Many health departments require a percolation test ("perc" test) to establish the suitability of drain field soil to receive septic tank effluent.  An engineer, soil scientist, or licensed designer may be required to work with the local governing agency to design a system that conforms to these criteria.
A more progressive way  to determine leach field sizing is by direct observation of the soil profile. In this observation, the engineer evaluates many features of the soil such as texture, structure, consistency, pores/roots, etc.
The goal of percolation testing is to ensure the soil is permeable enough for septic tank effluent to percolate away from the drain field, but fine grained enough to filter out pathogenic bacteria and viruses before they travel far enough to reach a water well or surface water supply. Coarse soils &ndash; sand and gravel &ndash; can transmit wastewater away from the drain field before pathogens are destroyed. Silt and clay effectively filter out pathogens but allow very limited wastewater flow rates. Percolation tests measure the rate at which clean water disperses through a disposal trench into the soil. Several factors may reduce observed percolation rates when the drain field receives anoxic septic tank effluent:
*Microbial colonies catabolizing soluble organic compounds from the septic tank effluent will adhere to soil particles and reduce the interstitial area available for water flow between soil particles.  These colonies tend to form a low-permeability biofilm of gelatinous slime at the soil interface of the disposal trench.
*Insoluble particles small enough to be carried through the septic tank will accumulate at the soil interface of the disposal trench; non-biodegradable particles like synthetic fiber lint from laundry, mineral soil from washing, or bone and eggshell fragments from garbage disposals will remain to fill interstitial areas formerly available for water flow out of the trench.
*Cooking fats or petroleum products emulsified by detergents or dissolved by solvents can flow through prior to anaerobic liquefaction when septic tank volume is too small to offer adequate residence time, and may congeal as a hydrophobic layer on the soil interface of the disposal trench.
*Rising groundwater levels may reduce the available hydraulic head (or vertical distance) causing gravitational water flow away from the disposal trench.  Effluent initially flowing downward from the disposal trench might ultimately encounter groundwater or impermeable rock or clay requiring a directional shift to horizontal movement away from the drain field.  A certain vertical distance is required between the effluent level in the disposal trench and the water level applicable when the effluent leaves the drain field in order for gravitational force to overcome viscous frictional forces resisting flow through porous soil. Effluent levels in the vicinity of the drain field will rise toward the ground surface to preserve that vertical distance difference if groundwater levels surrounding the drain field approach the level of effluent in the disposal trench.
*Frozen ground may seasonally reduce the cross-sectional area available for flow or evaporation.