Soil Testing for your Septic System

Tom Guyse on the job. For some reason there is a common misconception that all replacements of failing systems must be mound systems or that from a certain time forward, all new systems have to be mound systems. This is not the case. Soil conditions can differ and vary dramatically from lot to lot. Some require mounds, some conventional (subsurface) systems, as well as several other types of systems that fall somewhere between these two. What determines the type of system required on each lot or parcel of land are the results of a “Soil and Site Evaluation” by a Certified Soil Tester/Morphological Evaluator. This evaluation is commonly referred to as just a “soil test”, and just as commonly (although mistakenly) called a “perc test”.

Actual “perc tests” which used to be the common method for determining the design of “POWTS” (Private Onsite Wastewater Treatment Systems), was the method where water was literally poured into holes in the ground for the purpose of measuring just how far into the ground a certain amount of water would go in a certain amount of time, thus determining the soils percolation rate. The perc method is no longer the accepted method in Wisconsin of “soil testing”, as the information gathered is far too limited for making accurate determinations of important soil characteristics. Sure, one might know how fast the water goes into the ground, but the perc method does not indicate just how far this water would travel before it encounters a “limiting factor”.

A “limiting factor” would be high bedrock, or high groundwater. The “perc rate” may indicate how fast the water is moving, but if the water immediately (within inches or just a few feet) encounters high bedrock or high groundwater, the wastewater would not be treated properly: very likely contaminate the groundwater, move much more quickly to the groundwater (untreated) through the cracks and crevices in the bedrock, and cause a system to fail prematurely due to less than adequate depths to a limiting factor.

At this time, Wisconsin administrative codes mandate that any type of system must be designed so that the “bottom” of a soil absorption system is a minimum of 3 feet above high groundwater or bedrock. Groundwater in this instance does not necessarily refer to the actual water table your well is drilled to, but by code definition could mean any zones of soil saturation which include: perched water tables, shallow regional groundwater tables or aquifiers, or zones that are seasonally, periodically, or permanently saturated.

This answers the frequently asked question, “Why do I need a mound if my soil is all sand?---shouldn't it be a conventional?” The fact is that even if someone's soil is all sand, but the soil test shows indications of high groundwater at only 2 feet below the ground from existing grade, there is only one way to design a system which will be 3 feet above the groundwater. This is done by artificially creating a soil absorption system 1 foot above the ground: a mound system. Such a system is designed with the bottom of its' soil absorption area a minimum of 1 foot above the existing grade, to allow for the 3 foot vertical separation to the limiting factor on sites that have a limiting factor 2 feet below grade. Wisconsin administrative codes allow for a minimum of 2 feet to a limiting factor below exiting grade for approval of a mound system for new construction purposes, so if the limiting factor is observed less than 24 inches below existing grade, the site would “not pass” for a mound for “new construction.”. In some instances the limiting factor could be less than 24 inches for mound approval for replacement of a failed system for an existing building only. In such an instance, the mound design is changed accordingly.

Most soil testers are performed using a backhoe to dig “pits” large enough for the soil tester, and in most cases an inspector, to closely observe (and hand texture) each horizon (or layer) within the soil profile. This is done to both observe for the limiting factors discussed above, and to determine several other characteristics of the soil, all of which determine the type, depth, and square footage of a system for its' designed lot.

When a soil tester is observing the soil profile, trying to determine if the limiting factor of groundwater is present, the tester does not have to actually see the water. If the tester observes “mottling” in the soil, it is at the depth the mottling is observed that is considered the limiting factor of high groundwater (with few exceptions). Mottling is a discoloration or staining of a color which is not part of the dominant soil color. This is an indication that the groundwater had, at some point in time, fluctuated up to this particular elevation, and remained long enough to cause a chemical reaction (oxidation), which in turn, creates mottling. The soil tester must assume that because water had fluctuated that high at some point in the past, the potential exists for a recurrence. The groundwater elevation is now assumed to be the elevation at which the mottling was observed irrespective of whether the water is present at the time of the test.

The above information on mottling answers the frequently asked question---”Why do I need a mond when I didn't see any water?” Actual water does not have to be present, and the system must be designed according to the soil tester's observations.