Soil and perc tests in Snohomish County, Washington

How USDA rates the ground in Snohomish County

Very limited across most of the county — 98.6% of the rated acres. A rating describes the ground, not a parcel. The county's site evaluation decides each lot.

• Very limited — 542,337 acres — 98.6% of 550,154 rated acres.
• Somewhat limited — 7,817 acres — 1.4% of 550,154 rated acres.
• Not rated — 54,329 acres, excluded from the denominator — mostly urban land and water.

98.6% of 550,154 rated acres. 54,329 acres unrated. Survey area WA661 version 27, established 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.

• Depth to saturated zone — 433,106 acres. Reasons overlap — a component can carry several — so these acres exceed rated acres.
• Slow water movement — 257,132 acres. Reasons overlap — a component can carry several — so these acres exceed rated acres.
• Slope — 255,428 acres. Reasons overlap — a component can carry several — so these acres exceed rated acres.
• Seepage, bottom layer — 150,354 acres. Reasons overlap — a component can carry several — so these acres exceed rated acres.
• Flooding — 51,152 acres. Reasons overlap — a component can carry several — so these acres exceed rated acres.
• Ponding — 50,902 acres. Reasons overlap — a component can carry several — so these acres exceed rated acres.
• Filtering capacity — 47,456 acres. Reasons overlap — a component can carry several — so these acres exceed rated acres.
• Depth to bedrock — 43,174 acres. Reasons overlap — a component can carry several — so these acres exceed rated acres.
• Subsidence — 17,932 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.

• 72 — Tokul gravelly medial loam, 0 to 8 percent slopes — 61,376 acres in this county — rated Very limited. Dominant component: Tokul (85% of the map unit).
• 73 — Tokul gravelly medial loam, 8 to 15 percent slopes — 43,515 acres in this county — rated Very limited. Dominant component: Tokul (70% of the map unit).
• 1 — Alderwood gravelly sandy loam, 0 to 8 percent slopes — 41,874 acres in this county — rated Very limited. Dominant component: Alderwood (85% of the map unit).
• 16 — Elwell-Olomount-Rock outcrop complex, 30 to 60 percent slopes — 34,450 acres in this county — rated Very limited. Dominant component: Elwell (50% of the map unit).
• 5 — Alderwood-Urban land complex, 2 to 8 percent slopes — 27,451 acres in this county — rated Very limited. Dominant component: Alderwood (60% of the map unit).
• 55 — Puget silty clay loam — 24,334 acres in this county — rated Very limited. Dominant component: Puget (85% of the map unit).
• 77 — Tokul-Winston gravelly loams, 25 to 65 percent slopes — 20,393 acres in this county — rated Very limited. Dominant component: Tokul (60% of the map unit).
• 76 — Tokul-Ogarty-Rock outcrop complex, 25 to 65 percent slopes — 18,697 acres in this county — rated Very limited. Dominant component: Tokul (50% 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.

1. Snohomish County Health Department — Septic Permitting · checked 2026-09-23
2. Snohomish County Health Department — Septic · checked 2026-09-23
3. Snohomish County Board of Health Code § 5.11.020, Site Identification and Preparation · checked 2026-09-23
4. Snohomish County Board of Health Code § 5.55.060, OSS Contractor Certification · checked 2026-09-23
5. Snohomish County Board of Health Code § 1.40.090, On-Site Sewage Dispersal Systems · checked 2026-09-23
6. Snohomish County Health Department — Septic Application Turnaround Times · checked 2026-09-23
7. Snohomish County Health Department — Septic Permit Steps · checked 2026-09-23
8. Snohomish County Health Department — As-Built Records · checked 2026-09-23
9. Snohomish County Board of Health Code, Ordinance No. BOH26-02, Chapter 5.60 · checked 2026-09-23
10. Washington Administrative Code § 246-272A-0232 · checked 2026-09-23
11. Washington Administrative Code § 246-272A-0210 · checked 2026-09-23
12. USDA NRCS Soil Data Access (SSURGO) · checked 2026-09-16

Related pages

• Septic rules and requirements in Snohomish County, Washington
• Septic permits and fees in Snohomish County, Washington
• Home-sale septic inspections in Snohomish County, Washington
• Septic rules and requirements in Washington
• Septic rules in Snohomish County