Marin County, California
The dominant drainage class is Well drained and the dominant hydrologic group is D (very slow infiltration, high 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 Marin County, California.
| Soil Map Unit | Acres | Drainage | Hydro Group | Dwellings | Septic |
|---|---|---|---|---|---|
| Water | 59K | D | Not rated | Not rated | |
| Tocaloma-McMullin complex, 50 to 75 slopes | 25K | Well drained | B | Very limited | Very limited |
| Tocaloma-Saurin association, extremely steep | 25K | Well drained | B | Very limited | Very limited |
| Tocaloma-Saurin association, very steep | 18K | Well drained | B | Very limited | Very limited |
| Los Osos-Bonnydoon complex, 30 to 50 percent slopes | 13K | Well drained | D | Very limited | Very limited |
| Xerorthents-Urban land complex, 0 to 9 percent slopes | 10K | Not rated | Not rated | ||
| Blucher-Cole complex, 2 to 5 percent slopes | 9K | Somewhat poorly drained | C/D | Very limited | Very limited |
| Dipsea-Barnabe very gravelly loams, 50 to 75 percent slopes | 9K | Well drained | B | Very limited | Very limited |
| Maymen-Maymen variant gravelly loams, 30 to 75 percent slopes | 8K | Somewhat excessively drained | D | Very limited | Very limited |
| Reyes clay | 8K | Somewhat poorly drained | C/D | Very limited | Very limited |
| Reyes clay, 0 to 2 percent slopes | 8K | Somewhat poorly drained | C | Very limited | Very limited |
| Tocaloma-McMullin complex, 30 to 50 percent slopes | 8K | Well drained | B | Very limited | Very limited |
| Palomarin-Wittenberg complex, 50 to 75 percent slopes | 7K | Well drained | B | Very limited | Very limited |
| Tomales-Steinbeck loams, 5 to 15 percent slopes | 6K | Moderately well drained | D | Somewhat limited | Very limited |
| Saurin-Bonnydoon complex, 30 to 50 percent slopes | 6K | Well drained | C | Very limited | Very limited |
| Yorkville clay loam, 30 to 50 percent slopes | 6K | Moderately well drained | D | Very limited | Very limited |
| Los Osos-Bonnydoon complex, 15 to 30 percent slopes | 6K | Well drained | D | Very limited | Very limited |
| Tomales loam, 9 to 15 percent slopes | 5K | Moderately well drained | D | Somewhat limited | Very limited |
| Centissima-Barnabe complex, 50 to 75 percent slopes | 5K | Well drained | C | Very limited | Very limited |
| Bonnydoon gravelly loam, 30 to 75 percent slopes | 5K | Somewhat excessively drained | D | Very limited | Very limited |
What This Means
Building & Foundations
About 67% 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 71% 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.