Drainfield
A drainfield is the part of a septic system installed within the soil to receive treated sewage and disperse it into the ground. Virginia law calls it a subsurface drainfield, and the regulations keep structures off it and hold wells and springs set distances away.
Installed within the soil, and nothing built over it
Virginia's Code defines a subsurface drainfield in a single line.
a system installed within the soil and designed to accommodate treated sewage from a treatment works
Va. Code 32.1-163, definitions
Everything upstream of it prepares the sewage. The drainfield is where the soil takes over. In the statute's definition of a conventional septic system, one or more septic tanks send effluent by gravity, pumping or siphon to a gravity distributed subsurface drainfield. How the whole chain works, from the house to the last trench, is laid out in septic systems explained.
Depth is written into the rules. The regulation describes an in-ground system, conventional septic tank drainfields included, as one whose infiltrative surface sits 18 inches or more below the original ground. With septic tank effluent, the 18 inches of soil directly below the installation depth must show no limiting factor, such as bedrock, a seasonal or permanent water table, or pans.
The regulation is plain about what goes on top: "No structures shall be placed over the subsurface soil absorption system." A drainfield can also come with a second piece of ground. Where the estimated or measured percolation rate exceeds 45 minutes per inch, the state rule requires a separate reserve area equal to 50 percent of the required absorption area. That ground is defined under septic reserve area, and whether a particular lot needs one is worked through in what a septic reserve area is and when a lot needs one.
The well line first, then the spring line
Once a drainfield has a place on the plan, other things on the site keep their distance from it, and for a well its reserve counts as though it were already built.
Wells come first. Under the private well regulations, a private drinking water well has to be at least 50 feet from an active or permitted drainfield if it is a Class IIIA or IIIB well, and at least 100 feet if it is a Class IIIC well. The distance table in 12VAC5-630-380 lists the reserve drainfield alongside the active one, so the spare ground counts when the well site is drawn.
| Measured to | Minimum distance | Rule |
|---|---|---|
| Class IIIA or IIIB private well | 50 feet from an active or permitted drainfield, reserve drainfield included | 12VAC5-630-380 |
| Class IIIC private well | 100 feet from an active or permitted drainfield, reserve drainfield included | 12VAC5-630-380 |
| Developed spring, when the spring is down slope | 200 feet from the bottom or sidewall of an absorption trench | 12VAC5-610-597, Table 4.2 |
Springs come second. The table's spring row is measured from the absorption trench; the septic regulation also gives a developed spring a shorter distance in a different table, for other parts of the system, and which figure belongs to the drainfield is set out under spring.
Put those distances together with the rule against structures and the drainfield becomes a fixed line on a site plan. The house, the well and any outbuilding have to be arranged with the field and its reserve in view.
Not sure where the drainfield sits on the property you are selling?
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Questions about this term
How far below the ground does a conventional drainfield sit?
The regulation counts conventional septic tank drainfield systems among in-ground systems, whose infiltrative surface is placed 18 inches or more beneath the original surface of the ground. With septic tank effluent, the 18 inches of soil immediately below the installation depth must show no limiting factor, such as bedrock, a seasonal or permanent water table, or pans. The depth for a particular system is in its permit and design, which the local health department reviews.
Where this page's facts come from
Every rule, office and figure above traces to one of these. Rules change; check the office before you act on one.
- Va. Code 32.1-163: definitions, including subsurface drainfield and conventional onsite sewage system Code of Virginia
- 12VAC5-610-594: in-ground systems and limiting factors Virginia Administrative Code
- 12VAC5-610-700: no structures over the absorption system Virginia Administrative Code
- 12VAC5-610-710: reserve absorption area Virginia Administrative Code
- 12VAC5-630-380: private well siting distances Virginia Administrative Code
- 12VAC5-610-597: Tables 4.1 and 4.2, minimum separation distances Virginia Administrative Code