What is a percolation (perc) test and do I need one?

A percolation test, or perc test, measures how quickly water drains through the soil on a lot. It is used to design and locate a septic system, so you need one whenever the home will be on an onsite wastewater system instead of a municipal sewer. On most rural lots in the Jematell build area, including the Rio Verde Foothills and outlying parts of Maricopa and Pinal County, there is no sewer, which means a septic system, which means a soil and percolation evaluation is part of due diligence. The result decides whether a standard septic system works, whether you need a more expensive engineered alternative, and sometimes whether the lot can be developed at the density you want.

What a perc test actually measures

A percolation test involves digging or boring test holes at the proposed disposal area, saturating them, and timing how fast the water level drops. That drainage rate, combined with a soil profile showing the soil layers and any restrictive layers, tells a designer how well the ground can absorb and treat the effluent that a septic system discharges. Soil that drains too slowly (heavy clay) cannot accept the flow; soil that drains too fast (loose sand or fractured rock) does not treat the effluent enough before it reaches groundwater. Caliche, a cemented desert layer, and a shallow water table both complicate the picture. The test is the evidence that the leach field, also called the disposal field, will work where you want to put it.

Why and when you need one in Arizona

Onsite wastewater systems in Arizona are regulated by the Arizona Department of Environmental Quality under its rules for onsite wastewater treatment facilities (A.A.C. Title 18, Chapter 9), with permitting delegated to county environmental health departments such as Maricopa County Environmental Services. To get a construction authorization or permit for a septic system, the county requires a site investigation that includes a soil evaluation and, depending on the system, a percolation test performed and reported by a qualified professional. In practice you need a perc or soil evaluation when:

• The lot has no access to a municipal or community sewer, which is the norm on rural and unincorporated parcels.
• You are confirming feasibility before purchase, so you know the lot can actually support a system before you commit.
• You are sizing the system, because the soil result and the home's number of bedrooms together determine the required disposal area.

If the lot connects to a public sewer, you do not need a perc test for wastewater, though you may still want soils information for the foundation, which is a separate geotechnical question.

What happens if the soil does not perc well

A poor result is not always a dead end, but it costs money. Where conventional soil absorption will not work, the designer turns to an alternative or engineered system, such as a system with additional treatment before disposal, a different disposal method, or a larger or specially designed field. These cost more to install and sometimes more to maintain than a conventional septic system. A failed or marginal perc result can also limit how large a home the lot will support, because a bigger house with more bedrooms needs a larger disposal area. This is one of the practical reasons two similar-looking rural lots can carry very different all-in costs.

How it fits the rest of due diligence

The percolation and soil-absorption evaluation answers the wastewater question. It is separate from the structural geotechnical (soils) report, which tells your engineer how to design the foundation, although both involve looking at the ground and it often makes sense to plan them together early. It is also tied to the water question on a rural lot, because a parcel that needs a well and a septic system has to site the two with the required separation. Confirming septic feasibility, water, and access together before you buy is what keeps a rural lot from turning into an expensive surprise.

How Jematell Homes helps

On rural and unincorporated lots, we help buyers scope the wastewater question early, coordinate the soil and percolation evaluation with a qualified professional, and design the home so the disposal field, the well, and the structure all fit with the required separations. Because the rules, required test methods, and fees are set by ADEQ and administered by the county and can change, we confirm exactly what is required for your parcel with the local Authority Having Jurisdiction (Maricopa County Environmental Services, Pinal County, or your city, depending on where the lot sits) before relying on any result.