Percolation Testing and Septic System Design in el dorado County, California
Planning a residential or commercial septic system in El Dorado County, California requires understanding detailed local requirements for percolation testing, septic sizing, and septic design. El Dorado County’s Local Agency Management Plan (LAMP) is the county‑specific program under the state Onsite Wastewater Treatment System (OWTS) Policy that governs how septic systems are sited, tested, and designed to protect groundwater and public health.
El Dorado County’s Local Agency Management Plan (LAMP)
El Dorado County’s LAMP authorizes Environmental Management to regulate onsite wastewater systems locally rather than relying on state‑wide Tier 1 standards. The county’s LAMP allows for a range of soil and groundwater conditions while still maintaining strong protections for water quality.
Under this plan:
Septic systems must comply with state requirements and local design criteria.
Cesspools and holding tanks are generally prohibited, except for pre‑existing approved holding tanks.
Horizontal and vertical setback requirements around wells, water bodies, and property features apply to protect sensitive resources.
Percolation Testing & Soil Evaluation
Percolation testing is a fundamental step in evaluating whether your property can support a septic system and in determining the appropriate septic sizing and septic design.
In El Dorado County:
A minimum of four percolation tests must be conducted across the proposed dispersal area to represent soil conditions accurately.
Test pits (soil trenches) must be excavated and logged by a qualified professional (e.g., Registered Civil Engineer, Geologist, or Registered Environmental Health Specialist). County staff may observe these excavations.
Groundwater levels must be assessed, especially in areas with vegetation or historical evidence of high groundwater. Wet weather testing may be required to determine seasonal high groundwater.
These investigations define soil texture, depth to groundwater, presence of restrictive layers, and other factors that guide your septic system’s layout and septic sizing.
Test Pits and Soil Profile Investigation
Test pits are crucial for understanding the soil profile before a septic system design can be approved:
At least one test pit per lot is required to log soils and evaluate their suitability for effluent dispersal.
Additional test pits may be needed where soils are shallow or groundwater is a concern.
Soil observation pits help identify unsuitable soil conditions (e.g., impervious layers, fractured rock, or clay) that may require alternative system designs or supplemental evaluation.
Test pits are backfilled after inspection for safety and erosion control, and their logs are incorporated into the site evaluation required by the county.
Septic System Sizing and Design
Once soil conditions are understood through percolation testing and soil profiling, your septic system can be sized and designed according to El Dorado County standards:
Standard System Criteria:
Percolation rates between 5 and 30 minutes per inch (MPI) allow for conventional trench systems.
Percolation rates beyond this range (e.g., slower than 30 MPI or faster than 5 MPI under certain conditions) may require special design systems such as pressurized distribution, capping fill, pump systems, or mound systems.
Sizing Requirements:
Disposal fields must be sized based on projected sewage flow (e.g., number of bedrooms or commercial wastewater volume) and percolation rates. A minimum absorption area is specified (e.g., 300 square feet or greater) exclusive of impermeable formations.
Leach line sizing and configuration are determined by soil characteristics and percolation results.
Groundwater and Soil Depth Criteria:
A minimum of five feet of effective soil depth below the bottom of the leaching trench and at least five feet to anticipated highest groundwater are standard requirements.
Minimum soil depth standards increase with more permeable soils to protect groundwater quality.
Site Evaluation & Permitting Requirements
Before El Dorado County will accept a septic permit application:
A Site Evaluation Report must be prepared by a qualified professional and approved by Environmental Health. This report documents soil testing, dispersal areas, setbacks, and system recommendations.
The site evaluation report must include a scaled site plan, indicating all soils testing locations and proposed disposal areas.
The report is valid and transferable with the land, but changes to the property (grading, construction, new wells, etc.) may invalidate it.
El Dorado County recommends that soil and percolation testing be coordinated with Environmental Management early in the planning phase to avoid costly delays.
Residential vs. Commercial Septic Design
El Dorado County’s LAMP applies to both residential and commercial onsite wastewater systems:
Residential systems are sized based on percolation results and anticipated wastewater flow from the dwelling.
Commercial systems — especially those with flows above typical domestic volumes or unique waste characteristics — may require additional evaluation, reporting, and potentially supplemental treatment options or monitoring.
Large systems (e.g., projected flows greater than 2,500 gallons per day) have additional standards, such as dividing disposal and replacement areas and alternating flow fields to ensure performance and compliance.
Inspection and Construction
After permit issuance, El Dorado County requires inspections during construction:
Open trench inspections before backfilling.
Final inspections to verify that the system matches approved septic design plans and meets county standards.
Accurate “as‑built” drawings may be required on site for inspection, and special designs typically must have the engineer’s signed plot plan present.
How STL Can Handle Your El Dorado County Septic Project
At Soil Tests Laboratory, we provide full‑service solutions for percolation testing, soil evaluation, septic sizing, and septic design in El Dorado County, California. Whether you’re building a new residential home, an accessory dwelling unit, or a commercial facility, our team of qualified professionals can manage the entire process — from site evaluation and testing to design documentation required for county approval.
We work closely with El Dorado County Environmental Health to ensure your project complies with all LAMP standards and state policies, reducing delays and protecting your investment — so your septic system is designed right the first time.
Contact us for a price at: Info@soiltestslab.com or call (951) 345-3509
