What Are Percolation Tests and How to Carry One Out?

A percolation test, sometimes shortened to perc test, is one of the essential soil tests that you may need before building a home. Perc tests are often necessary when constructing a property on new, undeveloped land or installing or replacing a septic tank system on already owned land.

Understanding when you need a percolation test, what information the test provides, and why a perc test is recommended can help you choose the right professional for accurate results.

What is a Percolation Test?

A percolation test is a soil test designed to measure its water absorption rate. Perc tests are required before installing a septic system on the property to ensure it doesn’t flood or pollute natural groundwater.

A percolation test is performed by digging at least one hole into the soil and filling it with a predetermined quantity of water. This allows the tester to measure the water absorption rate, providing feedback about the suitability of the soil for building.

The water absorption rate is measured by calculating how fast the soil absorbs the water. Soil percolation test standards recommend digging multiple holes between 4 and 12 inches in diameter and 24 to 30 inches deep.

The holes must be roughened and pre-soaked before testing to ensure the results are as accurate as possible.

Interpreting Percolation Test Results

Percolation test results are measured by calculating the number of inches of water absorbed per minute.

Sandy soils have the fastest absorption rates, typically between 1 and 8 inches per hour.
Loamy soils, which contain a balance of sand, silt, and clay, have a moderate percolation speed, approximately 1 to 2.4 inches per hour.
Clay soils absorb water the slowest, rarely exceeding 0.17 inches per hour.

Professional percolation test results may use different units, such as minutes per inch (mpi). For example, 2 inches per hour equals 30 mpi.

Who Needs Percolation Tests?

Generally, any home or business not connected to the public sewer system must install a private sewage disposal system, such as a septic tank. These homes and businesses need to have a perc test conducted on their soil before installing an on-site wastewater treatment system. Examples include septic tanks for residential homes and septic systems suitable for commercial structures.

Who Performs a Percolation Test in Los Angeles?

Homeowners and business managers in Los Angeles have two primary options for conducting percolation tests: do-it-yourself (DIY) solutions and professional percolation tests conducted by licensed geologists.

Percolation Test: Do-It-Yourself vs. Licensed Geologist

There are potential benefits and drawbacks of DIY percolation tests versus professionally conducted tests. Understanding these factors is essential in determining the most suitable for your needs.

Expertise: Licensed geologists are accredited soil science and geotechnical engineering experts. They understand how each soil type behaves during a percolation test and can interpret the water absorption rates in the test results. Conducting a DIY percolation test requires basic knowledge of soil science and testing methodologies. While it may be sufficient for an informal or basic soil assessment, a professional can interpret the results more precisely.
Access to equipment: Conducting an accurate percolation test requires owning the right tools, such as augers, shovels, post hole diggers, and an appropriate water source. While it is possible to acquire the correct tools to conduct a perc test yourself, a professional geologist has access to a wider range of tools and has the expertise to use them efficiently.
Site evaluation: Professional geologists can conduct a thorough soil survey and assess the overall site conditions, such as groundwater levels, soil composition, and potential geotechnical issues that may hinder the installation of a septic system. This extra knowledge and service aren’t guaranteed with a DIY solution.
Regulatory concerns: Installing a septic system on your land in the County of Los Angeles requires a septic permit. Septic permits are issued by the Los Angeles County Department of Health – Environmental Health Division and ensure your proposed septic system complies with all local regulations. Licensed geologists can supply the local government with the information they need to verify your septic system’s viability and regulatory compliance.
Costs: Generally, a DIY percolation test solution is cheaper than one professionally conducted by a licensed geologist. A DIY test can potentially help you save money if you already have the tools and expertise. However, a licensed geologist can assume liability and offer warranties and assurance for their work, granting you peace of mind and preventing future costs that could arise due to inaccurate soil assessment.

Factors in Percolation Test Costs

Although professionally conducted percolation test costs between $750 and $1,850 on average, many factors can affect your test’s final cost.

Professional fees: Licensed geologists charge different base costs for a percolation test. These costs vary depending on the professional’s location, qualifications, and experience. When choosing a professional, look for someone with years of experience conducting percolation tests and designing viable, compliant septic systems, and contact them to obtain a quote.
Soil conditions: The soil type, composition, and overall soil condition on your property are driving factors in the cost of a professional percolation test. Challenging soils, such as dense clay or rocky soils, may require the geologist to use specialized equipment and spend more time on the test.
Test pit size: Professional percolation tests generally require multiple test pits of various diameters and depths to obtain precise results. The more test pits the geologist must dig, the higher the percolation test’s costs.
Costs of additional services: A geologist’s soil survey and assessing expertise can increase the test’s final costs. However, these thorough assessments and insights can easily prevent costly future problems.
Accessibility concerns: Licensed geologists may charge more if your site is challenging to access for their vehicle and equipment. Common accessibility concerns that can raise the costs of your test include difficult terrain, limited road access, steep slopes, distant locations, or sites that require additional licenses or permits to access.

Contact a Licensed Geologist for a Professional Percolation Test in Los Angeles

Founded in 1985 in Burbank, Applied Earth Sciences (AES) is a professional soil engineering and geologic company with over 35 years of experience serving Los Angeles and Southern California. AES is fully licensed and qualified to handle all geoscientific and geoengineering projects, from percolation testing to complete septic system design and installation.