Ranking · counties and states
Where soil is best for septic systems
Across the US, 80% of mapped land is rated very limited for conventional septic absorption fields. Crowley County, CO stands out: 45% of its land has no limitation at all, more than any other county.
- Very limited, US
- 80%
- of all mapped land
- Best county
- 45%
- not limited · Crowley County, CO
- Best state
- 9%
- not limited · Arizona
The 25 counties with the most septic-friendly soil
Share of land rated not limited
| County | Not limited | Somewhat limited | Very limited |
|---|---|---|---|
| 1. Crowley County, CO | 45% | 4% | 49% |
| 2. Escambia County, AL | 29% | 27% | 44% |
| 3. Winkler County, TX | 28% | 2% | 57% |
| 4. Ward County, TX | 27% | 14% | 58% |
| 5. Sedgwick County, CO | 26% | 12% | 56% |
| 6. Coffee County, AL | 26% | 21% | 53% |
| 7. Weld County, CO | 25% | 32% | 34% |
| 8. El Paso County, CO | 24% | 21% | 49% |
| 9. Barnwell County, SC | 24% | 29% | 47% |
| 10. Zapata County, TX | 23% | 24% | 39% |
| 11. El Paso County, TX | 23% | 22% | 44% |
| 12. Maricopa County, AZ | 21% | 15% | 47% |
| 13. Doña Ana County, NM | 20% | 13% | 54% |
| 14. La Paz County, AZ | 20% | 19% | 51% |
| 15. Lincoln County, CO | 20% | 27% | 51% |
| 16. Baca County, CO | 19% | 25% | 51% |
| 17. Aurora County, SD | 19% | <1% | 81% |
| 18. Hayes County, NE | 19% | 21% | 61% |
| 19. Yuma County, AZ | 18% | 13% | 57% |
| 20. Valencia County, NM | 18% | 35% | 47% |
| 21. Bethel Census Area, AK | 17% | 1% | 77% |
| 22. Lake and Peninsula Borough, AK | 17% | 24% | 60% |
| 23. Carter County, MT | 17% | 14% | 64% |
| 24. Costilla County, CO | 16% | 10% | 60% |
| 25. Phillips County, MT | 16% | 4% | 79% |
States with the most septic-friendly land
| State | Not limited | Very limited |
|---|---|---|
| 1. Arizona | 9% | 62% |
| 2. Colorado | 6% | 68% |
| 3. Wyoming | 6% | 60% |
| 4. New Mexico | 6% | 65% |
| 5. Alaska | 4% | 88% |
| 6. California | 4% | 79% |
| 7. Nevada | 4% | 77% |
| 8. Rhode Island | 4% | 85% |
| 9. Utah | 3% | 62% |
| 10. Alabama | 3% | 79% |
What "very limited" really means
The rating comes from NRCS and judges a soil for a standard gravity septic field: a trench about two feet deep where wastewater should soak in and be filtered before it reaches groundwater. Soil fails that test for several reasons: it drains too slowly (heavy clay), too fast (gravel and coarse sand that do not filter), sits over shallow bedrock or a high water table, floods, or is too steep.
"Very limited" does not mean a house cannot have a septic system. It means a standard system is likely to need a different design, such as a mound or pressure-dosed system, which costs more. Permits are decided by a site evaluation or percolation test on the actual lot.
Sources & data dates
- Source
- USDA NRCS Soil Survey Geographic Database (SSURGO), queried through Soil Data Access. Public domain.
- Data date
- SSURGO as published on November 3, 2026 (most recent survey area update October 26, 2026). NRCS refreshes SSURGO each October.
- Scope
- All mapped land in the 50 states and DC in the current soil survey.
- Method
- County rankings include counties with at least 100,000 acres of mapped land and soil ratings on at least 80% of it, so a small county with one unusual map unit cannot top the list. Counties without a full report on this site are named but not linked; the ZIP lookup covers them. Ratings are the NRCS "ENG - Septic Tank Absorption Fields" interpretation, overall class, weighted by area. Full methodology.