Soil and Site Evaluation for Onsite Wastewater Treatment

Bulletin 905

Threats to natural soil resources

Soil is a fragile and endangered resource. Each year in Ohio, many thousands of acres of soil are physically removed, eroded, covered with pavement and structures, compacted or contaminated. Ohio is fortunate to have large areas of valuable soil resources that support highly efficient and productive agriculture and forestry. Ohio’s soils are put to many, sometimes conflicting, uses. Deep, permeable soils, on low gradient slopes, are ideal for agriculture and also for the recycling of waste products including manure, wastewater, and municipal and industrial wastes. These same soils are also ideal for urban development. In Ohio, deep, permeable soil profiles are only present over about 6.4% of the land area. Many of these areas are found in or around the largest cities in Ohio or in rapidly developing peri-urban or ex-urban areas. Many of these soils are no longer available for agricultural use, and may not be available for future wastewater recycling.

As shallower, less permeable soil materials are being considered for use in wastewater treatment, care must be exercised to carefully evaluate soil and site limitations and opportunities. Soil and site evaluation is a critical step in the process of designing an appropriate and functional wastewater treatment system.

In Ohio, a number of soil profile limitations provide challenges for soil-based wastewater treatment. Most of these limitations are natural or induced restrictions to water and air movement in the soil profile and limit the depth and duration of unsaturated soil conditions. Identification of these limiting conditions is a critical step in the process of designing effective on-site wastewater systems that protect soil and water resources and the public health.

The major limitations include:

• high water tables, with saturated soil conditions present near the soil surface,
• restricted soil depth above dense, slowly permeable substratum materials, including unfractured bedrock and dense glacial till,
• restricted soil depth above dense slowly permeable subsurface soil layers, including fragipans, compacted soil, and heavy clay materials,
• other layers with inadequate permeability,
• poor drainage conditions,
• flooding,
• excessive slope,
• presence of excessive amounts of rock in the soil,
• fractured bedrock at shallow depth,
• sandy soil with excessive permeability, and
• sand and gravel layers below finer textured soil materials.

Restrictions to wastewater treatment and dispersal must be identified and appropriate systems designed to ensure adequate removal of pollutants before wastewater leaves the site to join the surface or groundwater resources.