Health Department has observed that hand dug test holes with post hole diggers tend to have better test results than test holes dug with power augers.)
  2. Dig a 48-inch test hole in the lowest part of the test area.
  3. The bottom and sides of each 2-foot test hole may be roughened with a saw blade, knife, or other sharp instrument. It is advisable to roughen those surfaces, which may become smeared with mud during the digging process. Smearing of those surfaces will tend to reduce the seepage rate. Remove all loose soil from the bottom of the holes.
  4. Fill each of the 2-foot test holes with water and refill as necessary to maintain a minimum depth of 12 inches for a period of at least four hours. This is the presoak. It must be conducted between 12 noon and 4 o'clock p.m. the day before the test. Health Department technicians may spot check the presoak to make sure it is being done properly. Note: DO NOT put any water in the 48-inch test hole.
  5. Water & Sewage - Soil Percolation BPrepare in advance of the test one wooden stake for each 2-foot test hole. Three nails should be pounded into each stake Diagram B). The first nail should be three inches from the bottom of the stake, the second nail must be exactly six inches above the first nail, and the third nail must be exactly 20 inches above the second nail.
  6. On the morning of the percolation test (the day following the presoak), the test holes should be cleaned out. Any loose soil or silt that accumulated at the bottom of the holes during the presoak should be removed. One of the stakes prepared per the instructions in #4 above should be driven into each hole so that the first nail rests on the bottom and the stake stands by itself.
  7. At a time previously arranged between the perc tester and the Health Department technician, each test hole is filled with water to the level of the second nail on the perc test stake (which should be exactly six inches). NOTE: Do not wait for the technician to arrive to start the test. After one hour, the perc tester shall measure how far the water has dropped in each test hole. The holes are then refilled with water to the level of the second nail. This process is repeated for at least three more hours. The Health Department technician will take the measurements for the second and all readings after that.
  8. In very porous soils, the water in the test holes may seep away in less than an hour. When this happens, the Health Department technician may go to half-hour or even ten-minute readings. A test may also be extended to five or more hours if the last three readings are inconsistent.

Perc Test Results

When the perc test is completed, most testers are anxious to know the results. The Health Department technician who witnessed the test may be able to say if a test fails, but never if it passes. Perc test results usually take a few days to process. Testers should advise the Health Department technician during the perc test about how he or she would like the results of the perc test reported.

Perc test results are calculated using one of the following methods. The method, which results in the slowest perc rate, is used.

  1. The average of the last hour readings for all test holes is divided into 60 minutes. This yields a minutes-per-inch figure called the "perc rate."
  2. If the last hour reading of one test hole exceeds three times the average of the readings of the remaining holes, then that one test hole is excluded from consideration and the perc rate is calculated on the basis of the remaining holes.
  3. The highest and lowest test hole readings are excluded from consideration and the perc test results are calculated based on the average of the remaining holes.
  4. If one or two test hole readings are inconsistent then they are excluded from consideration and the perc test results are calculated based on the remaining holes.

The following is a summary of when and how perc tests may be failed during the presoak.

  1. No 48-inch test hole dug.
  2. Evidence of seasonal high water table within 24 inches of the surface. (The test may be continued, but only at the request of the tester.)
  3. Improper presoaking - less than 12 inches of water in any test hole at any time.
  4. Filled lots when the Health Department has not been previously notified.
  5. Isolation distances or other location problems with the test area.
  6. There is evidence of impropriety.

The following is a summary of how and when a perc test may be failed during the test itself.

  1. At least half of the test holes fail to drop one inch or more in any of the hourly readings.
  2. There is evidence of impropriety.

Special Notes

  1. When a perc test fails because of poor soil conditions, the area of the failing test may not be retested until corrective measures have been taken.
  2. When a perc test fails due to a problem with seasonal high water table, the area may not be retested until the water table problem is corrected.
  3. Water table and soil quality problems are most frequently corrected by adding fill to the affected area. Filling a lot often requires an excavation and fill permit from the local zoning and drainage jurisdiction. The Health Department should be advised and consulted any time a lot is filled for the purpose of accommodating a perc test.
  4. When a lot receives between 6 and 18 inches of fill, it may be retested after the filling has been completed. When a lot receives over 18 inches of fill, the filled area must be allowed to settle for at least one year before a perc test may be conducted there.
  5. Any perc test scheduled for a lot with over 18 inches of fill, where the Health Department has not been notified in advance shall be cancelled and may not be rescheduled for one year.
  6. Perc test results are valid for one year. The validity of an expired perc test may be extended at the discretion of the Health Department.

Contact the health department if you have any questions concerning the perc test program.