Permeability of soil was determined by a percolation test, or "perk" test. Such a test involved excavating a round hole in the soil at the site, filling the hole with water in a standard fashion, and measuring the subsequent rate of fall of the water level in the hole.
The site investigation includes examination of the soils for a variety of characteristics that will help to predict septic system performance during all seasons of the year. The two most important soil attributes that must be described and understood in such a site investigation are: (1) soil permeability; and (2) wet-season water table.
In assessing soil permeability, more than one auger boring must be made and the soil described down at least to the 6-foot depth. Features studied and described include: 1) soil texture (proportions of sand, silt, and clay); 2) potentially restrictive layers, such as claypans or chemically cemented layers; 3) layers of organic matter (muck) or gravel; 4) zones of contrasting soil materials; and 5) variation in these sorts of soil features across the site.
If prohibited materials, such as hardpans, clay, muck, or gravel, are found in the 3-1/2 foot zone that will immediately underlie a proposed drainfield, then the unsuitable material the system bottom must be 3' above that layer. Alternatively, depending on the water table and other conditions, the drainfield may be elevated above the unsuitable material by building a mound of permeable soil brought to the site from elsewhere. Or, where site conditions are extremely limiting, the permit may be denied altogether.
Assuming that the soil is found to be suitable in the 3' zone under the proposed drainfield bottom, the suitability of these soil materials still must be known in order to determine the size of the drainfield. This is necessary to ensure that the drainfield will receive wastewater from the septic tank at a rate not exceeding the capacity of the soil to accept it. The designer must: 1) estimate the daily flow of wastewater; 2) determine the size of septic tank needed; 3) estimate the permeability of the soil in the 3' zone under the drainfield; and 4) compute the required drainfield size from the estimated flow rate and the estimated soil permeability at the site.
Generally, the more clayey soils will be found to have lower permeabilities and therefore will require larger drainfields
The other important soil feature that must be understood before a permit can be issued is the depth to which the water table rises in wet seasons. Once this is determined, the trench/bed bottom can be set at an elevation at the proper elevation above that wet-season water table level. The water table is the more-or-less horizontal boundary between unsaturated soil above and wet soil below. The depth to the water table varies greatly depending on location and on the nature of the soil and landscape at any given site.
Another key factor controlling depth of the water table is the weather. After an extended rainy period, the water table is likely to be much higher in the soil than it would be in the same soil following a long drought. The behavior of the water table is further complicated by the presence or absence of water-using vegetation on a site. Ground that seems rather dry when investigated while natural vegetation is intact may become wetter after the trees are cleared for development.
Site investigations need to proceed with the understanding
that the depth to the water table is a dynamic feature of the
soil. State septic system code requires that the depth to which
the water table is likely to rise during wet seasons of most
years be determined when site investigations are performed.
The seasonal high water is determined by looking for morphic redox features
(mottling), that is also considered a restrictive layer requiring the system
bottom to be 3' above that. If that layer is within 41" of the surface then your
system will have to be a At-grade or a mound.