Perc Tests: Soil Percolation & Soil Depth Requirements for Septic Absorption Systems / Septic Drainfields

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Septic soil percolation tests: the ability of a drain field, also called leach field, or drain field, to absorb septic effluent determines the size, location, and type of effluent absorption system which can be installed at a property. Septic drainfield percolation test procedures: this article describes the need for and process of "soil testing" or the preparation and use of soil test pits for septic system absorption system or drainfield design or repair.

  • How do we perform a soil perc test when designing or testing a septic system?
  • What is the difference between a soil percolation test or "perc test" and a "deep hole test" or test pit?

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WHAT ARE PERC TESTS: What is a septic system soil percolation test? What is a "deep hole test"?

Perc tests may also be performed in order to evaluate soils when a septic system is believed to have failed, and when repair or septic field replacement are being considered. Readers should also see our example of state-regulated soil percolation tests at the New York State Septic System Design Regulations 75-A.4 - Soil and site evaluation for septic system design page.

Citation of this article by reference to this website and brief quotation for the sole purpose of review are permitted. Use of this information at other websites, in books or pamphlets for sale is reserved to the author. Technical review by industry experts has been performed and is ongoing - reviewers welcomed and are listed at REFERENCES. This article is part of our series: Inspecting, Testing, & Maintaining Residential Septic Systems an online book on septic systems.

In specifying the size and type of absorption field (leach field, seepage pits, galleys, other) a septic engineer or health department official will require that a soil percolation test or "soil perc test" be performed. You may hear it described as a "deep hole test."

In brief, one or more holes are dug in the soil of the property where (or near where) a septic leach field is to be installed. Water is placed in the hole, and the engineer observes the amount of time it takes for the soil to absorb the water, or for the water to "percolate" through the soil. The engineer will also examine the exposed soil layers to obtain additional site design information. (Details follow).

What is a Soil Test Pit or Perc Test Pit or Deep Hole Test Pit?

Perc Test / Soil Percolation Test: A hole, 5-7 feet deep is dug in an area to be tested for future use as a drain field, or near the drain-field area in representative soils. Water is poured into the hole and and the soils or septic engineer or contractor observes the rate at which soil absorbs the water by noting the time that it takes for the level of water in the hole to drop one inch (for example). More precise "perc tests" may involve using a specific quantity of water or a perc test hole of specific dimensions to make these observations.

The first time I participated in a soil perc test procedure I found myself smiling with surprise at how low-tech the procedure actually was (in New York State.) After identifying the most-likely location on the lot for placement of a septic drainfield, the excavator used a backhoe to dig a very rough hole about 5 ft. deep. Happily no groundwater immediately filled in the hole (which would have been bad news). Perhaps this is why builders try to have this test done in July which is the period of most-dry weather and lowest groundwater table levels.

After digging this rough hole, the septic engineer poured a 5-gallon (joint compound) bucket of water into the hole. In some cases a few buckets might be dumped therein. After that sophisticated move, the observers simply watched the rate at which the water disappeared. a one-inch drop in water level in this hole in three minutes was considered very good. If the water was found still in the hole at no drop in level the next morning, this was considered seriously bad and probably requiring some soil exchange or other special design measures.

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