Understanding Your Drainfield
The drainfield is the most critical and expensive component of your septic system. Learn how it works and how to protect it.
What is a Drainfield?
A drainfield (also called a leach field or absorption field) is a series of underground trenches or beds where partially treated wastewater from your septic tank is distributed into the soil. As effluent percolates through the soil, naturally occurring bacteria remove harmful pathogens, nutrients, and contaminants before the water reaches groundwater.
90%+
Contaminants removed by healthy drainfield
20-30 yrs
Typical drainfield lifespan with proper care
$15-30K
Average replacement cost
Types of Drainfields
Conventional Gravel Trench
Traditional design with perforated pipes in gravel-filled trenches. Most common and least expensive option.
Advantages:
✓Lowest initial cost
✓Simple design
✓Easy to understand
Disadvantages:
✗Requires good soil
✗Large footprint
✗Gravel can shift over time
Chamber System
Plastic chambers replace gravel trenches. Creates void space for effluent storage and soil contact.
Advantages:
✓Easier installation
✓No gravel needed
✓Good for high water tables
Disadvantages:
✗Higher material cost
✗May require more area
✗Plastic degrades over decades
Pressure Distribution
Pump distributes effluent evenly throughout drainfield through small-diameter pipes with orifices.
Advantages:
✓Even distribution
✓Works on slopes
✓More efficient treatment
Disadvantages:
✗Requires pump/electricity
✗Higher maintenance
✗More complex design
Mound System
Elevated drainfield built above natural soil grade. Used when water table or bedrock is too high.
Advantages:
✓Works in poor conditions
✓Allows difficult sites
✓Visible for monitoring
Disadvantages:
✗Most expensive
✗Requires pump
✗Visible landscape feature
Drip Distribution
Uses drip irrigation tubing to distribute effluent into shallow soil. Highest level of treatment.
Advantages:
✓Excellent treatment
✓Flexible placement
✓Small footprint
Disadvantages:
✗Highest cost
✗Complex maintenance
✗Requires pretreatment
Drainfield Sizing Factors
Drainfield size is determined by multiple factors. A professional designer considers all these when creating your system specifications.
| Factor | Impact Level | Description |
|---|---|---|
| Soil Percolation Rate | Primary Factor | Slower percolation requires larger drainfield to handle the same wastewater volume. |
| Daily Wastewater Flow | Primary Factor | Based on bedrooms/occupants. More water usage requires more absorption area. |
| Soil Type & Texture | Important | Sandy soils absorb more per square foot than clay soils. |
| Groundwater Depth | Important | Shallow water tables may require raised systems or larger areas. |
| System Type | Moderate | Pressure distribution and drip systems can reduce required area by 25-40%. |
| Local Regulations | Variable | State and county codes may specify minimum sizes regardless of soil conditions. |
Drainfield Do's
✓Know your drainfield location and mark it
✓Plant only grass over the drainfield
✓Divert surface water and runoff away
✓Keep the area accessible for inspection
✓Pump tank regularly to protect drainfield
✓Conserve water to reduce drainfield load
✓Use septic-safe products in your home
Drainfield Don'ts
✗Drive or park vehicles on the drainfield
✗Build structures (patios, sheds) over it
✗Plant trees or deep-rooted shrubs nearby
✗Cover with plastic, concrete, or asphalt
✗Install sprinkler system over drainfield
✗Dig in the drainfield area without locating pipes
✗Overload system with excessive water use
Warning Signs of Drainfield Problems
Early detection can save thousands in repairs. Watch for these warning signs:
| Warning Sign | Severity | Recommended Action |
|---|---|---|
| Sewage odors near drainfield | High | Have system inspected immediately |
| Wet, soggy areas over drainfield | High | System may be failing - call professional |
| Unusually green grass over drainfield | Medium | Effluent may be surfacing - schedule inspection |
| Slow drains throughout house | Medium | May indicate full tank or failing drainfield |
| Sewage backup in lowest drains | High | Emergency - drainfield or pipe blockage |
| Standing water near distribution box | High | Distribution system problem - needs repair |
Reserve Drainfield Area
Most states require designating a "reserve area" equal to 100% of your primary drainfield size. This area must remain undeveloped and available for a replacement drainfield if your original system fails.
Important: Never build, pave, or plant trees in your reserve area. You'll need this space if drainfield replacement becomes necessary.
Ready to calculate your drainfield requirements?