Soil color is often used to determine seasonal high water table (SHWT) for system design. The soil treatment area bottom (or infiltrative surface) must be kept a certain distance above the SHWT according to code.
The most common way to determine the SHWT is to look for the first occurrence of gray colored (low chroma) redox depletions or mottles. Some codes may use the first occurrence (or a specified percentage) of red (high chroma) redox concentrations or mottles as well as the gray colors. It is important to know what methods are specified in the code in the area one works. Furthermore, if the entire soil is gray it is likely that soil is saturated for long periods of time throughout the year.