Percolation Testing and Septic System Design in San Bernardino County, California
If you’re planning a new home, cabin, or other structure in San Bernardino County, California, understanding the county’s requirements for percolation testing, septic sizing, and septic design is essential. While each California county implements its own Local Agency Management Program (LAMP) under state law, all septic systems must comply with the California Onsite Wastewater Treatment System (OWTS) Policy and local siting and design standards to protect public health and water quality. San Bernardino’s program balances state standards with local conditions across its diverse terrain and soil types.
What Is Percolation Testing?
Percolation testing is the process of measuring how quickly water drains through the soil on a property. These rates are used to determine whether a traditional septic system is feasible and guide septic sizing and septic design. San Bernardino County requires percolation testing for most new onsite wastewater treatment systems (OWTS) where there is no sewer connection, and where existing data are not adequate for review.
Test Pits: Inspection and Multiple Purposes
Before percolation testing can be conducted, test pits or exploratory borings are prepared to evaluate subsurface conditions. These excavations are critical to site evaluation and serve multiple purposes:
Soil Profile Evaluation: Inspecting soil layers allows professionals to classify texture, composition, and infiltration capacity — key factors for septic sizing and septic design.
Bedrock Identification: Pits reveal the depth to bedrock or other restrictive layers that could interfere with leach field placement.
Water Table Determination: Test pits help identify the historic and seasonal high groundwater level to ensure adequate vertical separation from the bottom of the disposal area to groundwater.
Site Suitability: Combined data on soil profile, depth to bedrock, and groundwater support an overall assessment of whether the soil conditions meet LAMP criteria and will support a septic system.
Who Must Inspect and Conduct Percolation Testing
All percolation testing and associated soil evaluations in San Bernardino County must be completed by a Qualified Professional (QP). Permitted types of professionals include:
Registered Civil Engineer (PE)
Registered Environmental Health Specialist (REHS)
Certified Engineering Geologist
Registered Geologist
Geotechnical Engineer
These professionals are recognized by San Bernardino County as qualified to perform soil testing, submit percolation reports, and contribute to septic septic sizing and septic design.
How Test Pits Are Dug
Test pits can be dug by a licensed contractor, the property owner, or by STL under the supervision of a Qualified Professional. To provide a complete soil profile and identify groundwater or bedrock, test pits in San Bernardino County are typically 18 to 24 inches wide and 7 to 8 feet deep (or deeper as needed), depending on soil conditions, groundwater depth, and design needs.
Percolation Testing Procedures and Notification
San Bernardino County requires that a Percolation Test Notification be submitted by a Qualified Professional at least two business days before the test is conducted. County staff may inspect the site during percolation testing and evaluate factors such as lot size, slope, and soil conditions. Percolation rates are measured by adding water to the test holes and documenting how many minutes per inch (MPI) it takes for water to drain. Uniform results (within 25% of the mean rate) help define the design rate; otherwise, the most conservative rate is used for sizing.
Septic Design and Sizing
Once percolation testing and soil evaluation are completed, the results feed directly into septic system design:
System Type: Based on soil absorption rate, depth to groundwater, slope, and other site characteristics, designs may be conventional gravity systems or require supplemental/alternative dispersal systems.
Septic Sizing: Leach field size and dispersal area are calculated using percolation rates and estimated wastewater flow (often tied to number of bedrooms or building use).
Setbacks and Separation: Local standards require horizontal setbacks from wells, structures, property lines, and certain water features, as well as minimum vertical separation below the dispersal system to groundwater or impermeable layers to protect water quality.
Unique Considerations in San Bernardino County
San Bernardino’s LAMP is comprehensive and contains several provisions that may be relevant to your project:
Site Evaluation and Inspections: DEHS may conduct site evaluations during percolation testing to assess slope, rock outcroppings, and other site constraints before approval.
Prohibition of Cesspools: Cesspools are not permitted; any existing cesspool must be replaced with a compliant OWTS.
Minimum Lot Size for New Developments: The LAMP includes minimum lot size requirements for subdivisions that rely on septic systems, often requiring at least one‑half acre per dwelling unit in unincorporated areas.
Special Watershed Interactions: Increased setbacks and notifications are required for systems near municipal water systems or sensitive water bodies to protect source water quality.
These requirements reflect San Bernardino County’s diverse geography — from desert soils with rapid percolation to mountainous terrain with shallow bedrock — and represent local adaptations of the California OWTS Policy to protect public health and local aquifers.
To view San Bernardino County’s full LAMP visit here:
https://wp.sbcounty.gov/wp-content/uploads/sites/7/2017/08/San-Bernardino-County-LAMP.pdf
How STL Can Handle It All
At Soil Tests Laboratory, we provide a complete onsite wastewater solution for San Bernardino County property owners. Our team includes licensed Professional Civil Engineers and other Qualified Professionals who manage every step of your project — from digging and inspecting test pits, handling percolation testing, to detailed septic sizing and septic design compliant with county and state requirements. With STL on your side, you get accurate results, proper documentation, and a septic system design that meets San Bernardino County’s standards and gets approved without unnecessary delays.
Contact us for a price at: Info@soiltestslab.com or call (951) 345-3509
