What the drain field does
The drain field is where most of the wastewater treatment happens. Effluent from the tank flows through perforated pipes or chambers into gravel or soil, and the soil filters out bacteria and other contaminants before the water reaches groundwater. A healthy drain field depends on soil that can absorb water and air that lets beneficial microbes work.
Why drain fields fail
- Solids from the tank clog the soil when the tank isn't pumped often enough or a baffle is broken.
- Too much water, from leaks, high water use, or roof drains and surface water directed onto the field, overloads the soil.
- Compaction from vehicles, livestock or construction crushes pipes and soil.
- Roots from trees and shrubs grow into pipes.
- High groundwater in wet seasons leaves the soil too saturated to accept effluent.
- Age. Over many years, a biological mat forms in the soil and can slow absorption.
Signs of a failing drain field
- Soggy ground, standing water or sewage odors over the field.
- Bright green, lush grass over the field in dry weather.
- Slow drains and backups, especially after heavy water use or rain.
- Sewage surfacing in the yard.
Surfacing sewage is a health hazard. Keep children and pets away and call for help.
Diagnosing the problem
A contractor should start by pumping and inspecting the tank, checking baffles and filters, inspecting the distribution box, and evaluating the field. A camera inspection can find crushed pipes or roots. Sometimes the problem is a broken pipe or distribution box rather than the soil, and a smaller repair solves it.
Repair and replacement options
- Repairing pipes, distribution boxes or baffles when damage is localized.
- Resting the field by reducing water use, or alternating between two fields where a system has them.
- Adding to or relocating the field when space and soils allow.
- Full replacement with a new conventional field.
- Alternative systems such as mound systems, sand filters, pressure distribution or aerobic treatment units, where soil, groundwater depth or lot size make a conventional field unsuitable. These need more maintenance and usually more frequent inspection.
Septic repairs and replacements generally require a permit from the county or local health or environmental agency, and new systems are designed around soil tests. Expect the permitting process to be part of the timeline.
Protecting a new or repaired drain field
- Pump the tank on schedule.
- Spread water use and fix leaks.
- Keep vehicles and heavy equipment off the field.
- Direct roof and surface water away.
- Plant only grass or shallow-rooted plants over the field, and keep trees well away.
- Keep records of the design, location and service.
Questions to ask
- What's causing the failure, and how did you determine it?
- Is a repair possible, or does the field need replacement?
- What permit is needed, and who handles it?
- If a new system is needed, what type, and what maintenance will it require?
- What warranty comes with the work?
How long repairs take
Small repairs, such as replacing a distribution box or a damaged pipe, can often be done in a day once the problem is found. Full drain field replacement takes longer, because soil testing, design and permits come first, and installation depends on weather and soil conditions. During that time, conserving water and having the tank pumped more often can keep the system working.