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.

How a perc test is done in Arizona

Arizona spells out the procedure in the state rule for onsite wastewater site investigations, A.A.C. R18-9-A310. The tester digs each hole in undisturbed soil, at least 12 inches deep and 12 inches square, or 15 inches across if the hole is round. The hole is then presoaked: filled with clean water to 12 inches above the bottom, filled again if it drains away in under an hour, and then kept at least 9 inches deep for four more hours. The soak lets the soil take on water the way it would under a working drain field.

The test itself runs 16 to 24 hours after the presoak. The tester times how many minutes the water takes to drop one inch from a fixed reference point, and repeats the reading until three measurements land within 10 percent of each other. The slowest of those three is the result, reported in minutes per inch. Readings between 60 and 120 minutes per inch, or readings that never settle, are worked out with a graphing method the rule sets out.

When a soil evaluation replaces the perc test

A perc test is not always enough. The same rule calls for a soil evaluation by an ASTM method, logged from test pits or borings, when the natural ground slopes more than 15 percent, when bedrock or a seasonal high water table sits within 12 feet of the surface, or when the perc results fall outside the limits the rule allows. The investigator looks for those limiting conditions to at least 12 feet down, or to rock if it is shallower. A seepage pit is tested a different way again, in a hole at least 18 inches wide drilled to the depth of the planned pit.

Who can perform a perc test

Not anyone with a shovel and a hose. R18-9-A310 limits site investigations to Arizona-registered professional engineers, geologists and sanitarians, people who hold a training certificate the Arizona Department of Environmental Quality recognizes, and other categories it names in writing. Counties add their own steps. Maricopa County, for example, requires an Arizona-registered engineer or geologist for a seepage pit evaluation, and some counties want notice before the test is run. Ask the county environmental services office what it needs before you schedule one.

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. For when a lot needs one and what happens if the soil fails, see what a perc test is and whether you need one.