Test Holes & Perc Testing in Kootenai County

The cheapest step, and the one everything else depends on

Before a septic system can be designed, priced or permitted, somebody has to find out what your soil actually does with water. That is what a test hole is for. An excavator opens a profile through the ground at the proposed drain field, and Panhandle Health District's Environmental Health Specialist reads it: how deep the usable soil runs, where you hit rock, clay or gravel, and how far down the groundwater sits.

What comes out of that half-day decides your drain field size, your tank size, your system type, and occasionally whether the parcel supports a septic system at all. It is the least expensive part of the whole project and the part that keeps the expensive parts from going wrong.

"Perc test" and what people usually mean by it

Ask around and you will hear this work called a perc test, a percolation test, a soil log, a test pit or test holes. In practice, in North Idaho, what Panhandle Health District needs is a soil profile evaluation read from an open excavated hole, not a timed water percolation test.

The distinction matters if you are reading advice written for another state. Idaho's rules size a drain field from an application rate in gallons per square foot per day tied to a soil group — roughly 1.0 for the most suitable soils down to 0.2 for the least — rather than from minutes-per-inch percolation figures. So if a lender, a realtor or an out-of-state engineer asks you for perc numbers, what they are going to receive is a soil log. That is the correct document here.

What the specialist is looking for

• Soil texture and structure through the profile — which design soil group you fall into, and therefore how many square feet of drain field your house needs.
• Depth to a limiting layer — impermeable layer, fractured bedrock, extremely permeable material, or groundwater. State rule sets a minimum effective soil depth above each of those, and the required depth changes with the soil group.
• Groundwater, normal and seasonal. A hole dug in August tells a different story than the same hole in April, which is why seasonal high groundwater is assessed separately.
• Site conditions that disqualify an area — draws, springs, wet ground, old fill, and slope over 20 percent.

What we typically find, by area

Every lot is its own answer, but the patterns around here are real:

Rathdrum Prairie — Hayden, Post Falls, Rathdrum, Dalton Gardens. A thin cap of ashy silt loam over extremely gravelly sand, cobbles and boulders from the Missoula Floods. Digs fast, drains ferociously. The catch is that Idaho classifies clean gravel and coarse sand as unsuitable for a drain field — water moves through too quickly to be treated — so prairie lots often need imported soil or an advanced system despite looking like perfect ground.

Athol, Spirit Lake and the north end. Same flood gravels, but the water table in the northern Prairie can sit more than 500 feet down, so groundwater is rarely the constraint. Granite and metamorphic bedrock comes up near the surface where the valley narrows against the Selkirks, and that can limit your depth.

Sagle, Cocolalla, Sandpoint. A genuinely different setting — the Purcell Trench, not the Prairie. Laminated lake clays and silts, compact lodgement till that is slow to dig and holds water, and peat and muck in old kettles and outwash channels. Here the problem is usually the opposite of the Prairie: water that will not move, and perched water tables. Mound and pressure systems are common.

How we run it

We call the Environmental Health Specialist and get the review scheduled first, then bring the excavator out and open the holes to spec — eight feet, three wide, one side sloped so the profile can be read safely from the surface. We stay on site through the review, because if the first location does not read well the specialist will often point at better ground and we can open another hole while we are still there rather than remobilising a week later.

Then we backfill and grade back out the same day. An open eight-foot hole is not something to leave on a property overnight.

Before any of it: call 811

Idaho law requires notice to Digline of at least two business days, and no more than ten business days, before excavation begins. The locate is free to you. The marks stay valid for four weeks, and within 24 inches of a marked facility the digging has to be done by hand. We place the locate on our own jobs. If a contractor offers to skip it, that tells you what you need to know about how they work.