What Is a Land Perc Test?

Rural areas usually don't have municipal sewer systems, which call for an independent septic system to process the wastewater generated by your rural home.

Septic tank installations require testing the soil's ability to absorb tank runoff.

But before you can build a septic system, state and local health departments and building jurisdictions typically require a passed soil percolation test before they issue the septic permit. These tests ensure the soil of your septic system's drainage or leach field can absorb the run-off from the septic tank without pooling raw sewage on top of the ground.

Test Regulations

Specific test standards and regulations vary from state to state and from town to town. In areas that require professional testing, the tests are conducted by licensed soil engineers or geologists during the wet-weather months of the year with test results valid for up to two years. But wherever you are, a failed perc test means you must either forego that building site or have an alternative and usually more expensive system designed. In many areas, septic systems can’t be installed if the soil is seasonally saturated or a layer of impervious bedrock or clay exists within 3 feet of the ground surface.

Analysis and Test Holes

The first part of the test involves analysis of the soil's drainage characteristics through a visual inspection of the hardpan and high water table markings in a hole dug 7 to 10 feet deep by a backhoe or other excavation machine. For the perc test, depending on the jurisdiction's standards, dig three or more holes 30 to 40 feet apart in the proposed leach field. The holes typically are 2 feet deep and from 6- to 12-inches in diameter with 2 inches of sand or gravel set at the bottom of the holes. The purpose of the test is to mimic the drainage conditions found in the leach field, which involves presoaking the holes for several hours before conducting the perc tests.

The Perc Test

After presoaking the holes with water for several hours or overnight, the professional or the homeowner, depending upon local requirements, typically begins the test by adding water to the hole at least 6 inches above the gravel bed and marking its level. After 30 minutes have passed, measure the water level again to determine how far it has dropped because of water absorption by the soil, which gives you the percolation rate: the time needed for the water level in the test hole to fall by 1 inch. If the test showed the water level dropped by 3 inches in 30 minutes, this would equal a percolation rate of 10 minutes per inch of drop. In most areas, the percolation rate must equal at least 1 inch of drop within 60 minutes to pass the test.

Buyer Beware

Before buying rural land that you can build a house upon, find out whether it has passed a percolation test at the local government's standards. You can also make the land's purchase contingent upon passing a perc test to protect you from buying land you can't build on. While many governmental jurisdictions might accept an alternative septic system design to meet its health requirements for independent wastewater disposal, these systems are often much more costly than standardized septic systems.