Retesting A Site That Failed to Perc?

Q: I am looking to purchase two properties — both of which failed to perc back in 2005.  I am not 100% sure if this was a perc test or a soil site evaluation, but after speaking with the county, they stated I could have it tested again to see if I could get different results.

This new test would for sure be a soil site evaluation (hope that is the correct lingo).  My questions is, is this even worth it?  Logically, in my mind what would even change for the results to be any different?  The process seems fairly expensive to hire a backhoe to dig the holes plus the cost of the test.  Any insight is much appreciated!  — Neil

A: It’s not unheard of for a site to fail on one occasion and pass on another. In most cases, this would be a site that is marginal — that is, it just flunked by a little bit, but might pass under different conditions. This might be a different time of year, a drier year, or a different location on the lot. It also could be that a different inspector interpreted their observations a little differently.

Also, test procedures vary a lot from one jurisdiction to another. Most require both a deep-hole” test where the layers of soil are observed in a test pit and a perc test where water is dumped in a hole under specified conditions and the absorption rate measured.  Areas that rely strictly on visual inspection of a test hole could get different results from different testers.

Large Lots

If the lots are big enough to have multiple locations for the leach field, this would be your most promising strategy. A site can certainly have dense soil or ledge in one area and more granular, well-drained soil in another. The water table may also be higher in one area than another.

Deep-Hole Test

In the deep-hole test, inspectors are looking for visual evidence of the seasonal high water table, and the upper limit of rock ledge or other impermeable soil. If this “limiting zone” of unsuitable soil is too close to the surface, then the site will fail.

In jurisdictions that do not require perc testing, the inspectors are also evaluating the soil for drainage characteristics, either visually or with the help of a lab tests.

In all these tests, a certain amount of human judgement is involved. For example, where the high water table is determined based on the visual inspection of “mottling” in the soil,  you could get a different result from a different tester. The same is true of other visual observations of the soil.

Marginal Sites

 In addition to pass/fail results, perc and soil tests determine the size of the leach field. So, in principle, poorer soils with larger leach fields should perform as well as systems with better soils.

However, a site that just squeaks by could have problems with septic system performance or longevity. For example, a site tested in a dry year or dry season could have problems with a high water table under wetter conditions. For that reason, many areas limit the tests to drier times of the year.

Or a site with bedrock, impermeable soil, or high water just barely at the acceptable depth, could have compromised performance under heavy usage (a big party or five loads of laundry) or very wet weather.

Cutting it too close can lead to poor performance or a shortened lifespan for the leach field. Better to design a system with extra capacity than too little.

Alternative Systems

If the problem is a high water table, sometimes the water table can be lowered around the leach field by using curtain drains. Input from a civil or geotechnical engineer would be advisable. Also, local authorities would have to approve the system.

There is also the option of an alternative septic system for a marginal site. Just be aware that this can increase the installation cost by 50% to 100% or more and increase annual maintenance costs as well. Most have moving parts that are subject to wear and tear and need careful monitoring. You will need to check with local authorities to see what types of systems are allowed in your area. And don’t be the first to try an innovative system. Only use something with a well-established track record in your area.

Get Local Advice

Finally, I would pick the brains of county folks who suggested trying another test. What is the basis for their comments? Have they seen failed sites pass on a second round of tests? What do they attribute this to? Have the regulations changed in this area? And, if so, are the new regulations more or less restrictive?

A lot to consider. Wish there was an easy answer, but at the end of the day, it’s a roll of the dice.

See also:  Marginal Perc Test Results       Perc Testing in Second Location     Site Perks After Failing 10 Years Ago