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APPENDIX A - PERCOLATION TEST PROCEDURE
Properly conducted percolation tests may be needed to determine absorption system site
suitability and to size the absorption system. If needed, percolation tests must be conducted
within the boundary of the proposed absorption system. The percolation test must be completed
by a qualified site evaluator approved by the reviewing authority. Some system designs may
dictate different test procedures than those outlined below. Please see applicable chapters for
further requirements.
Procedures outlined in ASTM D5093-02, Field Measurement of Infiltration Rate Using a Double-
Ring Infiltrometer with a Sealed-Inner Ring, may be required in addition to those listed below.
Test Hole Preparation
1. Dig or bore holes 6 to 10 inches in diameter with vertical sides. The depth of the
holes must be at the approximate depth of the proposed absorption trenches,
typically 24 inches below ground. If the hole is larger than 6 to 8 inches, place a
piece of 4-inch diameter, perforated pipe inside the hole, and fill the space between
the pipe and the walls of the hole with drain rock. It is recommended that a sketch
or photograph of the hole be provided to the reviewing authority.
2. Roughen or scratch the bottoms and sides of the holes to provide natural
unsmeared surfaces. Remove loose material. Place about 2 inches of 3/4-inch
washed gravel in the bottom of holes to prevent scouring during water addition.
3. Establish a reference point for measurements in or above each hole.
Soaking
1. Fill holes with clear water to a level of at least 12 inches above the gravel.
2. If the soil is coarser than sandy clay loam and the first 12 inches of water seeps
away in 60 minutes or less, add 12 inches of water a second time. If the second
filling seeps away in 60 minutes or less, the percolation test should be run
immediately in accordance with the sandy soil test. If both the first and second
fillings have percolation rates faster than 3 mpi, the test may be stopped.
3. If either the soil is sandy clay loam or finer, or the first 12 inches or the second 12
inches does not seep away in 60 minutes, the percolation test must be run in
accordance with the test for other soils. In these other soils, maintain at least 12
inches of water in the hole for at least 4 hours to presoak the hole.
Sandy Soils Test (percolation rate of 10 mpi or faster)
This test is applicable to sandy soils only (percolation rate of 10 mpi or faster).
Add water to provide a depth of 6 inches above gravel. Measure water level drop
at least four times, in equally spaced intervals, in a 1-hour time period. Measure to
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nearest 1/4 inch. Refill to 6-inch depth after each measurement. Do not exceed 6-
inch depth of water. Use final water-level drop to calculate rate.
Other Soils Test (percolation rate slower than 10 mpi)
This test is applicable to other soils (percolation rate slower than 10 mpi). Remove
loose material on top of gravel. Add water to provide a depth of 6 inches above
gravel. Measure water levels for a minimum of 1 hour. A minimum of 4
measurements must be taken. The test must continue until 2 successive readings
yield percolation rates that do not vary by more than 15 percent, or until
measurements have been taken for 4 hours. Do not exceed 6-inch depth of water.
Use final water-level drop to calculate rate.
Records
Record the following information on the attached form and include as part of the application:
Date(s) of test(s)
Location, diameter, and depth of each test hole,
Time of day that each soak period began and ended
Time of day for beginning and end of each water-level drop interval
Each water-level drop measurement
Calculated percolation rate
Name and signature of person performing test
Name of owner or project name.
Rate Calculation
Percolation Rate = Time interval in minutes/water-level drop in inches.
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MONTANA DEPARTMENT OF ENVIRONMENTAL QUALITY
PERCOLATION TEST FORM
Owner Name
Project Name
Lot of Tract Number Test Number
Diameter of Test Hole Depth of Test Hole
Date and Time Soak Period Began Ended
Date Test Began
Distance of the reference point above the bottom of the hole
Test Results
Start End Time Initial Distance Final Distance Drop in Percolation
Time Time of Interval Below Below Water Rate (mpi)
of Day Day (minutes) Reference Reference Level
Point Point (inches)
I certify that this percolation test was done by a qualified site evaluator in accordance with DEQ-4
Section 1.2.68 and Appendix A.
Name (printed) Signature Date
Company
2013 Edition
