06-09-1980 Planning Packet - Laserfiche WebLink

Home Browse Search

May 19, 1980 #556 Richard Hawkinson 4625 West Branch Road Page 2 If you were to bore a hole into the soil during a period of saturation it likely would fill with water and you could measure the depth to this "perched water table". More often than not, however, soil borings are done during dry periods and no observable "water table" is encountered. Over the years this has led to many soil reports indicating "no water table was observed", and drainfields often were installed in seasonally wet soils where they should not have been installed. To get around this problem, soil scientists use "mottling" as an indication ci’ vhe saturation zones in the soil. When a soil layer becomes saturated with water, the iron in the soil is "reduced", i.e. disassociates with oxygen. The iron particles go into solution. When soil drains and becomes desaturated, the iron particles tend to concentrate in the larger soil pores and become "oxidized", i.e. combine with oxygen, and taxe on the characteristic red or rust color. The areas the iron has drained (or "leached") from are left a gray or yellow color. This leaves a mottled gray/red/yellow color which readily identifies layers of soil where periodic saturation and desaturation occurs. The shallowest depth where mottling occurs in the soil is generally considered the top of the seasonal "water table" for designing septic systems. As a final thought on the theory of water tables, consider that there are two distinctly separate types: 1) a permanent water table below which all porous materials are saturated; and 2) zones in the soil layers above the permanent water table where less permeable layers periodically slow up the downward movement of water causing a temporary saturated condition. Usually an 8-12" layer of loamy topsoil lies above the clay layers. This topsoil often has a percolation rate which would allow septic tank effluent to be absorbed and treated satisfactorily by a mound system even when the clay layers below would be too impermeable or seasonally saturated for a trench system to function correctly. On the Hawkinson lot, soil mottling indicated the seasonally saturated zone starts at a depth of 14-16". Normally we would insist on a mound system for these conditions. While trench systems have been installed in soils such as these before, no studies have been done to document their success or failure. ir iillMf II MMr 11 I -1^