21.Despite section 20, an engineered landfill may be sited on land where the underlying unconsolidated deposits meeting the requirements of that section are at a greater depth, provided that the disposal areas have
(1) an impermeable sideslope liner system
— consisting of materials with a constant hydraulic conductivity of 1 x 10-6 cm/s or less;
— at least 1 m wide;
— extending upwards to ground level;
— the base of which extends at least 1 m into the unconsolidated deposits meeting the requirements of section 20; or
(2) an alternative sideslope liner system if the alternative liner system is at least as effective as the liner system described in subparagraph 1.
Excavation in a disposal area that has an impermeable sideslope liner system must in no case compromise compliance with the requirements of the first paragraph of section 20.