NAC445A.9686. Elevated mound system.

     1. An elevated mound system consists of a suitable fill material, an absorption area made of coarse aggregate and fill material with a distribution network, and a soil cap. In an elevated mound system, the effluent must be gravity fed, pumped or siphoned into the upper part of the absorption area through a distribution network. The effluent must pass through the aggregate and the fill material, with treatment of the wastewater occurring as it passes through the fill material and the unsaturated zone of the native soil.

     2. In preparing an elevated mound system, the site must be scarified, except that tree stumps and other herbaceous materials must be left in place after mowing or cutting to prevent excessive alteration of the soil structure. Mound construction must begin immediately after scarification, and each layer of the absorption system must be placed in such a manner as to prevent differential settling and to promote uniform density.

     3. An elevated mound system must not be constructed on a slope that is:

     (a) More than 6 percent if the soils comprising the slope have percolation rates slower than 60 minutes per inch; or

     (b) More than 12 percent if the soils comprising the slope have percolation rates equal to or greater than 60 minutes per inch.

     4. Percolation tests must be conducted at the depth estimated by the design engineer as being the point of interface between the native soil and the fill material and at a depth of 20 inches below the surface of the native soil. The size of the required area for the elevated mound system must be based on the slowest percolation rate obtained.

     5. The basal absorption area of an elevated mound system must be sufficiently large to absorb the wastewater before it reaches the perimeter of the elevated mound. The infiltration rates for determining the size of the basal absorption area are as follows:

Percolation Rate or Equivalent Soil Classification (minutes per inch)	Infiltration Rate (gallons per day per square foot)
0-30	1.0
31-45	0.5
46-60	0.3
61-120	0.2

     6. The absorption area required for an elevated mound system must be determined from the table set forth in subsection 5. In addition, if the site on which the elevated mound system will be located is:

     (a) Flat, the entire basal area, calculated as length multiplied by width, must be used to determine the absorption area of the elevated mound system.

     (b) Sloping, only the area below and down slope from the absorption bed must be used to determine the absorption area of the elevated mound system, calculated as W x (L+S), where:

          (1) “W” is the width of the absorption bed;

          (2) “L” is the length of the absorption bed; and

          (3) “S” is the required side slope of the elevated mound pursuant to subsection 9, as measured from the edge of the absorption bed to the perimeter of the mound.

     7. At least 4 feet of unsaturated soil or fill material, or any combination thereof, must be maintained between the bottom of the mound system and the top of the seasonal high groundwater or any impervious barrier, including, without limitation, any bedrock. On sloping sites, the depth of unsaturated soil and fill material must be increased to maintain a level bed.

     8. If practicable, the bed for an elevated mound system must be rectangular with a long axis that is parallel to the contour of the slope to minimize the possibility of seepage from the base of the elevated mound. If the natural soil has a percolation rate that is less than 60 minutes per inch, the bed must be made narrow and extend along the contour of the slope as far as practicable. The bed must be filled at least 9 inches deep with clean, graded aggregate.

     9. The side slopes of an elevated mound system must extend in a horizontal to vertical ratio that is at least 3 to 1.

     10. The aggregate in the absorption bed must be covered with untreated building paper, straw with a minimum thickness of 1 inch, geotextile fabric or a similar covering approved by the Division or other administrative authority.

     11. After placement of the covering required pursuant to subsection 10, the entire absorption bed must be covered with at least 1 foot of topsoil. The topsoil cap, which must be placed at the center of the mound, must maintain a minimum slope of 2 percent away from the crown.

     12. The following is a diagram of an elevated mound:

     (Added to NAC by Environmental Comm’n by R194-07, eff. 8-26-2008)