How can I perform a DIY septic system percolation test, and what do the results mean?

Question

Tyler Smith · Accepted Answer

Performing a DIY percolation test, often called a 'perc test,' is a preliminary step homeowners might consider to understand their soil's absorption capacity, especially if contemplating a new septic system or drain field repair. However, it's crucial to understand that an official perc test for permitting purposes must be conducted by a licensed professional, typically an engineer or health department official, following strict local regulations. This DIY guide is for informational purposes only and cannot replace an official assessment. Many buyers ask about this early in their system planning.

Steps for a DIY Percolation Test (for informational purposes only):

1. Choose a Site: Select an area in your proposed drain field location. Dig at least three test holes, spaced about 10-20 feet apart, to get a representative average. The depth of the holes should mimic the proposed depth of your drain field trenches, usually 24-36 inches deep, and be about 6-12 inches in diameter.
2. Prepare the Holes: Scrape the bottom and sides of each hole to remove any smeared soil, which can impede water absorption. Place 2 inches of coarse gravel at the bottom of each hole.
3. Presoak the Soil: Fill each hole with water and allow it to completely drain. This saturates the soil and simulates wet weather conditions, providing a more accurate reading. You may need to repeat this step if the soil is very dry. Let the holes sit overnight if possible.
4. Perform the Test: The next day, fill each hole with water to a depth of at least 6 inches above the gravel. Measure the water level immediately and record the time. After 30 minutes, measure the water level again. Record the drop in water level.
5. Calculate the Percolation Rate: Divide the drop in water level (in inches) by the time elapsed (30 minutes). Multiply this by two to get the inches per hour. For example, if the water level dropped 3 inches in 30 minutes, the rate is 6 inches per hour.

Interpreting the Results (general guidelines, not regulatory):
• Fast Percolation (e.g., >10-12 inches/hour): Indicates very porous, sandy soil. While good for absorption, very fast rates might mean contaminants aren't adequately filtered before reaching groundwater. This can sometimes require adjustments to drain field design.
• Moderate Percolation (e.g., 1-10 inches/hour): Generally ideal for drain fields. This allows for good absorption while providing sufficient contact time for wastewater treatment by the soil.
• Slow Percolation (e.g., <1 inch/hour): Indicates heavy clay soil or a high water table. This soil type is poor for conventional drain fields as it cannot absorb wastewater quickly enough, leading to system backups and failure. Alternative systems like aerobic treatment units or mound systems may be necessary.

Always remember that local health departments have specific requirements for perc test methodology and acceptable rates. This DIY exercise simply provides a rough estimate of your soil's suitability for a septic system. When it comes to investing in a new septic system, an official test is paramount for proper design and permitting, a critical point emphasized in all professional advice.