Perc Test (Percolation Test)

A tester digs one or more holes on your lot, fills them with water, and times how fast the level drops. That rate is the percolation rate, and the test that produces it is the perc test, short for percolation test. The number tells an engineer whether the soil can safely absorb the wastewater from a septic system and how large the drain field must be.

The test matters on land that has no connection to a city sewer, which describes many rural and large-lot properties around Rio Verde, Cave Creek, and the desert edges of Pinal County. If you cannot tie into a sewer, you need an onsite wastewater system (a septic system), and the soil has to drain well enough to handle it. Soil that drains too slowly, such as tight expansive clay, or too fast, like loose sand, can fail or force a more expensive engineered system.

For example, a buyer planning a custom home on five acres near Rio Verde would have a perc test done early, often before closing. A passing rate confirms a standard septic system will work and roughly how big the leach field needs to be. A failing rate means a costlier alternative system or, in some cases, that the lot cannot support a home.

In Arizona, onsite wastewater systems are regulated by the Arizona Department of Environmental Quality (ADEQ) and the county's environmental health office, who set how the soil evaluation is done and who must perform it. A perc test is part of the broader site evaluation, which also looks at slope, groundwater, and setbacks from wells. We schedule soil and septic testing before design, because the results decide where the house and drain field can sit on the lot.