By Caigan McKenzie
NESC Staff Writer
This article appeared in Small Flows Magazine, Fall/Winter 2009, Volume 9, No. 3
Editor’s Note: The drainfield is one of the most important parts of your septic system.
Maintaining it not only keeps the system functioning properly—which aids in public health—it prevents an expensive repair from damaging your wallet. The average cost of replacing a drainfield can run from $6,000 to more than $12,000. And no one wants a smelly drainfield to ruin outdoor activity. The following Q&A offers advice about preserving your drainfield to provide you with years of uninterrupted service.
What is a drainfield?
There are two main parts to a basic septic system: the septic tank and the drainfield. A conventional drainfield is an arrangement of perforated pipes or chambers buried underground that channel pretreated wastewater (effluent) from the septic tank out over a large area of the soil for final treatment. A reserve drainfield, required by many states, is an area on the property that is suitable for a new drainfield system should the current drainfield fail.
How does a drainfield work?
The drainfield is where the real treatment of the wastewater occurs. As the effluent slowly moves through the soil, the drainfield acts as a natural filter by absorbing the organic materials, reducing or removing bacteria and viruses, and removing some nutrients. The naturally purified water is then moved to the groundwater or evaporates from the soil.
What is a drainfield’s life expectancy?
Drainfields should last at least 25 to 30 years or more if they are designed, installed and maintained correctly, according to Zane Satterfield, engineering scientist with the National Environmental Services Center. What causes a drainfield to fail? The two most common causes of drainfield failure are hydraulic and biological overloading.
Hydraulic overloading occurs when too much water is sent to an under-designed system while too much organic matter in the effluent causes biological overloading.
However, there are many and varied causes for drainfield failure, including,
• not maintaining your septic tank;
• solids from the septic system clogging the small holes in the drainfield pipes;
• additives because they can damage the system by breaking up the sludge and scum
layers, causing them to flush out of the tank and clog the drainfield;
• flushing fats, oils, greases, chemicals, solvents, paint and other improper substances
down the drain;
• improper drainfield siting, design, construction;
• overuse of water-generating appliances;
• garbage disposals because the extra solids introduced into the tank can increase the
accumulation of sludge by more than 50 percent
• excessive rainfall;
• downspouts, gutters, meds online without prescription basement sump pump drains and other rainwater or surface
water drainage systems that divert water over the drainfield;
• tree roots that interfere with the drainlines;
• vehicles driving over the drainfield and cracking the pipes, and
• age of the system.
How can I tell if my drainfield is failing?
The most obvious sign of drainfield failure is surfacing effluent. If the soil can no longer absorb the effluent, it will either rise to the ground surface and pool or, if the drainfield isn’t flat, it will ‘blow out’ the end of the last trench. You might also notice muddy soil around your septic system or in your basement, toilet or sink back-ups when you flush or do laundry, or strips of bright green grass growing over the drainfield.
Can I drive on, pave over, or build a structure on my drainfield?
No. These activities can cause the soil to compact, exposing system components and
possibly untreated sewage to the ground surface. Paving over all or a portion of a drainfield may prevent air from getting into the soil, impairing the treatment process, as well as limiting access for repairs, maintenance, or inspection.
What can be planted over or near a drainfield?
The best covering for your drainfield is grass because it helps to remove water and
nutrients from the soil and helps to prevent soil erosion. Some people may choose to landscape their drainfield because of limited space or because they think it is an eyesore. The challenge is to find plants that will meet your landscaping needs but not clog the drain pipes. Thurston County, Washington, offers the following planting tips:
- The deep roots of some vegetables may damage the drainfield pipes as can digging
and rototilling the soil to prepare it for planting vegetables. Hand cultivate only.
- Parts of the system may be only six inches under the surface. Adding two to three
inches of topsoil to the drainfield should be fine but more than that could prohibit the
exchange of air and water, decreasing the drainfield’s ability to treat the wastewater.
- Plants should be low in maintenance and water needs and should be shallow rooted
- Base your plant selection on the amount of sunlight the area receives and the flowers
- For wildflower seeds, remove grass in small areas (six inches in diameter) and sow the seed in those areas. Keep the grass out of the area until the seed has germinated and is large enough to compete with the grass. Trees or shrubs should not be planted within 25 feet of the drainfield, according to the North Carolina State University, because their roots could clog or damage the drain lines.
A properly operating drainfield will not contaminate the soil with disease-causing organisms, but it is very difficult to determine if a field is operating as it should; therefore, vegetables should not be planted over the drainfield. In addition,
vegetables need watering, and excess water in the soil reduces its ability to treat the wastewater.
How can I maintain my drainfield?
Maintenance begins with water use and waste disposal habits. The drainfield does not have an unlimited capacity, so the more water you use, the greater the likelihood of
problems with the drainfield. Water conservation practices can help reduce the amount of wastewater generated in the home.
“The drainfield should be inspected, but typically, it is overlooked when it comes to onsite system inspection and maintenance unless there are obvious signs of failure or an inspection is required for property transfer,” Satterfield says. “Repair and rejuvenation can be costly, so it is a good idea to maintain your drainfield so small problems can be taken care of before they become bigger problems.”
When is the best time to inspect my drainfield?
“Drainfield inspections are most often performed at the same time a septic tank is being inspected or pumped because the outlet of the tank is easier to identify, which
makes it easier to locate the piping to the drainfield,” explains Satterfield. “This enables the inspector to watch the outlet pipe when the septic tank is pumped to see if any water drains back into the tank from the drainfield.”
What happens during a drainfield inspection?
“Inspecting a drainfield is not as straight forward as inspecting a septic tank,” explains Craig Mains, engineering scientist, NESC. “Because the drainfield is buried, it is difficult to visually determine its condition. If there are no visible problems, the drainfield inspection consists mostly of locating the drainfield area and checking the soils to make sure the system is designed and installed correctly.”
The soil is checked with a soil probe, Mains further explains, which comes in different shapes, sizes, and materials. “The probe helps determine the length of the drainfield
and the length and width of its trenches. Checking each of these parameters ensures the proper amount of drainfield area is present.
“The probe is pushed into the ground until it touches the top of the drainfield pipe. Once the pipe is located, the inspector inserts a flag or marker, takes a few steps back, and repeats the procedures. “The inspector may also want to check the
trench lines for biomat growth.”
What is a biomat?
A biomat is the black, jelly-like layer that forms along the bottom and sidewalls of the
drainfield trench. It is caused by the bacterial growth that develops beneath the distribution lines. Biomats tend to restrict the flow of effluent through the drainfield but are crucial because they filter out viruses and pathogens. Biomat formation cannot, and should not, be prevented, but septic tank filters, proper organic loading, and proper maintenance of the septic tank can slow the rate at which it forms.
City of Columbia Health Department. Columbia, Missouri. “Septic Tank and Drainfield Maintenance.” Download at: www.gocolumbiamo.com/Health/Wastewater_Systems/
Mains, Craig, Zane Satterfield. Interviews with author in August 2009.
National Environmental Services Center. Pipeline. “Drainfield Rehabilitation.” Winter 2005. Volume 16, Number 1.
North Carolina State University. “Soil Facts: Septic Systems and Their Maintenance.” Download at: www.soil.ncsu.edu/publications/Soilfacts/AG-439-13/#What_Maintenance_Is_Needed
Ibid. “Soil Facts: Why Do Septic Systems Fail?” Download at: www.soil.ncsu.edu/publications/Soilfacts/AG-439-44/ag439_44.pdf
Public Health and Social Services, Thurston County,
Washington. “Landscape your Drainfield.” Download at:
U.S. Environmental Protection Agency, “A Homeowner’s
Guide to Septic Systems.” Download at:
Virginia Cooperative Extension. “Planting on Your Septic
Drain Field.” Download at: pubs.ext.vt.edu/426/426-617/426-617.html