County soil summary
Soil types in Contra Costa County, California
The most widespread soils are Los Osos, Altamont and Diablo; the most common drainage class is well drained (54% of the land), and typical topsoil pH is 6.7.
- Well or moderately well drained
- 71% of land · state 81%
- Typical topsoil pH
- 6.7 neutral · state 6.7
- Septic fields very limited
- 83% of land · state 79%
- Subject to flooding
- 16% of land, any frequency · state 11%
Most common soils in Contra Costa County
Share of mapped land
A county is a patchwork of many soils. These 8 cover 46% of the mapped land; 88 named soil series are mapped here in all.
| Soil | Share of land | Drainage | Topsoil pH |
|---|---|---|---|
| Los Osos | 9.4% | Well drained | 6.5 |
| Altamont | 7.6% | Well drained | 7.0 |
| Diablo | 6.7% | Well drained | 7.3 |
| Millsholm | 5.6% | Well drained | 6.5 |
| Los Gatos | 5.0% | Well drained | 6.5 |
| Clear LakeGeneral soil group, not a named series | 4.0% | — | — |
| Capay | 4.0% | Moderately well drained | 7.2 |
| Rincon | 3.4% | Well drained | 7.0 |
What this means
What the soils in Contra Costa County mean for gardens, lawns and septic systems
For your garden
Typical topsoil in Contra Costa County is neutral: pH 6.7 (most land between 5.8 and 7.5), the range most vegetables, flowers and lawn grasses prefer.
Loamy topsoil (loam, silt loam and similar) covers 45% of the land: generally the easiest soil to garden, holding moisture without staying waterlogged. The most common surface texture is clay (28% of the land).
71% of the land is moderately well to excessively drained, which suits most garden plants.
For your lawn and yard
On 81% of the land, rain soaks in slowly (hydrologic groups C and D, including dual groups that are D until drained). Expect runoff and puddles after heavy rain: core-aerate compacted lawns and keep ground sloping away from foundations.
For a septic system
For conventional septic absorption fields, NRCS rates 83% of the land in Contra Costa County very limited, <1% somewhat limited and 0% not limited (17% not rated, such as urban land, pits or water).
That is close to the state as a whole (79% very limited).
Where the soil is very limited (slow permeability, a high water table, flooding, shallow bedrock or steep slopes), alternative systems such as mound or sand-filter designs are common and cost more.
Flooding is mapped on 16% of the land (<1% occasional, 2% frequent).
Only a licensed site evaluation or perc test at the actual lot decides whether a system is allowed and which design.
Drainage
How long the soil stays wet
The most common drainage class is well drained (54%). 71% of the land is moderately well drained or better, compared with 81% across California.
Show as a table
| Category | Contra Costa County | California |
|---|---|---|
| Excessively drained | 1.0% | 7.7% |
| Somewhat excessively drained | 5.7% | 17.6% |
| Well drained | 53.5% | 50.7% |
| Moderately well drained | 10.4% | 4.7% |
| Somewhat poorly drained | 0.5% | 2.5% |
| Poorly drained | 9.0% | 2.0% |
| Very poorly drained | 7.5% | 0.7% |
| Not rated | 12.5% | 14.2% |
Septic absorption fields
NRCS rating for conventional drain fields
The rating looks at the soil from 24 to 60 inches deep: permeability, depth to a seasonal water table or bedrock, flooding and slope. “Very limited” means at least one feature is unfavorable enough that a conventional field is hard to make work.
Show as a table
| Category | Contra Costa County | California |
|---|---|---|
| Not limited | 0.0% | 4.3% |
| Somewhat limited | 0.4% | 3.6% |
| Very limited | 82.5% | 79.2% |
| Not rated | 17.1% | 12.9% |
Rain, runoff and water table
Hydrologic soil group, flooding, seasonal water table
Hydrologic soil group
How fast rain soaks in when the soil is already wet. Group A soaks in fastest, D slowest.
Show as a table
| Category | Contra Costa County | California |
|---|---|---|
| A: fast infiltration (mostly sandy or gravelly) | 3.7% | 20.5% |
| B: moderate infiltration | 1.3% | 12.5% |
| C: slow infiltration | 46.5% | 22.7% |
| D: very slow infiltration (clayey, shallow or wet) | 30.3% | 30.3% |
| A/D: D as is, A if drained | 3.2% | 0.3% |
| B/D: D as is, B if drained | 0.0% | 0.3% |
| C/D: D as is, C if drained | 1.3% | 0.9% |
| Not rated | 13.7% | 12.6% |
Flooding
Show as a table
| Category | Contra Costa County | California |
|---|---|---|
| No flooding | 78.9% | 77.1% |
| Rare or very rare | 13.7% | 8.2% |
| Occasional | 0.1% | 1.6% |
| Frequent or very frequent | 2.0% | 1.0% |
| Not reported | 5.4% | 12.1% |
Seasonal high water table
Show as a table
| Category | Contra Costa County | California |
|---|---|---|
| Within 18 inches at some time of year | 5.3% | 2.6% |
| 18 inches to 3.3 feet | 1.6% | 1.9% |
| Deeper than 3.3 feet or none reported | 92.3% | 86.9% |
| Not reported | 0.8% | 8.6% |
Topsoil pH and texture
First mineral layer of each soil
Typical topsoil pH is 6.7 (neutral); 80% of rated land falls between 5.8 and 7.5. Median topsoil organic matter is 2.4%.
Show as a table
| Category | Contra Costa County | California |
|---|---|---|
| Strongly acidic (below 5.5) | 2.0% | 3.1% |
| Moderately to slightly acidic (5.5–6.4) | 29.1% | 22.8% |
| Near neutral (6.5–7.3) | 39.1% | 25.2% |
| Alkaline (7.4 and up) | 9.8% | 30.6% |
| Not reported | 20.1% | 18.2% |
Texture
Show as a table
| Category | Contra Costa County | California |
|---|---|---|
| Sandy | 3.7% | 21.3% |
| Loamy (loam, silt loam and similar) | 45.4% | 56.7% |
| Clayey (40%+ clay) | 28.9% | 3.9% |
| Organic (muck or peat) | 5.0% | 0.2% |
| Not reported (rock, urban land, etc.) | 17.1% | 18.0% |
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
- Contra Costa County, California (FIPS 06013): 460,644 acres of mapped land, 47,000 acres of open water excluded. State comparisons pool all mapped land in California.
- Method
- Shares are by mapped land area: each map unit’s acres in the county times each soil component’s percentage. Open water is excluded. Topsoil values use the first mineral horizon of each component. Full methodology.