Percolation Testing and Septic Design in Mono County California
If you’re planning to build a home, cabin, or other structure in Mono County, California, understanding the county’s requirements for percolation testing, septic sizing, and septic design is critical. While each California county has its own Local Agency Management Program (LAMP), all septic systems must meet state‑mandated standards to protect public health and groundwater quality. Following proper procedures ensures your system functions as intended and receives county approval.
Percolation Testing: What It Is and Why It Matters
Percolation testing measures how quickly water moves through soil on your property. This is foundational to determining whether your site is suitable for a septic system and drives decisions about septic sizing and septic design. In Mono County, percolation testing and soil evaluations are required for most new individual onsite sewage systems unless sufficient existing data proves uniform soil conditions.
Test Pits: Inspection and Multiple Purposes
Before percolation testing can be performed, test pits must be prepared. In Mono County, these test pits are inspected by a qualified professional to gather key subsurface information. Test pits serve several critical purposes:
Soil Profile Evaluation: They expose soil layers so that texture, composition, and suitability for effluent dispersal can be assessed.
Bedrock Identification: Test pits reveal the depth to bedrock or other restrictive layers that could limit leach field placement.
Water Table Determination: Inspecting pits shows how close the groundwater is to the planned system and ensures adequate vertical separation.
Compliance with Separation Standards: Mono County’s LAMP requires minimum separation from the bottom of the disposal area to groundwater or impervious layers (like bedrock or clay) to protect public health and water quality.
Who Must Inspect and Conduct the Perc Test
All percolation testing — including test pit inspections — must be conducted under the supervision or inspection of a qualified professional such as a Registered Environmental Health Specialist (REHS), a licensed Professional Civil Engineer (PE), a registered Geotechnical Engineer, or a licensed Professional Geologist (PG).
How Test Pits Are Prepared
Test pits can be dug by a licensed contractor, the property owner, or by STL. STL can handle the excavation under professional oversight to ensure compliance with Mono County requirements, including proper depth and location for accurate evaluation. Test pits in Mono County are typically 18 to 24 inches wide and 7 to 8 feet deep or deeper depending on soil conditions and required separation distances.
Performing the Percolation Test
Once test pits are inspected, percolation tests are conducted by introducing water to the holes and measuring how quickly the water drains through the soil (usually expressed in minutes per inch). These percolation rates determine the septic sizing and inform the appropriate system type and design criteria. All percolation test data and soil evaluations must be documented and included in the permit application submitted to Mono County Environmental Health.
Septic Design and Sizing in Mono County
Septic system design in Mono County must follow the county’s LAMP requirements while complying with applicable state standards:
System Type: Based on percolation rates and soil/site conditions, the OWTS design may be a conventional gravity system, pressure distribution system, or approved alternative that meets local requirements.
Septic Sizing: Leach field and septic tank sizing are calculated using anticipated wastewater flow (often based on number of bedrooms) and soil application rates derived from percolation testing and soil profiles.
Minimum Separations: Mono County’s LAMP outlines minimum vertical separation to groundwater, bedrock, or restrictive layers, ensuring long‑term operational safety.
Setbacks: Horizontal setbacks to wells, structures, property lines, and surface waters are enforced and may exceed general state standards depending on site conditions.
Unique County Rules, Unified State Requirements
While Mono County implements its own LAMP with specific criteria for siting, percolation testing, and septic design, all California counties must comply with the State Water Resources Control Board’s Onsite Wastewater Treatment Systems Policy (OWTS Policy). This state policy ensures septic systems protect public health and groundwater quality across California, even as local programs adapt requirements to local geology, water resources, and development patterns. Follow this link to read Mono County’s LAMP: https://www.waterboards.ca.gov/lahontan/water_issues/programs/owts/docs/lamp_tracking/mono_county_lamp_final.pdf
How STL Can Handle It All
At Soil Tests Laboratory, we provide a comprehensive solution for property owners in Mono County, California. Our team includes licensed Professional Civil Engineers who can manage every step of your project — from digging and inspecting test pits, performing percolation testing, to complete septic sizing and septic design tailored to your site. We handle the technical details, county requirements, and documentation so your septic permit application is complete, compliant, and ready for approval. Let us make your septic project smooth and successful from start to finish.
Contact us for a quote: Info@soiltestslab.com or call (951) 345-3509
