A percolation, "perc" or "perk" test is used to determine whether the soil absorption rate is sufficient for the anticipated capacity of a septic system or leach field. The perk test also indicates the best location and layout for the drain field, or whether an engineered septic system is required.
The perk test is relatively straight forward - a series of holes is dug and saturated (usually overnight), a specified amount of water is added to the holes and the rate at which the water is absorbed is measured. The speed at which the water is absorbed indicates how well the soil "percolates."
The number, location and depth of holes varies depending on location and on local health codes, as well as visual inspection of the soil. Perk tests are performed by a soil evaluator, and may also be attended by someone from the county health or sanitation department, to ensure the accuracy of the results.
The final result of the perk test depends on several other factors:
Evaluation of the soil type
Loamy soil or loamy clay is considered a better filter for nutrients and bacteria, but may require a larger field for proper absorption. On the other hand, coarse sandy soils absorb more water faster, but may do a poor job of filtering.
A deeper hole, used to find the water table, is drilled, and if ground water is reached within a certain depth, the design of the field may also be affected.
The size of the structure and number of bedrooms is also required, as well as a list of water-using appliances (dishwasher, water softener, garbage disposal, humidifier, washing machine).
While the soil evaluator may be able to give a preliminary assessment upon the completion of the test, all of the information collected must be analyzed and considered before final results are given.
There are three reasons why a perk test is important:
Requirement for building approval
First, a perk test is required before receiving approval to build a new structure, and in some cases, it may be required before purchasing an existing building. It may also be prudent to obtain the results of a perk test before purchasing property. Building without proper approvals may result in fines, or even the demolition of the structure.
Identifying the best septic solution
Second, if the soil passes the perk test, recommendations as a result of the test indicated the best size layout for a traditional septic system. If the perk test fails, there is a variety of options available for septic fields even when the soil fails the perk test. Before building or purchasing property, it's important to understand which is the best sanitation option for the situation, and a perk test is a critical part of making that decision.
Avoiding ongoing septic problems
Accepting the existing septic system when buying, or opting for the traditional septic tank and drain field system without testing, may initially be the least expensive option. However, a system that is too small for the outflow, is in soil which doesn't absorb water quickly enough, or if the water table is too high and there is insufficient filtration between the field and the water table, will create problems in the future which may be more expensive than the test in the long run.
While a perk test may seem an unnecessary expense, it is essential to determine the soil's absorption capabilities and type before committing to the expense of purchasing or building. Given the number of potentially expensive problems a perk test can help a prospective buyer or builder to avoid, this test is a sound investment.