Soil Testing & Perc Tests
Understanding your soil is essential for proper septic system design. Learn about percolation tests and how soil type affects your system.
Why Soil Testing Matters
Your soil is the final treatment system for septic effluent. As wastewater percolates through soil, bacteria and other organisms remove harmful pathogens and pollutants before water reaches groundwater. The soil's ability to absorb and treat wastewater determines what type of septic system you need.
System Design
Soil type determines drainfield size and system type required
Treatment Quality
Proper soil ensures adequate pathogen and nutrient removal
Legal Requirement
Most jurisdictions require perc tests before permit approval
Soil Types & Percolation Rates
Percolation rate measures how quickly water drains through soil, expressed in minutes per inch. This rate directly affects drainfield sizing and system design.
| Soil Type | Perc Rate | Suitability | Notes |
|---|---|---|---|
| Gravel/Coarse Sand | < 1 min/inch | Too Fast | Water drains too quickly for proper treatment. May require engineered system. |
| Sand | 1-5 min/inch | Good | Excellent drainage with adequate treatment time. Ideal for conventional systems. |
| Sandy Loam | 5-15 min/inch | Good | Good balance of drainage and treatment. Works well for most systems. |
| Loam | 15-30 min/inch | Acceptable | Adequate drainage. May need larger drainfield for proper absorption. |
| Clay Loam | 30-60 min/inch | Marginal | Slow drainage requires significantly larger drainfield or alternative system. |
| Clay | > 60 min/inch | Poor | Too slow for conventional systems. Requires mound, ATU, or other alternative. |
The Percolation Test Process
Contact Health Department
Schedule a perc test with your local health department. They will specify requirements and may need to witness the test.
Dig Test Holes
Excavate holes at the proposed drainfield location. Typically 6-12 inches in diameter and 24-36 inches deep, matching the planned trench depth.
Pre-Soak the Holes
Fill holes with water 12-24 hours before the test to simulate saturated conditions. This ensures accurate results.
Conduct the Test
Fill holes to a specific depth and measure how long it takes for water level to drop one inch. Repeat multiple times for accuracy.
Calculate Results
Average the readings to determine percolation rate in minutes per inch. This determines drainfield sizing requirements.
Review with Officials
Health department reviews results, soil observations, and site conditions to approve system design or recommend alternatives.
Site Evaluation Factors
Beyond the perc test, professionals evaluate several site conditions that affect septic system design and placement.
Depth to Bedrock
CriticalMinimum 2-4 feet of soil required above bedrock for proper treatment and absorption.
Water Table Height
CriticalDrainfield must be above seasonal high water table to prevent contamination.
Slope
ImportantAffects system type. Steep slopes may need pressure distribution or terraced systems.
Soil Layers
ImportantDifferent soil layers at various depths affect water movement and treatment.
Flood Zone
ImportantSystems in flood-prone areas need special design considerations.
Available Space
PracticalMust have room for primary drainfield plus 100% reserve area for future replacement.
Typical Perc Test Costs
Expect to pay between $250-$1,000 for a standard percolation test:
• Basic perc test: $250-$400
• Deep pit evaluation: $400-$700
• Complete site assessment: $700-$1,000
What if Your Site Fails?
A "failed" perc test doesn't mean you can't build. Options include:
• Mound or raised bed systems
• Aerobic treatment units (ATUs)
• Sand filter systems
• Alternative drainfield designs
Tips for Successful Soil Testing
Schedule Wisely
Test during wet season for accurate high water table readings. Results from dry periods may not reflect worst-case conditions.
Get Multiple Holes
Soil conditions can vary across your property. Multiple test holes ensure accurate representation of site conditions.
Use Licensed Professionals
Many states require licensed soil scientists or sanitarians to conduct and certify perc tests for permits.
Test Before Buying Land
If buying rural property, make purchase contingent on passing a perc test. Poor soil can make building very expensive.
Ready to estimate your septic system size?