Question 16 of 24: Secure Landfill — Description and Cross-Section Sketch

Question 16: Secure Landfill — Description and Cross-Section Sketch (5 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Definition. A secure landfill is an engineered hazardous-waste containment facility designed to isolate the buried waste from the surrounding environment (groundwater, surface water and air) essentially permanently, through multiple redundant engineered barriers plus active leachate management and long-term monitoring — distinct from an ordinary municipal solid-waste landfill, which relies on a single composite liner system.

Key components (point form), bottom to top:

1. Native subgrade / prepared foundation — compacted, structurally stable base graded to drain toward the leachate collection sump.
2. Composite bottom liner system — a low-permeability compacted clay layer (typically ≥0.6 m, k ≤ 1×10⁻⁹ m/s) overlain by an HDPE geomembrane, providing two independent barriers to leachate migration.
3. Leachate collection system (LCS) — a granular drainage blanket with a network of perforated collection pipes sloped to a sump, pumping leachate to on-site treatment or off-site disposal before it can build up head on the liner.
4. Waste cells with daily cover — waste placed and compacted in discrete lifts, each covered daily with soil or an alternative cover to control odour, vectors and windblown litter.
5. Gas collection/venting system — vertical wells or trenches to collect and safely vent or flare landfill gas generated by waste decomposition.
6. Final cap — a low-permeability cap (compacted clay and/or geomembrane) topped with a drainage layer and vegetated topsoil, minimizing rainwater infiltration once the cell is closed.
7. Groundwater monitoring wells — a ring of wells upgradient and downgradient of the cell, sampled regularly to detect any leachate breakthrough at the earliest possible point.
8. Perimeter berm and surface-water controls — diversion of clean run-on away from the active cell and controlled collection of run-off, keeping uncontaminated stormwater out of the leachate stream.