What is Involved in a Site Evaluation for a Sewage System?
​Proper design and operation of an on-lot sewage system hinges on the ability of the soil to absorb and renovate the wastewater flowing from the treatment tank.
Therefore, before any on-lot sewage system can be selected and designed, the soil properties at the site must be evaluated. For the soil to properly renovate the treatment tank's effluent, two soil-related events must occur:

• the wastewater must be absorbed into the soil profile, and
• the wastewater must percolate through the suitable soil, thus coming into contact with an aerobic (oxygenated) soil environment.

To maximize the likelihood that an on-lot sewage system will perform properly for many years, several soil evaluations are required before the type of system can be chosen and the system can be designed and built.

The first soil evaluation is known as the deep probe or probe. The probe consists of digging a minimum of two deep pits ("test pits") near the intended absorption area. The purpose of the probe is so the Sewage Enforcement Officer (SEO) or a Certified Professional Soil Scientist (CPSS) can evaluate the soil profile and locate the limiting zone.
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The second soil evaluation is designed to determine how rapidly the septic-tank effluent will percolate into and through the soil (the Water Movement Rate). Depending on the depth to the Limiting Zone and the type of disposal system chosen, this evaluation may require either (1) a Percolation Test or "Perc Test" or (2) a soil scientist's soil morphological evaluation. Note that the probe and water movement rate evaluations should be completed successfully before a prospective buyer agrees to purchase a lot. Failure of either of these evaluations may make it very expensive to properly dispose of the wastewater generated on the property.