Percolation Testing and Septic Design in kern County California
If you’re planning a new home, guest house, or other structure in Kern County, California, understanding the county’s requirements for percolation testing, septic sizing, and septic design is essential. While each California county operates under its own Local Agency Management Program (LAMP), all septic systems must comply with state‑level standards that protect public health and water quality. Adhering to proper protocols ensures your system functions safely and receives county approval.
Percolation Testing: What It Is and Why It Matters
Percolation testing measures how quickly water drains through the soil in your proposed septic area. These rates help determine whether your site can support a conventional system and directly influence septic sizing and septic design. Kern County’s LAMP incorporates these requirements into the local onsite wastewater treatment system (OWTS) regulations, which were developed in alignment with the California State Water Resources Control Board’s OWTS Policy.
Test Pits: Inspection and Multiple Purposes
Before percolation testing is conducted, test pits must be prepared and inspected by a qualified professional. These small excavations serve several essential purposes:
Soil Profile Evaluation: Exposing the soil layers allows professionals to classify texture, composition, and absorption characteristics that are crucial for septic sizing and septic design.
Bedrock Identification: Test pits reveal the depth to bedrock or other restrictive layers that could limit suitable placement of leach lines or require alternative systems.
Water Table Determination: Inspecting test pits helps identify the depth to groundwater or perched water, ensuring adequate vertical separation is maintained.
Site Suitability: The combined soil profile, bedrock depth, and groundwater data verify whether the site meets Kern County’s siting criteria in the LAMP.
Who Must Inspect and Conduct Percolation Testing
All percolation testing — including soil evaluation and logging — must be overseen or reviewed by a Qualified Professional (QP) such as a Registered Environmental Health Specialist (REHS), a licensed Professional Civil Engineer (PE), a California Professional Geologist, a Certified Engineering Geologist, or a Registered Professional Soil/Geotechnical Engineer.
How Test Pits Are Dug
Test pits for percolation evaluation can be dug by a licensed contractor or the property owner, but they must be inspected by a QP and meet county standards. STL can excavate these pits under professional supervision so they are ready for inspection and testing. Typical dimensions for these pits are 18 to 24 inches wide and 7 to 8 feet deep or deeper where required to evaluate soil and groundwater conditions.
Performing the Percolation Test
Once test pits are inspected and soil conditions are verified, the percolation test itself is performed. Water is placed in the test holes after a presaturation period, and the rate at which it filters into the surrounding soil is measured in minutes per inch (mpi). In Kern County:
Minimum of two holes: A minimum of two percolation test holes are required per system, and at least three tests per hole must be recorded.
Representative placement: Tests must be conducted within approximately 35 feet of the proposed leach field and positioned to represent the entire area.
Use of results: The slowest percolation rate observed — unless five or more tests are performed — is used to size the leach field.
Documentation: All tests must be recorded on an approved data log, submitted with your permit application, and wet‑stamped and signed by the QP who supervised or performed the testing.
These percolation rates are also used to classify soil types for design purposes — for example, soils that percolate slower than 1 mpi or faster than 60 mpi may require alternative systems or special engineering.
Septic Design and Sizing
Septic system design in Kern County follows detailed siting and design criteria found in the LAMP and associated Kern County Onsite Systems Manual:
System Type: Designs are based on soil conditions, percolation rates, and expected flows. Conventional gravity systems may be permitted where soil and separation criteria are met, while alternative systems or supplemental treatment may be required for borderline or slow‑percolating soils.
Septic Sizing: The leach field size and application rates are calculated using percolation test results and projected wastewater flow based on structure use (e.g., number of bedrooms).
Minimum Separations and Setbacks: Designs must ensure adequate vertical separation to groundwater, bedrock, and other restrictive layers and meet horizontal setbacks from wells, buildings, and property features.
Site Constraints: Ground slope, floodplain location, and other environmental constraints are evaluated as part of site evaluation to ensure long‑term performance.
Unique County Rules, Unified State Standards
While Kern County’s LAMP contains specific local criteria for siting, soil evaluation, and septic design, all California counties must implement septic regulations consistent with the state’s OWTS Policy. The LAMP framework allows counties like Kern to tailor requirements to local soil, hydrology, and development patterns while maintaining the statewide goal of protecting public health and water quality. A full copy of Kern County’s LAMP can be found here: https://www.waterboards.ca.gov/lahontan/water_issues/programs/owts/docs/lamp_tracking/kcehd_lamp_final.pdf
How STL Can Handle It All
At Soil Tests Laboratory, we offer a complete, start‑to‑finish solution for property owners in Kern County, California. Our team includes licensed Professional Civil Engineers and other Qualified Professionals who can handle every step — from digging and inspecting test pits, performing percolation testing, to precise septic sizing and septic design — all in compliance with Kern County’s LAMP and state requirements. With STL on your side, you get thorough, professional results and guidance that keep your septic project moving smoothly from concept to approval.
Contact us for a price at: Info@soiltestslab.com or call (951) 345-3509
