Wis. Admin. Code NR § 506.06
Unless otherwise approved by the department in writing, any portion of a landfill which has been used for solid waste disposal but will not receive additional solid waste for a period exceeding 6 months shall be covered with one foot of fine grained intermediate cover or other material approved by the department. A specific soil type may be specified by the department for this one foot layer. The intermediate cover shall be compacted and adequately sloped to allow storm water runoff. The slopes shall be no less than 5% and no greater than 33%. The department may require that intermediate slopes be vegetated depending on the length of time they will remain open. This section does not apply to high volume industrial waste nor does it apply to wood residue approved as a construction material or to provide protection of the liner from frost under s. NR 506.07(3) (b), unless specifically required by the department.
Wis. Admin. Code Department of Natural Resources § NR 506.06