Before a septic tank system can be designed for a particular dwelling on a particular building site, the ability of the soil to absorb the estimated amount of sewage that will be produced must be determined. On undeveloped land especially, far from a municipal sewer connection, ignorance of the underlying soil conditions may greatly increase the construction costs, as it may turn out that a much larger or more elaborate septic tank system will be required than was first estimated.
If the soil close to the house is impermeable or rocky, it may be necessary to lay a sewer line to a more suitable location where the absorption system can be installed, and this location may be a considerable distance from the house. It may turn out that the entire site is so stony, with frequent outcrops of rock, that excavating for a septic tank is either impossible or extremely expensive. Or the water table may be so high that the soil cannot be dug into more than a few feet without water being struck. Or the site may turn swampy during rainy weather, which will also make it impossible to install a septic tank system. The existence of any of these conditions can be discovered by boring test holes at the site of the proposed absorption system. In the absence of more definite information, such test borings should be made before the property is purchased.
However, even if none of these conditions exists, a percolation test must still be performed to ascertain the actual permeability of the soil for, as already noted, the size and design of the dispersal system will depend on the soil’s permeability.
In principle, a percolation test is simple. One digs or bores a hole into the soil that is from 4 to 12 in. in size and as deep as the drain tile will be laid. Water is poured into this hole until the soil is as a completely soaked as it is ever likely to become during the most prolonged stretch of rainy weather. Finally, the rate at which a measured quantity of water seeps from this hole into this saturated soil is measured, and this result is the percolation rate.
In practice, local regulatory authorities differ in their requirements as to the size of the test hole, the length of time the soil must be presoaked, the meaning of the readings obtained, and who can conduct the test in the first place. In some communities, the percolation test can be performed only by officials of the local department of health or of the buildings department, although other communities will accept test results obtained by a licensed engineer, or by a firm that specializes in the construction of septic tank systems.
In most communities, if it takes the water in a hole longer than 60 mm to fall 1 in., then the soil is deemed to be too impermeable for use in a subsoil absorption system, and the potential builder or homeowner will have to consider the installation of a more expensive sand filter system or decide against buying the property.