How central Arkansas ground decides your septic system

The septic tank only does half the job. The rest happens in the soil: EPA describes treated water leaving the tank, spreading through the drain field and soaking into the ground, where the soil strips out harmful bacteria, viruses and some nutrients. That makes the soil under a lot just as important as the tank, and around Little Rock the soil changes a great deal from one side of town to the other.

Where the regions meet

According to the Encyclopedia of Arkansas, Little Rock and North Little Rock sit on the boundary between the Ouachita Mountains and the Gulf Coastal Plain.

• West and northwest, into the Ouachitas. The rock here is mostly sandstone and shale that began as sea-floor sediment. Ridge tops tend to have shallow soil, while the broad valleys between ridges hold deeper, more productive ground. Shallow rock is a classic septic problem, because the rules demand a set thickness of soil between the bottom of a trench and bedrock.
• South and southeast, on the Coastal Plain. The surface is unconsolidated sand, gravel, silt and clay laid down on an old ocean floor. Sandy pockets drain readily; clay pockets do not.
• East toward Lonoke. The Prairie Terrace runs south from Searcy past Lonoke. The encyclopedia describes the eastern Arkansas terraces where rice is grown as sitting on a clay subsoil or "pan", ground that holds water rather than passing it through.
• North toward Conway. Conway lies partly in the Arkansas River Valley, a region of rolling plains with scattered mountains.

These are broad strokes. The only answer that counts for a given lot comes from the soil pits a Designated Representative digs for the permit.

What the Arkansas rules measure

Depth to rock and to water

The ADH onsite rules set minimum separations below the bottom of the absorption trench. To bedrock: 18 to 24 inches in low or moderate conductivity soil, depending on signs of seasonal saturation, and 36 inches in high conductivity soil. To a true water table: 24 inches in low or moderate conductivity soil and 36 inches in high conductivity soil. Sandy ground needs more separation because water moves through it faster.

Clay content

Soils with 40 to 60 percent clay fall in the low hydraulic conductivity class, which still works, just with a larger field. Above 60 percent clay the rules give no loading rate at all, so a standard trench system has nothing to be sized from.

Seasonal water

Designers read the seasonal water table from redoximorphic features, the gray and rust-colored patterns left where soil has stayed saturated long enough for iron and manganese to be chemically reduced. A lot that looks dry in August can carry those marks a foot below the surface.

When the soil falls short

• Capping fill. Absorption trenches cannot be installed in fill, but a cap of loamy soil, no more than 27 percent clay, can be added over trenches dug in natural ground. It credits up to 7 extra inches of depth to the seasonal water table, on slopes of 12 percent or less.
• Interceptor drains. On slopes of 3 percent or more with soil under 30 percent clay, a curtain drain can lower a brief seasonal water table to a maximum depth of 18 inches.
• Alternate systems. Low-pressure distribution, sand filters and aerobic units have their own design rules, and they must go in through a licensed installer.
• Surface discharge. Systems that release disinfected water above ground are allowed only where the soil fails the standard for a conventional system, and they need a state discharge permit.

Protecting a field you already have

Whatever the soil, a drain field lasts longer when it is treated gently. EPA advises keeping vehicles off it, planting trees far enough away that roots stay out of the lines, and sending roof drains, sump pumps and other runoff somewhere else. A tank pumped on time keeps solids from sealing the soil; see septic tank pumping. If the field is already soggy, start with septic repair.

Questions people ask

My lot is mostly clay. Can it still have a septic system?

Often, yes. Clay between 40 and 60 percent is allowed with a larger field, and capping fill, interceptor drains and alternate systems exist for tougher sites. Above 60 percent clay a standard trench system cannot be sized, so the design moves to one of those alternatives.

Can fill dirt be brought in to make a lot work?

Not as a place for the trenches: the ADH rules forbid absorption systems in fill. A capping fill of loamy soil over trenches set in natural ground is allowed and earns a limited water-table credit.

Is a perc test enough to know if my land will pass?

Arkansas relies on soil pits. The department calls the perc test only a rough estimate, while pits show rock, tight layers and seasonal water directly.