Which option defines the 'lower and upper flexible membrane liners' in a landfill design?

• A. Waste and upper compacted clay liner
• B. Native soil and low permeability soil
• C. Upper liner and filter medium
• D. Lower and upper flexible membrane liners

Answer: (D)

The main idea here is that a landfill uses two synthetic barriers as containment: a lower flexible membrane liner at the bottom and an upper flexible membrane liner above the waste. This pair acts as primary and secondary containment, preventing leachate from migrating into surrounding soil and groundwater and working with the leachate collection system to capture any leaks. The lower liner provides base isolation, while the upper liner adds an extra barrier to keep contaminants from moving upward or sideways. Other options mention waste placement or soil-based barriers, or a different type of barrier, which do not describe two layers of flexible membrane liners. So the description that matches two explicit flexible membrane liner layers—one at the bottom and one above the waste—is the correct idea.