Site Evaluation and System Design Strategies for Severe Sites.

Site Evaluation and System Design Strategies for Severe Sites

Severe soil receiver sites for single-family home wastewater dispersal present characterization challenges often not addressed in prescriptive guidelines.  The site evaluator / soil scientist / designer must consider the attributes and limitations of a site in formulating a successful design strategy.  Large flow systems have similar considerations.

Chapter 5 “Treatment System Selection” of the On-Site Wastewater Treatment Systems Manual (U.S. Environmental Protection Agency.  EPA/625/R-00/008 Washington, D.C.:  2002.)  provides an excellent outline of soil / site evaluation considerations utilized in the selection of an appropriate design solution. 

“An appropriate onsite wastewater treatment system concept for a given receiver site—proposed location of the system, regional geologic and hydrologic features, and down gradient soils used for treatment—depends on the prevailing design conditions. Designers must consider and evaluate the design conditions carefully before selecting a system concept. Design conditions include the characteristics of the wastewater to be treated, regulatory requirements, and the characteristics of the receiver site. With sufficient knowledge of these factors, the designer can develop an effective preliminary design concept. This chapter focuses on general guidance for evaluation of the receiver site, identification of the site’s design boundaries and requirements, and selection of suitable designs to meet the performance requirements.”

It is the responsibility of site evaluator to delineate the site and identify its attributes and limitations.  In conjunction with the information required by rule, the report should communicate to the designer site-specific details of the delineated area and include a preliminary system layout and design that complies with the requirements of the rule.  The report should identify, comment on, and offer recommendations to address, as necessary, site specific conditions such as soil quality, slope, stoniness, vegetation, surface drainage, site preparation, depth of installation, etc. that may, in the judgment of the evaluator, affect the design and/or field installation.  

Review of Mass Loadings and utilizing the boundary design methodology approach as outlined in of the On-Site Wastewater Treatment Systems Manual, Chapters 4 and 5, will be the general format.  Fundamental elements of soil evaluation and site characterization will be reviewed in regards to sites with one or more limiting features.

Traditional infiltrative surface loading rates, as well as corresponding area (hydraulic footprint) loading sizing, will be reviewed for Ohio and compared to other states. 

Instantaneous loading rates, as applied to the infiltrative surface will be presented, and as a tool to properly address a sites characteristics.

The concept of landscape linear loading (gallons per topographic linear foot per day, saturated flow) as it is applied to shallow horizontal boundary sites will be discussed.  An assessment calculation in evaluating the “stacking” of infiltrative surfaces (mounds) will be presented.