Soil and perc tests in Williamson County, Texas

USDA rates most of the soil in Williamson County Very limited for septic absorption fields — 97.3% of the rated acres. Williamson County requires a site evaluation, not a percolation test, before a permit. A licensed evaluator or an engineer must perform it. USDA NRCS Soil Data Access (SSURGO) · checked 2026-09-22

Very limited

USDA rating, 97.3% of rated acres

checked 2026-09-22

Slow water movement

Most common limiting reason

checked 2026-09-22

Yes

Site evaluation required

checked 2026-09-23

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

Williamson County requires a site evaluation, not a percolation test, before a permit. A licensed evaluator or an engineer must perform it.

The state sets this same requirement for every Texas county. Williamson County's own guide restates it in full.

Williamson County Department of Infrastructure — On-Site Sewage Facility (OSSF) Guide · checked 2026-09-23

How USDA rates the ground in Williamson County

Very limited across most of the county — 97.3% of the rated acres. A rating describes the ground, not a parcel. The county's site evaluation decides each lot.

Very limited — 607,473 acres — 97.3% of 624,121 rated acres.
Somewhat limited — 16,648 acres — 2.7% of 624,121 rated acres.
Not rated — 103,400 acres, excluded from the denominator — mostly urban land and water.

97.3% of 624,121 rated acres. 103,400 acres unrated. Survey area TX491 version 26, established 2025-09-05.

USDA NRCS Soil Data Access (SSURGO) · checked 2026-09-22

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 — 432,106 acres. Reasons overlap — a component can carry several — so these acres exceed rated acres.
Depth to bedrock — 386,051 acres. Reasons overlap — a component can carry several — so these acres exceed rated acres.
Large stones — 106,840 acres. Reasons overlap — a component can carry several — so these acres exceed rated acres.
Flooding — 49,537 acres. Reasons overlap — a component can carry several — so these acres exceed rated acres.
Depth to saturated zone — 9,502 acres. Reasons overlap — a component can carry several — so these acres exceed rated acres.
Slope — 4,711 acres. Reasons overlap — a component can carry several — so these acres exceed rated acres.
Depth to cemented pan — 4,038 acres. Reasons overlap — a component can carry several — so these acres exceed rated acres.
Ponding — 350 acres. Reasons overlap — a component can carry several — so these acres exceed rated acres.
Filtering capacity — 96 acres. Reasons overlap — a component can carry several — so these acres exceed rated acres.
Seepage, bottom layer — 96 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.

EaD — Eckrant cobbly clay, 1 to 8 percent slopes — 53,954 acres in this county — rated Very limited. Dominant component: Eckrant (85% of the map unit).
EeB — Eckrant stony clay, 0 to 3 percent slopes, stony — 48,640 acres in this county — rated Very limited. Dominant component: Eckrant (85% of the map unit).
HoB — Houston Black clay, 1 to 3 percent slopes — 42,715 acres in this county — rated Very limited. Dominant component: Houston Black (80% of the map unit).
BrB — Branyon clay, 1 to 3 percent slopes — 41,107 acres in this county — rated Very limited. Dominant component: Branyon (85% of the map unit).
BrA — Branyon clay, 0 to 1 percent slopes — 40,227 acres in this county — rated Very limited. Dominant component: Branyon (85% of the map unit).
DoC — Doss silty clay, moist, 1 to 5 percent slopes — 33,497 acres in this county — rated Very limited. Dominant component: Doss (85% of the map unit).
GsB — Georgetown stony clay loam, 1 to 3 percent slopes — 31,477 acres in this county — rated Very limited. Dominant component: Georgetown (90% of the map unit).
DnB — Denton silty clay, 1 to 3 percent slopes — 30,271 acres in this county — rated Very limited. Dominant component: Denton (88% of the map unit).

USDA NRCS Soil Data Access (SSURGO) · checked 2026-09-22

Sources

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

Williamson County Department of Infrastructure — On-Site Sewage Facility (OSSF) Program Fees · checked 2026-09-23
Williamson County, TX — On-Site Sewage Facilities (OSSF) · checked 2026-09-23
Williamson County Department of Infrastructure — On-Site Sewage Facility (OSSF) Guide · checked 2026-09-23
Texas Administrative Code, Title 30, Chapter 285, §285.91(2), Table II · checked 2026-09-23
USDA NRCS Soil Data Access (SSURGO) · checked 2026-09-22

Verify before you act. Septic rules are set locally and they change. Check anything here before you spend money on it. Call the county environmental health office, on 512-943-3330.