1. (a) Landfill capping systems shall be designed to meet the performance standards in Env-Sw 807.04 so as to:

   1. (1) Reduce leachate generation by limiting to the extent practicable precipitation and surface water infiltration of the waste, through placement of either impermeable or low-permeable cover materials over landfilled areas;
   2. (2) Promote drainage of stormwater and other surface waters away from and around the facility, by properly grading the facility and surrounding areas, and by constructing drainage structures, including berms, trenches, swales, ditches, and detention ponds;
   3. (3) Limit erosion and sedimentation by controlling stormwater drainage and by seeding, vegetating, riprapping, or otherwise stabilizing surface soils against the effects of wind and water;
   4. (4) At facilities that will generate methane gas, control the release of methane gas from the facility by incorporating vents designed to function properly in all anticipated weather conditions, including heavy or drifting snow, or by incorporating recovery wells or similar methods, to assure compliance with Env-Sw 805.18 and Env-Sw 806.07;
   5. (5) Protect or isolate the underlying waste materials from exposure to the environment to limit the attraction of vectors, production of leachate, production of odors, risk of injury or fire, and other threats to the public health and safety;
   6. (6) Resist damage due to the influences of freeze-thaw cycles, settlement, loading, or other anticipated conditions which, not properly considered during design, will result in over stressing the cap; and
   7. (7) Remediate environmental damage resulting from the facility, if determined necessary by the department on the basis of groundwater and surface water quality at the site or on the basis of other conditions involving environmental degradation at the site prior to closure.
2. (b) Lined landfill capping systems shall be designed in cross section according to the specifications provided in (e) below.
3. (c) Unlined landfill capping systems shall be designed in cross section according to the specifications provided in either (e) or (g) below.

4. (d) For unlined landfills, the type of capping system required pursuant to (e) or (g) below shall be based on which system type will meet the performance standards in Env-Sw 807.04, using the following factors to make the determination:

   1. (1) Type and quantity of waste received by the facility;
   2. (2) Size of facility footprint;
   3. (3) Compliance and performance history of the facility;
   4. (4) Extent of groundwater to waste contact zones, if any;
   5. (5) Groundwater and surface water quality at the facility site;
   6. (6) Proximity to drinking water supplies, surface waters, and floodplains;
   7. (7) Age of the facility;
   8. (8) Site topography; and
   9. (9) Geologic and hydrogeologic characteristics of the site.

5. (e) Impermeable landfill capping systems shall, at a minimum, be comprised of the following layers, from the top of the waste to the top of the cap:

   1. (1) Layer 1, immediately overlying the waste, shall:

      1. a. Serve to provide a stable, properly graded base for the placement of layer 2;
      2. b. Be designed to resist raveling, or the sifting of fines downward into the underlying wastes; and
      3. c. Consist of no less than 12 inches of unspecified soil;

   2. (2) Layer 2 shall:

      1. a. Serve to protect layer 3 from potential damage by underlying coarse soil materials;
      2. b. Serve to transmit decomposition gases to gas vents if active gas extraction wells are not used; and
      3. c. Consist of at least 12 inches of sand with 100% passing the one inch sieve and with no more than 12% passing the number 200 sieve on a weight basis if for a facility without an active gas extraction system or at least 6 inches of soil with 100% passing the one inch sieve if for a facility with an active gas extraction system;

   3. (3) Layer 3 shall consist of an impermeable barrier which shall:

      1. a. Minimize the infiltration of water into underlying wastes so as to limit continued leachate production and the associated adverse impacts to the quality of groundwater and surface waters; and
      2. b. Consist of a geomembrane with a minimum thickness of 40 mils;

   4. (4) Layer 4 shall:

      1. a. Provide for the removal of water which infiltrates through the topsoil layer, or layer 5;
      2. b. Protect layer 3 from penetration or other damage, including frost and desiccation;

      3. c. Consist of:

         1. 1. At least 18 inches of free-draining sand which has a saturated hydraulic conductivity of no less than 1 x 10 -3 cm/sec, will pass on a weight basis no more than 10% through the number 200 sieve, and contains no more than 15% calcium carbonate; or
         2. 2. Drainage geocomposite and no less than 12 inches of drainage sand, specified based on the results of a hydraulic calculation supporting the design and containing no more than 15% calcium carbonate; and
      4. d. Terminate in a drainage system at the toe of the slope of the cap that is designed to readily remove water transmitted by layer 4; and

   5. (5) Layer 5 shall:

      1. a. Serve to stabilize the capping system against the forces of wind and water erosion;
      2. b. Provide a low-maintenance surface;
      3. c. Promote evapotranspiration, meaning loss of water from the soil by both evaporation and transpiration from plants growing thereon;
      4. d. Provide ease of visual inspection of the capping system surface; and
      5. e. Consist of no less than 4 inches of topsoil, which is fertilized and vegetated.
6. (f) Alternative or supplemental materials to the topsoil specified in (e)(5) above shall be used if determined necessary by the permittee and agreed to by the department on the basis of actual site features and conditions, to achieve the required stability or erosion control.

7. (g) Low permeable capping systems, formerly referred to by the department as “extended interim closure,” shall be comprised of the following layers, at a minimum, from the top of the waste to the top of the cap:

   1. (1) Layer 1 shall conform to the requirements for layer 1 for impermeable capping systems as specified in (e)(1) above;

   2. (2) Layer 2 shall:

      1. a. Serve to isolate wastes from the environment and to protect public health;
      2. b. Reduce the infiltration of water into underlying wastes; and
      3. c. Consist of no less than 18 inches of soil with a saturated hydraulic conductivity not greater than 1 x 10-5 cm/sec; and
   3. (3) Layer 3 shall conform to the requirements for layer 5 for impermeable capping systems as specified in (e)(5) except as allowed in (f) above.
8. (h) Landfill capping systems shall be constructed in accordance with a quality assurance/quality control plan established pursuant to Env-Sw 805.16.

9. (i) Landfill capping systems shall be designed to manage properly stormwater resulting from the 50-year/24-hour storm event and incorporate at least one of the following:

   1. (1) Intermittent swales on the side slopes of the capping system, placed at an interval of 20 vertical feet or minimum spacing of 100 feet as measured from the center line of the swales, with a minimum slope of 2 percent to allow for settlement; or
   2. (2) An erosion-resistant moisture retention layer, placed directly under the vegetated layer, designed to sustain vegetative growth and effectively resist gully erosion.
10. (j) Stability calculations for the landfill capping system shall be submitted by the applicant to support the proposed design.
11. (k) Excluding the drainage swales noted in (i)(1) above, landfill capping systems shall be designed at a slope of 5% or greater, as necessary to accommodate the anticipated effects of settlement and consolidation and assure positive drainage and stability of the capping system during the post-closure period.

12. (l) The landfill capping system shall:

    1. (1) Extend beyond the confirmed limits of landfilled waste and the underlying liner systems; and

    2. (2) Terminate or be anchored in a manner which:

       1. a. Stabilizes the capping system against creep, pull-out, and material failure due to excessive stress;
       2. b. Limits erosion or wash-out at the toe-of-landfill slope(s); and
       3. c. Limits surface water from infiltrating into the waste.
13. (m) The landfill capping system and the post-closure period stormwater management system shall be compatible in design. Hydraulic calculations to support the proposed design shall be provided, including calculations to demonstrate that the drainage media shall function as intended in the design.
14. (n) Unless the facility only receives inert waste such as concrete debris, the landfill capping system shall incorporate a decomposition gas management system meeting the design criteria of Env-Sw 805.18 and the performance criteria of Env-Sw 806.07.
15. (o) Landfill capping systems shall be designed to support such loading as might occur during and following construction, including activities anticipated to occur during post-closure care, such as maintenance vehicles driving over the cap.
16. (p) On-the-ground markers or other type of clearly visible control points shall be placed at the facility site to allow facility personnel and inspectors to readily identify and locate the limits of the capping system.
17. (q) Except in areas where berms, swales, other stormwater infrastructure, or access roads are constructed, the maximum slope of capping systems shall not exceed 2.5 horizontal to 1 vertical.

Source. #5172, eff 7-1-91; ss by #6535, INTERIM, eff 7-1-97, EXPIRES: 10-29-97; ss by #6619-B, eff 10-29-97; (See Revision Note at chapter heading for Env-Sw 800); ss by #8459, eff 10-28-05 (formerly Env-Wm 2505.10); ss by #10597, eff 7-1-14; ss by #14156, eff 12-21-24