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Percolation Testing For Septic Development – Sonoma County

Before installing a new soak-away system or replacing an existing one, perform a percolation test or perc test to measure the rate at which the soil at the proposed site absorbs liquid. A perc test provides necessary information about the ground on which the septic tank will be built. Your septic tank contractor will analyze the results of the perc test to determine if the proposed site is suitable for a septic tank.

The Importance of Performing a Perc Test

Your septic tank contractor will use the test results to develop an effective septic tank design, so your tank works properly and doesn’t pollute the environment. In addition to helping check the suitability and permeability of the soil, a percolation test can assist you in determining if the drainage field will allow discharge to soak into the soil properly. In many cities, property owners must have a perc test performed before installing a sustainable drainage system.

How to Perform a Simple Perc Test?

Before you perform a perc test, choose the right location for your septic tank. Determine where your outlet pipe will be and where your drainage field is likely to start. Dig a 300 mm x 300 mm hole below the proposed invert level of the outlet pipe. Excavate a larger area where the invert level is quite deep. Remove any loose debris. Fill the 300 mm section of the test hole with water and allow it to seep away overnight.

The following morning, refill the test hole up to a depth of 300 mm. Observe how swiftly the water seeps away. When the water level reaches 225 mm – that is, the hole is ¾ full, use a stopwatch to see how long it takes for the water level to reach 75 mm. Divide this number by 150, that is the decrease in water level (225-75) to get the average time it takes the water level to drop 1 mm. The result is the Vp.

An example: Say it takes 60 minutes or 3,600 seconds for the water level to drop 150 mm.

Vp = 3,600/150 = 24 seconds.

Perform the test at least three times per hole at different times of the day. The average Vp is the percolation rate of your soil. If the Vp values obtained from tests vary widely, consider performing the test three more times in different locations to ensure your average figure is as accurate as possible. 

A Word of Caution: Performing a percolation test is more complex than it might seem. If you do not understand the concept of percolation or have never performed a test before, it can be easy to miscalculate or make a mistake.  Instead, hire a Sonoma County septic service expert to assist you and perform a percolation test. 

Analyzing the Result of a Perc Test

If the average Vp is 15, the percolation rate of your soil is slow enough to allow the effluent to be treated before it reaches groundwater. If the value is below 15, untreated effluent may drain too quickly. A maximum value of 100 guarantees a percolation rate that is fast enough to allow effluent to disperse effectively.

L.J. Construction is a renowned septic tank contractor in Sonoma County. We are committed to helping homeowners ensure smooth, hassle-free septic system installations. To learn more, call (707) 823-0247.