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Large septic systems are used for places such as schools and apartments in the rural areas. The objectives of this paper are to discuss data collection and present a mounding analysis protocol as part of a multi-stage soil/site evaluation for designing large septic systems. One large septic system in an area with shallow water table and one in an area with a slowly permeable layer in the vadose zone are discussed in this paper. A design for each system was prepared after a preliminary soil/site characterization as specified by state regulations. After selecting a loading rate for the type of the septic system, a comprehensive characterization of the drainfield area of each system, including measurement of the saturated hydraulic conductivity (Ksat) of various horizons, was performed. For each system, the rates of water flow from the trenches and through the vadose zone were compared with theoretical flow rates calculated by Darcy's Law using measured Ksat values. Using measured data, a ground water mounding analysis was performed for the site with shallow water table. For the site with a slowly permeable layer under the drainfield, lateral water flow above the layer and vertical flow through it were assessed. Based on the analyses performed on these systems, a set of steps for assessing soil/site suitability for septic systems is recommended.
Darcy's Law, hydraulic conductivity, impermeable layer, saturated zone, vadose zone, wastewater disposal
Professor, North Carolina State University, Raleigh, NC
President, Soil and Environmental Consultants, Inc., Raleigh, NC