Contra Costa County, California
The most common soil here is Los Osos, on about 8% of the land: a loam, a balanced mix that crumbles in your hand. Altamont and Diablo come next.
About 58% of the land drains well, and 16% stays wet for long stretches; groundwater comes within two feet of the surface under 5% of it.
USDA rates 86% of the land very limited for a standard septic drain field, and 75% very limited for homes with basements. That means extra cost or special design, not that it can't be done.
Worth knowing: 2% of the land floods occasionally or more, and bedrock lies within 40 inches under 38% of it.
Soils change over short distances, so look up an address below for the soil at that exact spot and the reasons behind each rating.
The soils you're most likely standing on
The biggest named soils in Contra Costa County, California, each drawn five feet deep from its official USDA description, in its recorded colors.
Dots mark gravel; white flecks mark lime. Shares are by acres across the survey area, and each mapped patch usually mixes several of these soils.
The biggest mapped areas
The dominant drainage class is Well drained and the dominant hydrologic group is C (slow infiltration, moderate runoff). The most common soil order is Mollisols — grassland soils with a thick, dark, fertile topsoil rich in organic matter. 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. Search your address to find the exact soil composition, drainage, and series details at your specific location.
| 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 |
Soil Orders in This Area
The USDA classifies every soil into one of 12 soil orders. Here are the dominant orders found in this survey area.
What This Means
Building & Foundations
About 75% 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 86% 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.