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Missoula City-County Health Department
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Health Division: Environmental Health
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Environmental Health: Land Services

Soil Percolation Tests

The percolation test (perc 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 Missoula County, the Health Code stipulates that perc test must be performed by persons approved by the department. 

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".

Approved persons conduct the perc test following established procedures and submit the results to the Health Department.  The Health Department uses perc test results in combination with the results from the soil profile to design a septic system that will function properly in the soils on your site.

 The Missoula City-County Health Code makes the following reference to percolation tests.

 Percolation Tests and Exceptions.

  1. A percolation test must be completed if the soils in the area of the proposed system are of the following types:
    1. Soils finer than silt loam
    2. Soils coarser than medium sand, unless a sand-lined, pressure distribution system is to be installed
    3. Compacted silt loam
    4. If a potential impervious layer is present less than six feet below ground level.
  2. Three percolation tests shall be conducted, when required, in accordance with DEQ 4, Appendix A on absorption system sites. Test holes shall be evenly spaced throughout the area of the proposed absorption site.
  3. Percolation tests must be conducted by persons approved by the Department.

Generally, persons approved by the department will be the following:

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