1. (1) Each disposal unit must be provided with a separate, independently supported protective enclosure that will function from the beginning of operations until disposal unit closure.

2. (2) Protective enclosure design. The protective enclosure must be designed to meet the following requirements:

   1. (i) the protective enclosure must be freestanding and provide weather and frost protection to the disposal unit;
   2. (ii) the protective enclosure must be capable of being removed at disposal unit closure without adversely affecting the performance of the disposal units; and
   3. (iii) the protective enclosure must be watertight and capable of being easily inspected and repaired.

3. (3) The roof of the protective enclosure and of the disposal unit must be sloped at a gradient of no less than two percent and sufficient to facilitate drainage and prevent ponding of water.

   (b) Water infiltration resistance.

   The exterior of the disposal units must be protected against water infiltration by at least two barriers to water infiltration. Each barrier must function independently (i.e., the effective performance of each barrier must not require the presence or proper performance of any other barrier). The barriers may be sealant materials, bentonite impregnated panels, polymeric sheet membranes, or other materials or structures.

   (c) Cover.

4. (1) A cover must be installed over the disposal unit during disposal unit closure. The cover must be designed and constructed to:

   1. (i) protect the disposal unit against frost, weathering, erosion, and burrowing animals;
   2. (ii) provide resistance to water infiltration;
   3. (iii) direct percolating or surface water away from the waste; and
   4. (iv) protect against inadvertent intrusion, if required by section 382.33(a) or 382.34 of this Title.
5. (2) The cover must be sufficiently thick to place the roof of the disposal unit below the maximum depth of frost penetration.
6. (3) The design of a multi-layer cover must include stability analyses to demonstrate that no sliding or sloughing will occur at any interface between the various components under the most severe conditions anticipated. Such analyses must be based on the results of tests of interfaces between the same materials as proposed in the cover design.

7. (4) The requirement for a cover may be satisfied by either a multi-layer cover meeting the requirements of subdivision (d) of this section or an alternative design meeting the requirements of paragraphs (1) through (3) of this subdivision.

   (d) Multi-layer earthen cover requirements.

8. (1) Unless an alternative design is approved by the department, a multi-layer earthen cover must meet the following requirements. The cover must be composed of, at a minimum, the following layers from the disposal unit upward:

   1. (i) Compacted low permeability barrier layer. A low permeability barrier layer consisting of a low permeability soil material must be constructed to resist the infiltration of water into the underlying waste disposal unit(s).
   2. (ii) Polymeric sheet membrane layer. A polymeric sheet membrane layer must be installed over the compacted low permeability barrier layer to resist water infiltration into the underlying disposal unit area.
   3. (iii) A subsurface drainage layer. Immediately above the polymeric sheet membrane, a subsurface drainage layer, consisting of a free-draining gravel material, must be installed. Above the subsurface drainage layer, a geosynthetic filter fabric must be installed and covered by a protective sand layer.
   4. (iv) An anti-erosion and burrow barrier layer. An anti-erosion and burrow barrier layer must be installed above the subsurface drainage layer to protect the multi-layer earthen cover against frost action, erosion, root penetration, and damage caused by burrowing animals.
   5. (v) Common fill layer. A common fill layer must be constructed over the anti-erosion and burrow barrier layer providing a separation between the anti-erosion and burrow barrier layer and the surface topsoil layer. The common fill layer will help maintain adequate moisture content within the topsoil layer for promoting growth of the vegetation cover.
   6. (vi) Topsoil layer. A topsoil layer must be designed and constructed to maintain vegetative growth over the multi-layer earthen cover.
9. (2) The installation and grading of adjacent material layers within the earthen cover must incorporate geosynthetic filter fabric and/or other design provisions to minimize the risk of soil particles migrating into and clogging the underlying layers.
10. (3) The surface and intermediate material layers composing the earthen cover must be graded to provide for positive drainage away from the disposal unit. The surface of the earthen cover must be at a slope that will not promote erosion. The minimum slope must not be less than four percent and the maximum slope must not be more than 25 percent.

(a) Protective enclosure.