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Septic System Test Holes in Canyon County, Idaho

How to Do a Perc Test to Evaluate the Septic System

Today I’m out on two lots where we are conducting the due diligence for the owners of two side by side lots that have entered escrow. One lot is an acre, and the other is an acre and a quarter. I’m evaluating the septic system, specifically conducting a perc test today with the health department, Southwest District, out here in Wilder. We’re looking to see if the soil is good in the areas where we want to put the septic system, and we have to dig to find out.

Digging Septic Test Holes in Canyon County

We’re digging two test holes, which must be within 50 feet of the future primary septic system area. I want to show what we did to locate where these systems belong in relation to other wells and septic systems. Other systems on other properties could adversely crossover with the preferred well locations. The entire subdivision has 100-foot setback requirements from any well to any septic system. Since these lots are small, the adjacent properties affect each other and drive where you can put your well and where you can put your septic system.

Plotting the Septic Location

I created a document to identify where we can dig both the well and septic system. To do so, I pulled the septic as-builts from the county health department and plotted those areas on the printout. The printout displays where the 100 foot setbacks of each septic system reach (blue), and the setbacks to where each well reaches (yellow.) You can see the printout explanation in the video at one minute in. The blue and yellow areas show how the placement of each well or septic system impacts the locations of the others.

Digging Septic Test Holes

The test hole on lot one currently reaches 10 feet. The soil appears to identify as B-1, which means it has a little bit of gravel in it, but it’s like a sandy loam. This is a decent soil type and should help with the size of the system. We won’t have to install quite as big of a system for this home.

Our septic contractor is digging with a little mini-excavator, and we must dig within 50 feet from the final location of this primary system. You can see the subdivision and how close the houses impact the location of where we can put our well and our septic system. Each of these homes have a setback off their systems. This neighbor’s system in particular is much closer, but their septic system on this side is close to ours, and when you’ve got two septic systems touching each other, it doesn’t really impact you too much.

Wrapping Up Test hole #1

The mini-ex is backfilling test hole number one, which revealed great soil for a septic system. It is a sandy loam soil. We dug down about 10-11 feet just to verify that we broke through the hardpan and confirmed that we had good soil. The inspector confirmed the soil type, marked the test hole location with GPS coordinates, and then notated it against the parcel number. He will email us over a system design recommendation based on the size of the drain field and soil type that we have here. Now we are going to head over and dig test hole number two on the second lot and repeat the same process.

Surprise Lot Circumstances in Canyon County

The seller recently informed us that lot two received a boundary adjustment. This gravel area was designated for a fire department turnaround, so we can’t put the septic system in right there. So we moved it over 50 feet. This primary system still fits all the necessary setbacks and works with the fire department as well.

Septic Impact on Steep Embankments

One side of the lot has an embankment. I spoke with the septic inspector about it and he wasn’t too worried about the grade if we needed to push the system right up next to the drop off. He explained that the embankment would need to angle off continuously at 45-degrees or more for anyone to be concerned about septic effluent leaching out of the ground. So, he was okay with this area as a backup option.

Lower areas on a steep angle, such as in the adjacent canyon, can lead to a seepage problem. This would require a setback ranging from 25, 50, or 75 feet, depending on the depth and steepness of that hill or ravine.

Soil Types at Test Hole #2

The excavator will now dig through the hard pan and again reach a minimum depth of 10 feet. You can see how the inspector checks the soil as we dig deeper and deeper.  He notates what type of soil exists at different depths on his report, which helps him to design the system that’s needed. You can see the inspector shaking water through the soil in his hand. He uses this water to see if the sand consistency will help him determine what type of soil is coming out of the ground. You can see the layering of the earth as he penetrates through the hard pan and digs deeper. That lighter colored layer of dirt is the hardpan that I keep referring to.

Conversation with the Inspector

The video documents the dialogue between myself, the excavator and the inspector. We basically discussed the different depths and determined how much further digging we wanted to spend in order to discover what type of soil exists here. We found some pretty fine sand at the bottom of the second test hole There’s roughly 6 feet of sand, which is a significant amount of sand. After enough discussion, the inspector cleared us to backfill.

The inspector determined that lot one has a B2 soil and lot two has a B1 soil. Those soil types will determine the size of the system and the depth that we have to install the septic system. He should have that info to us in about a week from now. Once we get his email, we’ll be able to move forward and draw up a plot plan. After that, we can pull a permit to get the septic systems put in when we build our houses here.

Septic System Test Holes in Canyon County, Idaho

This is the process of digging a test hole and why it’s important to complete this process, especially if you’re in a small subdivision that has smaller lots. The other properties can impact your wells and septics on your lot.

If you need help buying land, or if you’re looking to build a home, take a peak around the website.

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