A percolation test helps size a leachfield for wastewater disposal. Different soils have different percolation rates and this will affect how large your leachfield should be. Some soils are sandy and have relatively quick perc rates, while other soils are clay-rich and have relatively slow perc rates.

Imagine you are running dishwasher, flush the toilet, and take a shower all at once. The wastewater flows into your septic tank, then out to the leachfield. If the soils don't perc fast enough, the water will back up into your shower, and you don't want that! You also don't want the water running too quickly through the soil as the bacteria in the soil will not have time to treat the water. This could pollute the groundwater table.

Methods

Perc test methods vary county by county, but the concept is generally the same:

- Dig a hole 24-36 inches deep and about 6 inches around.

- fill the hole with water to presoak the soils for roughly 4-24 hours.

- fill the hole to a desired level and time water falling in the hole. This gives a minutes/inch rate, which translate into a leachfield size. Usually it takes 1-3 holes per site to determine a sufficient perc rate.

For most applications, the leachfield size is related to the number of rooms in the structure, such 300 sq. ft./bedroom. This is then calculated for the intended application.