CPWM Landfill Design and Operations 5 — Questions and Answers
Question 1: What is the primary advantage of using geosynthetic clay liners (GCLs) in landfill liner systems compared to compacted clay liners (CCLs)?
- GCLs have higher hydraulic conductivity allowing better drainage
- GCLs are much thinner while achieving equivalent or better barrier performance (Correct answer)
- GCLs eliminate the need for any geomembrane component
- GCLs are more resistant to puncture from heavy equipment
Correct answer: GCLs are much thinner while achieving equivalent or better barrier performance
GCLs (typically 5–10 mm thick) can achieve hydraulic conductivity of ≤5×10⁻⁹ cm/s, matching or exceeding CCL performance while requiring far less material and installation space.
Question 2: Under what condition must a municipal solid waste landfill owner install a gas collection and control system (GCCS)?
- When the facility accepts more than 100 tons of waste per day
- When surface methane concentrations exceed 500 ppm at the property boundary
- When the total design capacity exceeds 2.5 million Mg and the NMOC emission rate equals or exceeds 50 Mg/year (Correct answer)
- When the landfill is within 1 mile of a residential area
Correct answer: When the total design capacity exceeds 2.5 million Mg and the NMOC emission rate equals or exceeds 50 Mg/year
EPA's New Source Performance Standards (40 CFR Part 60, Subpart WWW/XXX) require a GCCS when design capacity ≥2.5 million Mg and non-methane organic compound (NMOC) emissions ≥50 Mg/year.
Question 3: Which slope stability analysis method is most commonly applied to landfill side slopes to evaluate the factor of safety against circular failure?
- Rankine earth pressure method
- Bishop's modified method of slices (Correct answer)
- Coulomb wedge analysis
- Newmark sliding block method
Correct answer: Bishop's modified method of slices
Bishop's modified method of slices is widely used for landfill slope stability analysis because it accounts for interslice forces and provides accurate factors of safety for circular failure surfaces.
Question 4: A landfill operator must relocate a solid waste boundary because excavation revealed the site is within a 100-year floodplain. This restriction primarily aims to:
- Prevent interference with floodwater conveyance and protect waste containment integrity (Correct answer)
- Reduce equipment operating costs during wet seasons
- Comply with stormwater NPDES permit minimum acreage requirements
- Avoid conflicts with wetland mitigation banking credits
Correct answer: Prevent interference with floodwater conveyance and protect waste containment integrity
Siting restrictions prohibit MSW landfills in 100-year floodplains to prevent flooding from disrupting waste containment systems and transporting contaminants off-site.
Question 5: What operating record must a landfill maintain to document the quantity of waste received each operating day?
- Leachate volume log
- Waste acceptance and weight/volume records (Correct answer)
- Daily gas pressure readings
- Groundwater sampling chain-of-custody forms
Correct answer: Waste acceptance and weight/volume records
Federal regulations require owners to maintain daily records of the types and quantities (weight or volume) of solid waste received to track airspace consumption and regulatory compliance.
Question 6: When designing a landfill cell, which factor most directly determines the required capacity of the leachate collection sump pumping system?
- The depth of compacted waste above the liner
- The peak leachate generation rate based on water balance modeling (Correct answer)
- The number of gas extraction wells in the cell
- The geomembrane seam welding method used
Correct answer: The peak leachate generation rate based on water balance modeling
Sump pump capacity must match the peak leachate generation rate, typically estimated using water balance models (e.g., HELP model) that account for precipitation, evapotranspiration, and waste moisture.
Question 7: What is the term for the practice of mining older sections of a landfill to recover buried materials, reclaim airspace, and retrieve soil amendments?
- Landfill reclamation (landfill mining) (Correct answer)
- Cell reconstruction
- Airspace banking
- Waste reconsolidation
Correct answer: Landfill reclamation (landfill mining)
Landfill mining (reclamation) involves excavating previously disposed waste to recover recyclables, compost, and soil-like material while reclaiming valuable permitted airspace for additional waste disposal.
CPWM Landfill Design and Operations 5
What is the primary advantage of using geosynthetic clay liners (GCLs) in landfill liner systems compared to compacted clay liners (CCLs)?