What Is a Drain Field?

A drain field (also called a leach field or drainfield) is the part of a septic system that spreads the liquid leaving the tank into the soil. The soil then filters and treats it before the water reaches groundwater.

Explanation

The EPA describes a drain field as a shallow, covered excavation made in unsaturated soil. Liquid from the tank (effluent) flows through pipes onto porous surfaces, and as it soaks down through the soil, bacteria, viruses and nutrients are removed. Florida's septic rule defines a drainfield as perforated or open-jointed piping, approved alternative distribution units or other facilities that spread effluent for "filtration, oxidation and absorption by the soil" in the zone of aeration, the unsaturated soil above the water table (Rule 62-6.002(20)).

In a standard Florida system, the effluent reaches the trenches or bed through a distribution box or a header pipe, and all of the drain field's sidewalls sit below the level of the undisturbed native soil (Rule 62-6.002(55)). If part of the sidewalls sits above native soil, it is a filled system (Rule 62-6.002(26)); if any part of the bottom is at or above native soil, it is a mound system (Rule 62-6.002(40)).

Florida's site rules (Rule 62-6.006, F.A.C.) set the conditions a drain field site must meet:

• 24-inch separation: the water table at the wettest season of the year must be at least 24 inches below the bottom of the drain field.
• Soil depth: suitable soil must extend 42 inches or more below the bottom of the drain field.
• No paving or traffic: the drain field site must not be covered with asphalt or concrete or be subject to vehicle traffic.

The EPA notes that if a drain field gets more liquid than it can take, it can flood and send sewage to the ground surface. It also says that after 25 to 30 years the natural biomat at the bottom of the trenches can thicken and reduce drainage. Florida law requires a permit to repair a system, so drain field repair or replacement usually needs one. Published ranges put drain field repair at about $1,000–$5,000 and replacement of a conventional drain field at about $3,000–$15,000.

When to seek help

• Standing water or damp spots over the drain field, or sewage odors around it (EPA warning signs).
• Bright green, spongy grass over the drain field, even in dry weather.
• Slow drains, gurgling plumbing or sewage backing up into the house.
• You plan to pave, park or build over the area. Ask your permit office first, since the rule keeps the drain field site free of paving and traffic.

Learn more in how a septic system works in Florida, septic repair and drain field costs and why drain fields struggle in the rainy season. Read the signs a septic tank is full, find your county permit office, or compare drain field repair companies.

Related terms

• Distribution box
• Mound system
• Percolation test
• Effluent filter