What a Percolation Test Tells You About Your Property

By Hawaiʻi Cesspool HelpPublished July 31, 20267 min read

A percolation test measures how quickly water moves through soil at a proposed disposal depth. It is one part of the site evaluation for an individual wastewater system (IWS). The result helps a licensed civil engineer evaluate a soil absorption system, but it does not by itself select a system, prove that a lot is buildable, or guarantee approval.

Why is the test needed?

An IWS must treat and dispose of wastewater without creating an unacceptable risk to groundwater, surface water, or public health. Soil conditions affect whether an absorption trench or bed can receive wastewater and how much disposal area may be needed.

Hawaiʻi Administrative Rules section 11-62-31.2 requires the engineer to evaluate more than the percolation rate. The site evaluation also covers soil depth and texture, seasonal high groundwater, bedrock or another limiting layer, slope, flooding hazard, and the amount of suitable area available. These conditions work together. A rate that looks workable cannot overcome an inadequate setback, shallow rock, insufficient vertical separation, or too little usable area.

What happens during a Hawaiʻi percolation test?

The current Hawaiʻi Department of Health (DOH) site evaluation and percolation test form uses a falling head procedure. The person performing the work prepares a test hole about four to twelve inches in diameter, with vertical walls, near the proposed depth of the soil absorption system. The sidewalls and bottom are scratched to remove smeared soil, loose material is removed, and coarse sand or gravel is placed at the bottom.

The procedure begins by adding twelve inches of water and recording how long it takes to seep away, then repeating that step. Later readings measure water-level drops over set time intervals. The test approach differs for sandy soils and for soils such as silt, loam, or clay:

• In sandy soil, readings are generally taken at least every ten minutes for at least one hour, with a shorter interval if water moves very quickly.
• In other soils, the hole is presoaked for at least four hours and allowed time to swell before the test readings. Depending on the first drop, readings continue at ten-minute or thirty-minute intervals.

The final interval is used to calculate the percolation rate in minutes per inch. The engineer records the test information, soil profile, observed groundwater or bedrock, and other site information on the DOH form.

How should a homeowner read the result?

Minutes per inch describes the time water takes to drop one inch in the test hole. A smaller number means faster movement during that test. A larger number means slower movement. Faster is not automatically better.

Under section 11-62-34, absorption trenches and absorption beds cannot be constructed in soil with a rate slower than sixty minutes per inch. The same rule also warns that rapid percolation may be unsuitable where it could contaminate a water-bearing formation or surface water. The engineer must apply the rate together with the full site evaluation and the applicable design tables.

A single number is therefore not a homeowner pass or fail score. It is a design input. Ask the engineer what depth was tested, whether the tested material matches the proposed disposal depth, and which other site conditions control the recommendation.

Why might another test be required?

The rules require a soil percolation test at a minimum depth of three feet. They also require the soil profile to be observed to at least five feet. If the soil at five feet differs from the soil at three feet when construction begins, another test must be performed at the bottom depth of the proposed absorption system. DOH may also require additional testing to identify a limiting layer.

This matters on Hawaiʻi Island because conditions can change across a short distance or with depth. A test hole is evidence about the material actually tested. It does not establish that every part of the parcel has identical soil or rock.

What can change after testing?

Field conditions discovered during excavation may not match the information used for approval. Section 11-62-31.2 says that when actual construction conditions differ from the approved site conditions, the design engineer must revise the wastewater plans and submit the revised design for approval.

That is why it is risky to buy equipment, fix a construction price, or promise a system location from a preliminary rate alone. The design still depends on the approved plans and the conditions found in the field.

How should you prepare?

1. Provide the property address and complete tax map key.
2. Share any survey, site plan, building floor plans, and existing wastewater records.
3. Point out wells, water lines, drainage paths, streams, wetlands, steep areas, filled ground, and known rock or groundwater conditions.
4. Keep the proposed test and disposal area accessible, but do not excavate test holes without direction from the responsible professional.
5. Tell the engineer about existing and future dwellings, bedrooms, additions, and access needs before the site evaluation.
6. Ask what the result means for disposal area, system options, uncertainty, and possible additional testing.

DOH's IWS review requirements list the site evaluation form among the materials submitted for agency review. The test is therefore part of a documented engineering and approval process, not a stand-alone service that authorizes construction.

Official sources

• Hawaiʻi Administrative Rules Chapter 11-62, Wastewater Systems
• Department of Health IWS Site Evaluation and Percolation Test Form
• Department of Health Requirements for Review Process of Individual Wastewater Systems
• Department of Health individual wastewater systems resources

Requirements and agency guidance can change. Confirm the current design and approval path with the Hawaiʻi Department of Health and a Hawaiʻi licensed civil engineer before relying on this information for a project.

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