Soil and perc tests in Maricopa County, Arizona

USDA rates most of the soil in Maricopa County Very limited for septic absorption fields — 56.1% of the rated acres. Maricopa County calls this a site and soils evaluation. It measures how fast the soil drains through test holes. USDA NRCS Soil Data Access (SSURGO) · checked 2026-09-16

Very limited

USDA rating, 56.1% of rated acres

checked 2026-09-16

Slow water movement

Most common limiting reason

checked 2026-09-16

Yes

Site evaluation required

checked 2026-09-23

What is a perc test and does Maricopa County require one?

Yes. Maricopa County calls this a site and soils evaluation. It measures how fast the soil drains through test holes.

The county's own staff usually run this test. A registered engineer, a geologist, or a sanitarian may run it instead. The department must say yes to them first.

Maricopa County Environmental Services Department — Site Investigation and Soils Evaluation Instructions · checked 2026-09-23

How USDA rates the ground in Maricopa County

Very limited across most of the county — 56.1% of the rated acres. A rating describes the ground, not a parcel. The county's site evaluation decides each lot.

Very limited — 1,944,812 acres — 56.1% of 3,468,598 rated acres.
Somewhat limited — 832,448 acres — 24.0% of 3,468,598 rated acres.
Not limited — 691,338 acres — 19.9% of 3,468,598 rated acres.
Not rated — 918,025 acres, excluded from the denominator — mostly urban land and water.

56.1% of 3,468,598 rated acres. 918,025 acres unrated. Spans 6 survey areas. The largest are AZ645 (v21), AZ653 (v21), AZ651 (v20), plus 3 more. Versions established 2025-08-27 to 2025-08-28.

USDA NRCS Soil Data Access (SSURGO) · checked 2026-09-16

What limits absorption here

The reasons USDA gives for the rating, by the acres they affect. A component can carry several, so the acres overlap.

Slow water movement — 1,707,996 acres. Reasons overlap — a component can carry several — so these acres exceed rated acres.
Slope — 769,260 acres. Reasons overlap — a component can carry several — so these acres exceed rated acres.
Depth to bedrock — 626,131 acres. Reasons overlap — a component can carry several — so these acres exceed rated acres.
Flooding — 352,723 acres. Reasons overlap — a component can carry several — so these acres exceed rated acres.
Depth to cemented pan — 347,192 acres. Reasons overlap — a component can carry several — so these acres exceed rated acres.
Filtering capacity — 162,620 acres. Reasons overlap — a component can carry several — so these acres exceed rated acres.
Large stones — 78,154 acres. Reasons overlap — a component can carry several — so these acres exceed rated acres.
Depth to saturated zone — 26,667 acres. Reasons overlap — a component can carry several — so these acres exceed rated acres.
Seepage, bottom layer — 392 acres. Reasons overlap — a component can carry several — so these acres exceed rated acres.

Ratings by soil map unit

The largest map units in the county, each with its rating. A soil survey names the ground. The county's evaluation of your lot is what a permit turns on.

21 — Denure-Rillito-Why complex, 1 to 5 percent slopes — 129,826 acres in this county — rated Not limited. Dominant component: Denure (40% of the map unit).
54 — Quilotosa-Rock outcrop complex, 15 to 55 percent slopes — 105,404 acres in this county — rated Very limited. Dominant component: Quilotosa (60% of the map unit).
GgA — Gilman loam, 0 to 1 percent slopes — 100,378 acres in this county — rated Somewhat limited. Dominant component: Gilman (100% of the map unit).
34 — Gunsight-Chuckawalla complex, 1 to 15 percent slopes — 84,350 acres in this county — rated Not limited. Dominant component: Gunsight (40% of the map unit).
RS — Rock outcrop-Cherioni complex — 80,720 acres in this county — rated Not rated. Dominant component: Rock outcrop (65% of the map unit).
8 — Cherioni-Coolidge complex, 1 to 15 percent slopes — 80,533 acres in this county — rated Very limited. Dominant component: Cherioni (40% of the map unit).
52 — Gachado-Lomitas-Rock outcrop complex, 7 to 55 percent slopes — 76,240 acres in this county — rated Very limited. Dominant component: Gachado (45% of the map unit).
LcA — Laveen loam, 0 to 1 percent slopes — 75,909 acres in this county — rated Somewhat limited. Dominant component: Laveen (100% of the map unit).

USDA NRCS Soil Data Access (SSURGO) · checked 2026-09-16

Sources

Each figure on this page has a government source and a checked-on date.

Maricopa County — Onsite Wastewater Systems, Frequently Asked Questions · checked 2026-09-23
Maricopa County Environmental Services Department — Onsite Wastewater Forms / Applications · checked 2026-09-23
Maricopa County Environmental Services Department — Locations & Hours · checked 2026-09-23
Maricopa County Environmental Health Code, Chapter II Section VIII — Onsite Wastewater Treatment Facilities · checked 2026-09-23
Maricopa County Environmental Health Code, Chapter I — Fee Schedule · checked 2026-09-23
Maricopa County Environmental Services Department — Site Investigation and Soils Evaluation Instructions · checked 2026-09-23
Maricopa County Environmental Services Department — Notice of Intent to Discharge Instructions · checked 2026-09-23
Maricopa County Environmental Services Department — Online Septic Research · checked 2026-09-23
Arizona Administrative Code, Title 18, Chapter 9 — Department of Environmental Quality, Water Pollution Control · checked 2026-09-23
USDA NRCS Soil Data Access (SSURGO) · checked 2026-09-16

Verify before you act. Septic rules are set locally and they change. Check anything here before you spend money on it. Call the Maricopa County Environmental Services Department, on 602-506-6666.