NAC445A.9676. Absorption trench system.

     1. An on-site sewage disposal system may use absorption trenches as the effluent absorption system, unless limiting conditions exist at the site which preclude such use, including, without limitation, high groundwater, highly permeable stratum, sloping terrain, shallow bedrock or a layer of semi-impervious soil with a percolation rate that is slower than 120 minutes per inch. In such a case, an alternative effluent absorption system may be used in accordance with NAC 445A.9678 to 445A.969, inclusive.

     2. An absorption trench system used in an on-site sewage disposal system must meet the following design and construction criteria:

     (a) The trench must not be excavated if the soil is saturated. Surfaces in a trench which are smeared or compacted must be scarified to the depth to which the soils are smeared or compacted, and all loose material must be removed.

     (b) The bottom of the trench must be level and must not be less than 1 foot or more than 3 feet in width.

     (c) The length of each trench and the number of trenches needed must be determined by calculating the total absorption area required pursuant to subsection 3.

     (d) The distance between trenches must be at least 4 feet, with an additional 2 feet added for each foot of depth of the trench below the bottom of the distribution piping. The depth must be measured from the centerline of the trench.

     (e) The distribution pipe in each trench must be placed in clean, graded aggregate ranging in size from 3/4 inch average diameter to 2 1/2 inches in diameter. The aggregate must extend from at least 2 inches above the distribution pipe to at least 12 inches below the distribution pipe. If the trench is more than 6 feet below the finished grade, the aggregate must extend not less than 12 inches below the ground surface to avoid anaerobic conditions in the trench.

     (f) The aggregate in the trench 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. The top of the trench must be backfilled with not less than 4 inches or more than 6 inches of soil, and fines must be kept from entering the trench from poorly cleaned gravel or poorly cleaned cover. As used in this paragraph, “fines” means any small, crushed or powdered material which may seriously impair the absorption ability of the aggregate used in an absorption trench.

     3. The total absorption area of the trench must be determined by measuring the size of the effective area of each sidewall of the trench beneath the distribution pipe. Not more than 4 feet of aggregate below the distribution pipe may be used to calculate the effective area of the sidewall unless approved by the Division or other administrative authority. To determine the required length of the absorption trench to meet the total absorption area requirement for the on-site sewage disposal system as calculated pursuant to NAC 445A.9674, the total absorption area required, in square feet, must be divided by 2 times the depth, in feet, of the aggregate beneath the distribution pipe. The depth of the aggregate must be multiplied by 2 to account for both sidewalls of the trench. For example, if the required absorption area is calculated at 3,000 square feet and the aggregate below the distribution pipe is 4 feet deep, the equation would be 3,000 feet squared/(4 feet x 2) = 375 linear feet of distribution pipe, and thus absorption trench, required.

     4. The following is a diagram of an absorption trench:

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