To maintain a healthy environment for plant growth, plants need aerobic soil. Therefore, the application of treated wastewater must be carefully considered to keep the soil from getting too wet, even in a wetter than average year. Ohio State University Extension Bulletin 860, Reuse of Reclaimed Wastewater Through Irrigation, recommends a 1-foot-deep soil layer between the soil surface and a limiting soil condition. A soil limiting condition includes ground or perched water tables, hard, unfractured bedrock, dense glacial till, compacted zones, dense clays, pans such as fragipans, sand and gravel layers, and fractured rock.
Soil dispersal uses irrigation systems to apply a shallow depth of wastewater with each application. For onsite systems, this should never exceed 1 inch per week for normal lawn and landscaping applications. Do not confuse irrigation systems designed for wastewater dispersal with swale or wetland systems that concentrate several inches of water in a small area. Saturated soils can only support a limited variety of wetland plants. Cattails, for example, are a sure sign of wet, saturated soil.
Table 3 lists by name the soil series that are in the range of soil depths and permeabilities to be suited for dispersal systems. The 123 soil series are present in 49% of Ohio's land area. Remember, these soils series are used to describe soils over a range of depths. As shown in Figures 9 and 10 two of the soils listed range from 8 inches to 20 inches of depth above a limiting condition. Some of these depth differences are natural and some are the result of erosion and human activities. It is necessary to check the soil depth to a limiting condition in a soil pit before designing, building, or approving a soil dispersal system.