Perc tests around Fort Smith: how they work and what happens if you fail

Before a county health unit in Arkansas will approve a septic system on a lot with no sewer, someone has to prove the ground can take the water. A percolation test, or perc test, is the classic way to do that: dig holes where the drain field would go, soak them, and time how fast the water level falls. On the heavy clay found across much of Sebastian and Crawford County, the result often decides whether a lot gets a plain trench field, an engineered system, or a hard rethink.

Everything below comes from the Arkansas Department of Health's Rules Pertaining to Onsite Wastewater Systems (effective September 5, 2024). The rule numbers are there so you can check them.

Who is allowed to do it

You can't run your own perc test for a permit, and neither can a general contractor who isn't certified. The test has to be done by a Designated Representative (DR), a person ADH certifies to make perc tests, design systems and inspect them (rule 2.9). DRs are typically professional engineers, land surveyors, master plumbers or registered sanitarians. They pass a state exam, register every year and attend an approved training course each year to stay registered (rule 13.1). Ask to see the registration; the rule says a DR has to show proof of it when the health unit asks (rule 13.2).

Soil pits come first

A perc test isn't the first thing that happens. The DR starts by digging at least two soil pits: one in the proposed primary field and one in the backup area. They log how deep the seasonal water table sits, whether rock turns up, and what each soil layer looks like, then leave the pits open so the county's environmental health specialist can look too (rule 7.1). If the pits show the soil is unusable, there's no point timing water in it.

How the test is run

1. Holes. At least three holes spread evenly across the primary field and one in the secondary area, dug to the depth the trenches will be. A standard hole is 18 inches deep and 12 inches across (rules 8.5.2.1.1 and 8.5.2.1.2).
2. Prep. The sides and bottom are scratched with a knife or sharp tool so smeared, slick soil doesn't skew the reading, loose dirt is cleaned out, and 2 inches of coarse sand or fine gravel go in the bottom (rule 8.5.2.1.3).
3. The long soak. Each hole is kept filled with at least 12 inches of clear water, preferably overnight and for no less than 4 hours. The rule draws a line between saturation, which happens fast, and swelling, which is slow in clay and is the whole reason for the wait. The goal is soil that behaves the way it will in the wettest part of the year (rule 8.5.2.1.4).
4. Timing. The water is set to 6 inches above the gravel and the drop is measured from a fixed point over 30 minutes, refilling as needed. In fast soil where the first 6 inches vanish in under 30 minutes, readings are taken every 10 minutes for an hour and the last 10-minute drop counts (rules 8.5.2.1.5 and 8.5.2.1.6).

What the number means

The result is a percolation rate in minutes per inch (mpi). Appendix A of the rule turns it into how many gallons a day each square foot of field may receive, from 0.75 at 15 mpi down to 0.37 at 75 mpi. Anything slower than 75 mpi is unsuitable for a standard system (rule 8.5.2.2).

Here is what that does to a three-bedroom house, which ADH sizes at 370 gallons a day (Appendix B). At 15 mpi the field needs about 490 square feet of absorption area (370 ÷ 0.75). At 45 mpi it's about 740 (370 ÷ 0.50). At 75 mpi it's 1,000 square feet, roughly double the fast-soil field, before the backup area is counted. That's why slow clay costs more even when it passes. The field also can't come out smaller than the size the seasonal water table data calls for on the same site (rule 8.5.2.3).

Perc test cost

There's no state price for the test itself. The DR's fee for the soil pits, perc holes and design is a private charge between you and the DR, so get it in writing before they dig. It depends on how many holes the lot needs, access for a backhoe, and whether the design turns out to be an alternative system.

The one fixed state cost is the ADH permit review fee, paid when the application goes in: $30 for a house of 1,500 sq ft or less, rising to $150 above 4,000 sq ft, with garages and porches left out of the count (rules 4.9.1 and 4.9.2). The full table is on our septic installation page.

If the lot fails

A failed perc test doesn't always mean no house. The usual paths:

• Size by the water table instead. A Soil Qualified Designated Representative, a DR who has also passed the soils exam, can size a standard field from seasonal water table depths rather than perc data (rules 2.59 and 8.5).
• Prove the soil is drier than it looks. Mottled soil colors can suggest a water table that no longer exists. The owner can ask, in writing, to monitor observation wells in the primary and backup areas through the wet season (December 1 to May 15): 8 straight weeks, at least 16 readings, done by a soil classifier or DR, never by the owner (rule 7.4.3).
• Capping fill. Loamy fill spread over the natural ground can add storage and earn a credit of up to 7 inches on the water table depth, but the trenches still go in natural soil and the slope can't top 12% without a variance (rules 7.4.4.2 and 8.7).
• Secondary treatment. An ADH-authorized aerobic treatment unit or a recirculating sand filter cleans the effluent before it reaches the soil, and comes with a lifetime monitoring contract (Section 11 and Section 12).
• Surface discharge. Treated, disinfected effluent released above ground is allowed only on lots of 3 acres or more, with the outlet at least 300 feet from neighbors' homes, and under an NPDES discharge permit (rules 9.4 to 9.7).

One option that's off the table: a holding tank that just gets pumped. ADH doesn't approve holding tanks for homes, full- or part-time (rule 9.9.3).

Buying land? Test before you close

On rural acreage outside Fort Smith, Van Buren or Greenwood, make the purchase contingent on a passing soil evaluation, or at least have the DR dig the pits before the option runs out. A lot that needs an aerobic system or a long pumped line to the only usable soil can change the budget a lot. If the land already has an old system, our drain field problems page covers what failure looks like.