What does mottling in soil generally indicate?

Mottling in soil generally indicates seasonal saturation. It occurs when there are alternating periods of saturation and drying, resulting in the presence of different colored spots or blotches within the soil profile. These color variations typically reflect changes in the soil's oxidation state due to fluctuating moisture levels. When soil becomes saturated, it undergoes anaerobic conditions, leading to reduced iron and gray hues, while when it dries out, the iron can oxidize, causing orange or reddish mottles to form.

Understanding mottling is crucial in the context of On-Site Sewage Facilities because it helps in assessing the drainage capabilities of the soil. If mottles are present at shallow depths, it often indicates that the soil can retain water for significant periods, which could hinder the performance of an on-site sewage system by increasing the risk of system failure due to saturation. Recognizing this characteristic helps professionals determine the suitability of a site for the installation of sewage systems.