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Land Use Planning Information>Septic Systems>Locating a Septic System - Topography

Locating a Septic System - Topography
The design, size, and location of a septic system is dependent on both the amount of wastewater anticipated and the characteristics of the site. These include topography, soil conditions, geology and drainage. As the properties are identified, decisions regarding the design and construction can be made.

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Critical to the location of the absorption field is the topography of the site. A contour map shows varying elevation is which each line represents a set elevation change.

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This topographic profile shows the basic types of landforms. Depressional areas and floodplains must be avoided since these areas may become saturated and be unable to adequately treat the effluent flow.

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When locating the absorption field on the site, areas where water naturally converges (note blue flow lines) should be avoided. Areas where flow will diverge (note red flow lines) are preferable.

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A "usable area" of soil absorption should be located in an area of diverging flow. This area can be triangular or trapezoidal in shape, but is most commonly rectangular. The larger the area chosen, the easier the system layout will be.

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The absorption trenches should be placed perpendicular to the slope. Another way of saying this is that the trenches will be parallel to the contour lines.

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The exact slope of the site must be determined. Knowing the elevation change from the house to the absorption field is critical. If the field is located uphill from the house, hydraulic pumps will be need to be included in the design.

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