1. A. Disposal trenches shall conform to the following:

   (1) the trench width shall be no less than one foot or no more than three feet;

   (2) a minimum of six inches of aggregate shall be placed below the invert of the distribution pipe; and

   (3) up to a maximum of three feet of aggregate may be placed below the distribution pipe.

2. B. Absorption beds shall conform to the following:

   (1) a minimum of six inches of aggregate shall be placed below the invert of the distribution pipes; and

   (2) up to an additional one foot of aggregate may be placed below the distribution pipes.

3. C. For conventional disposal trenches and absorption beds, the distribution lines shall have an inside diameter of no less than four inches. Perforated pipe shall have two rows of holes and a minimum perforated area of one and one-half square inches per linear foot. Perforations shall be located not less than 30 degrees or more than 60 degrees from the vertical on either side of the center line of the bottom of the pipe. All plastic pipe and fittings shall conform to the current and appropriate ASTM standards. End caps shall be installed on all distribution lines.
4. D. Before placing aggregate or drain lines in a prepared excavation, all smeared or compacted surfaces shall be removed from trenches by raking to a depth of one inch and the loose material removed. Aggregate shall be placed in the trench to the depth and grade required. Drain lines shall be placed on the aggregate. The drain lines shall then be covered with aggregate to a minimum depth of two inches and then covered with untreated building paper, straw or similar porous material to prevent closure of voids with earth backfill. When geotextile fabric is utilized, no aggregate cover of the drainlines is required. No earth backfill shall be placed over the aggregate cover until authorized by the department.
5. E. The department shall allow drainfields for proprietary systems to be sized in accordance with recommendations by the wastewater technical advisory committee that have been approved by the secretary. The wastewater technical advisory committee shall make its recommendations upon standardized, objective evaluations in accordance with Section 9-7A-15 NMSA 1978. Drainfields for proprietary systems shall not be reduced in size by more than 30% in comparison to a conventional system.
6. F. Capped inspection ports shall be constructed, at a minimum, of four inch diameter, SDR 35 or better pipe installed at the end of each trench, provide inspection access to the bottom of the trench and terminate at finished ground level. Inspection ports may be installed below grade if located in a protective enclosure and locatable with GPS coordinates or a metal detector.
7. G. If seepage pits are used in combination with disposal fields, the aggregate in the trenches shall terminate at least 10 feet from the pit excavation and the line extending from such points to the seepage pit shall be constructed of approved pipe with watertight joints.

8. H. Where two or more drain lines are installed, an approved distribution box of sufficient size to receive lateral lines shall be installed at the head of each disposal field. The inverts of all outlets shall be level and the invert of the inlet shall be at least one inch above the outlets. Distribution boxes shall be designed to ensure equal flow and shall be installed on a level base in natural undisturbed or compacted soil or on a concrete footing. Access to the distribution box shall be provided at the ground surface. However, the installer, after approval by the department, may install in lieu of a distribution box a tee fitting and a distribution header to multiple trenches provided that the tee and header pipe are level.

   (1) Concrete distribution boxes shall be coated on the inside with bituminous coating or other approved method acceptable to the department.

   (2) All laterals from a distribution box to the disposal field shall be approved pipe with watertight joints. Multiple disposal field laterals, wherever practicable, shall be of uniform length.

   (3) Connections between a septic tank and distribution box or drainfield shall be laid with approved pipe with watertight joints on natural ground or compacted fill or appropriate bedding material. Such approved pipe shall be SDR 35 or better.

9. I. When more than 500 lineal feet of distribution line is required, a low-pressure dosed system shall be used.

10. J. Disposal fields shall be constructed as follows:

    MINIMUM MAXIMUM

    Number of drain lines 1 per field

    Length of each line -- 160 ft.

    Bottom width of trench 12 in. 36 in.

    Depth of earth cover of lines 9 in. --

    Grade of lines level 3 inch/100 ft.

    Aggregate under drain lines 6 in. 3 ft.

    Aggregate over drain lines with:

    geotextile fabric 0 in. --

    other material 2 in. --

11. K. Minimum spacing between trenches or absorption beds shall be four feet plus two feet for each additional foot of depth below the invert of the distribution line. Distribution drain lines in absorption beds shall not be more than six feet apart on centers and no part of the perimeter of the bed shall be more than three feet from a distribution drain line.
12. L. When necessary to prevent line slope in excess of three inches per 100 feet, absorption trenches or beds shall be stepped. The lines between each horizontal section shall be made with watertight joints and shall be designed so each horizontal trench or bed shall be utilized to the maximum capacity before the effluent shall pass to the next lower trench or bed. The lines between each horizontal absorption section shall be made with approved watertight joints and installed on natural or unfilled ground.

13. M. Sites with type Ia or type IV soils may use soil replacement. Sites with failed disposal systems may also use soil replacement. In addition to other design, setback and clearance requirements of 20.7.3 NMAC, the following conditions are required:

    (1) The replacement soil shall be type Ib, or a higher level, as described in Table 703.1.

    (2) Replacement soil shall be placed to a depth of at least 48 inches below the bottom of each trench.

    (3) Replacement soil is placed to a width of at least 24 inches on both sides and ends of each trench.

    (4) The application rate used for design of the trench shall be 2.00 square feet per gallon per day.

14. N. Disposal systems, including both conventional and alternative disposal, shall not be paved over or covered by concrete or any material that can reduce or inhibit any possible evaporation of effluent. Disposal systems shall not be subjected to vehicular traffic of any kind.

    [For specifications for drainfield pipe, see the most recent versions of the ASTM standards: For Type PSM Poly (Vinyl Chloride)(PVC) Sewer Pipe and Fittings, for Poly (Vinyl Chloride)(PVC) Pipe and Fittings, for Corrugated Polyethylene (PE) Tubing and Fittings, and for Smoothwall Polyethelene (PE) Pipe for Use in Drainage and Waste Disposal Absorption Fields]

[20.7.3.701 NMAC - Rp, 20.7.3.701 NMAC, 9/1/13; A, 9/1/13; A, 9/15/14]