Contra Costa County, California
The dominant drainage class is Well drained and the dominant hydrologic group is C (slow infiltration, moderate runoff). This report summarizes the major soil map units across the survey area to help you understand what to expect when buying, building, or gardening in Contra Costa County, California.
| Soil Map Unit | Acres | Drainage | Hydro Group | Dwellings | Septic |
|---|---|---|---|---|---|
| Water | 47K | D | Not rated | Not rated | |
| Los Osos clay loam, 30 to 50 percent slopes | 32K | Well drained | D | Very limited | Very limited |
| Altamont-Fontana complex, 30 to 50 percent slopes | 27K | Well drained | C | Very limited | Very limited |
| Clear Lake clay, 0 to 15 percent slopes, MLRA 15 | 20K | Poorly drained | C | Very limited | Very limited |
| Los Gatos loam, 30 to 50 percent slopes | 19K | Well drained | C | Very limited | Very limited |
| Altamont clay, 15 to 30 percent slopes, MLRA 15 | 17K | Well drained | C | Very limited | Very limited |
| Diablo clay, 15 to 30 percent slopes, MLRA 15 | 16K | Well drained | C | Very limited | Very limited |
| Diablo clay, 30 to 50 percent slopes, MLRA 15 | 15K | Well drained | C | Very limited | Very limited |
| Rock outcrop-Xerorthents association | 15K | D | Not rated | Not rated | |
| Millsholm loam, 20 to 60 percent slopes, moist, MLRA 15 | 12K | Well drained | D | Very limited | Very limited |
| Millsholm loam, 15 to 50 percent slopes, moist, MLRA 15 | 11K | Well drained | D | Very limited | Very limited |
| Rindge muck, 0 to 2 percent slopes, partially drained, MLRA 16 | 11K | Very poorly drained | A/D | Very limited | Very limited |
| Capay clay, 0 to 3 percent slopes, MLRA 17 | 11K | Moderately well drained | C | Very limited | Very limited |
| Los Osos clay loam, 15 to 30 percent slopes | 10K | Well drained | D | Very limited | Very limited |
| Dibble silty clay loam, 16 to 54 percent slopes, MLRA 15 | 9K | Well drained | C | Very limited | Very limited |
| Delhi sand, 2 to 9 percent slopes | 9K | Somewhat excessively drained | A | Not limited | Very limited |
| Cropley clay, 2 to 5 percent slopes | 8K | Moderately well drained | C | Very limited | Very limited |
| Brentwood clay loam | 8K | Well drained | C | Very limited | Very limited |
| Urban land | 8K | D | Not rated | Not rated | |
| Lodo clay loam, 30 to 50 percent slopes, very rocky, MLRA 15 | 7K | Somewhat excessively drained | D | Very limited | Very limited |
What This Means
Building & Foundations
About 74% of soils are rated "very limited" for dwellings with basements. Specific challenges include shallow bedrock, steep slopes, poor drainage in some areas, flood-prone areas. A geotechnical assessment is recommended before building.
Septic Systems
About 77% of soils are rated "very limited" for septic absorption fields. Despite good drainage overall, limitations may stem from shallow bedrock, steep slopes, or seasonal conditions. Engineered or alternative systems are frequently required. Always get a professional perc test before purchasing land that needs septic.
Gardening & Agriculture
Well-drained soils on sloped terrain — good for most plants but watch for erosion. Terracing, contour planting, and mulching help retain moisture and topsoil. Drip irrigation is more effective than sprinklers on slopes.