Bulletin #2442, Vegetable Gardens and Septic Fields Don’t Mix

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According to U.S. Census Bureau data, Maine is the most rural U.S. state, with roughly 61 percent of our population residing in rural areas.1 Because many rural-area homes are not connected to municipal sewer systems, many Mainers rely instead on septic systems to discharge home wastewater.

Planning Around Your Septic System

septic fieldA septic system consists of an underground tank and a soil absorption field, also known as a septic system disposal field, and sometimes called a “leach field.” The disposal field cleans the wastewater through filtration and the action of microorganisms in the soil, preventing polluted water from entering lakes, streams, and groundwater.

The existence of a septic system on your property means that you may have to plan your land use around it. Driveways, play areas, sheds, patios, and gardens must all be designed with your septic system in mind. This particularly applies to vegetable gardens. In fact, the Maine Department of Agriculture explicitly advises homeowners to avoid locating vegetable gardens on or near septic system disposal fields.

What if You Don’t Know Where Your Septic System Is?

Locating your system is not always an easy task. Even if you find your septic tank, the disposal field can be many feet away.

To locate your system, refer to your septic system design form (known as the HHE-200 form). If you can’t find a copy, contact your local plumbing inspector to see whether he or she has a copy on record. You can also contact the Maine Subsurface Wastewater Program through their Maine Subsurface Wastewater Team page on the Division of Environmental and Community Health (Maine.gov) website,1 or by calling 207.287.5689. This program receives and holds all septic system records from towns since 1974 (the newer the system, the more likely it is that they will be able to find the record). If no record can be found, you can contact a Site Evaluator to locate your system.

How Septic System Wastewater Can Contaminate Your Garden

Untreated wastewater, also known as sewage, can be a health hazard. Septic system disposal fields are designed to treat, or renovate, this wastewater. The most important reason you should not install a vegetable garden on top of, or right next to, a septic system disposal field is because the plants can become contaminated by wastewater that has not yet been renovated by the field. Plants on disposal fields can absorb wastewater pathogens. Eating vegetables that have absorbed wastewater pathogens can make you very sick.

What You Should Know About Disposal — Field Design

Most septic system disposal fields designed since 1974 are installed either partly or completely above the original ground surface. This is because most of our soils in Maine have hardpan, bedrock, and/or a shallow seasonal groundwater table. The bottom of the disposal field must be elevated above any of these limiting factors in order for the wastewater to drain into the soil and be renovated.

How a disposal field is constructed

The main body of the disposal field — the lowest layer — consists of components such as plastic or concrete chambers, fabric-wrapped pipe, geo-textile sand filters, or stone. The purpose of these components is to provide storage capacity for the wastewater, which is usually generated in spurts, faster than the soil can absorb it: people usually generate the most wastewater in the morning before work and school, and in the evening after coming home.

Just above the stone or other disposal-field components is a layer of compressed hay or filter fabric, which prevents fine soil particles from entering spaces among the stones or in other devices. If the spaces in the stone or other components become filled with soil, they will not be able to store wastewater, and the septic system will fail.

Above the compressed hay or filter fabric is a layer of fill material, usually eight to twelve inches deep. The lower part of this fill is supposed to be a gravelly, coarse sand material. This is to allow for the free exchange of air into the disposal field so that microbes can quickly attack and renovate the wastewater. Generally, only the top four or so inches of this fill material has silt or clay and organic matter in it.

Why septic system disposal fields are unsuitable garden sites

  • In a brand-new septic system disposal field, the wastewater level in the disposal field is usually quite low. Over time, however, as the disposal field matures, ponding of wastewater can be expected. This results from the partial clogging of the soil pores by particles escaping from the septic tank, and the living and dead bodies of microorganisms. The thicker this clogging layer gets, the higher the wastewater level in the disposal field becomes. The wastewater level will also rise over time as family members increase and grow up, and as a result of heavy-use events. Eventually, the wastewater in a disposal field will very likely reach high enough levels for even shallowly rooted plants to come in contact with it.
  • Water (including wastewater) will wick up into soil due to capillary attraction. If wastewater rises high enough in the disposal field to come in contact with the fill material on top of it, capillary attraction could cause the wastewater to wick up to as high as 18 inches, depending on the texture of the fill. This is why a vegetable garden should not be placed on a disposal field fill extension, especially near the disposal field. Even though there may be no wicking up to the top of the disposal field or fill extension material at first, it may occur as the disposal field matures.
  • Generally, the soil over the top of a septic system disposal field is very permeable, particularly soon after the disposal field is installed. Therefore a garden planted on top of a septic system disposal field would require watering in order for the plants to prosper. The problem is that adding water to the top of a disposal field, particularly if the disposal field is only marginally functional, could cause it to fail.
  • Rototilling the top of a disposal field can damage the compressed hay or filter fabric. If the compressed hay or filter fabric is damaged, it could allow soil particles to migrate down into the stone or other devices in the disposal field, reducing the wastewater holding capacity.
  • Placing additional fill over the top of a disposal field, in order to create a safe zone for vegetable plants to grow, is not a good idea. The additional fill material might suffocate the disposal field by inhibiting the free exchange of air. An anaerobic (oxygen-free) disposal field is much more likely to clog up and fail than a well-oxygenated one. In addition, placing additional fill material on the disposal system could result in damage to disposal field components.
  • Any plants installed on the top of the disposal field will send roots down in search of water and nutrients, neither of which will be found in the gravelly sand fill material. Ultimately, septic system disposal fields make unsuitable garden sites because roots that come in contact with wastewater can take up pathogens, such as viruses, which can then infect anyone who eats the plants.

Better Choices for Covering Disposal Fields

The most suitable vegetation to grow on top of septic system disposal fields and fill extensions is grass. Flowers may also work, but only if you avoid rototilling the soil and watering the plants. Woody-rooted plants should not be planted on disposal fields or fill extensions, because the roots might clog up pipes and other devices in the disposal field. If you do not want vegetation to grow over your disposal field, covering the bare soil with bark mulch is an acceptable solution.

Adapted with permission from:

David Rocque, Maine Department of Agriculture Policy on Establishment of Vegetable Gardens on Septic System Disposal Fields (Augusta: Maine Department of Agriculture, Conservation and Forestry, 2012).

John M. Jemison, Jr., Your Septic System, Bulletin #7080 (Orono: University of Maine Cooperative Extension, 2002, 2010).

1“Census: Maine most rural state in 2010 as urban centers grow nationwide,” Bangor Daily News, Matt Wickenheiser, March 26, 2012

Information in this publication is provided purely for educational purposes. No responsibility is assumed for any problems associated with the use of products or services mentioned. No endorsement of products or companies is intended, nor is criticism of unnamed products or companies implied.

© 2012

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