A minimum of 10 feet of vertical separation is required between bedrock and the base of the constructed liner at all points along the liner system, except as provided in section 363-6.11(a)(4) of this Part. The material between the base of the constructed liner and bedrock, whether natural or backfilled and must consist of low permeability soils with silty and clayey characteristics and with the ability to attenuate and absorb contaminants and is subject to department approval.