CPWM Landfill Design and Operations 3 — Questions and Answers
Question 1: What is the purpose of a groundwater monitoring network at a permitted landfill?
- To supply water for dust suppression operations
- To detect potential liner leaks and groundwater contamination early (Correct answer)
- To measure precipitation for stormwater management
- To monitor methane migration pathways
Correct answer: To detect potential liner leaks and groundwater contamination early
A groundwater monitoring network with upgradient and downgradient wells detects liner performance failures and potential contamination before it spreads significantly.
Question 2: In landfill gas management, what percentage of landfill gas (by volume) is typically methane at a mature municipal solid waste site?
- 10–20%
- 30–40%
- 50–60% (Correct answer)
- 75–85%
Correct answer: 50–60%
Mature municipal solid waste landfills typically generate gas that is approximately 50–60% methane and 40–50% carbon dioxide by volume.
Question 3: What minimum separation distance from the historical high groundwater table is required for the base of an MSW landfill liner system under federal Subtitle D regulations?
- 1 foot (Correct answer)
- 2 feet
- 5 feet
- 10 feet
Correct answer: 1 foot
RCRA Subtitle D requires the base of the liner system to be at least 2 feet (not 1 foot) above the historical high groundwater table—but the correct answer here is 2 feet.
Question 4: Which document establishes federal minimum criteria for municipal solid waste landfills in the United States?
- RCRA Subtitle C
- RCRA Subtitle D (40 CFR Part 258) (Correct answer)
- CERCLA Section 104
- Clean Water Act Section 402
Correct answer: RCRA Subtitle D (40 CFR Part 258)
40 CFR Part 258, promulgated under RCRA Subtitle D, establishes the federal minimum criteria for MSW landfill location, design, operation, groundwater monitoring, and closure.
Question 5: A final cover system for a closed MSW landfill must include a layer with hydraulic conductivity no greater than:
- 1×10⁻⁵ cm/s
- 1×10⁻⁷ cm/s (Correct answer)
- 1×10⁻³ cm/s
- 1×10⁻⁹ cm/s
Correct answer: 1×10⁻⁷ cm/s
Federal regulations require the low-permeability layer of a final cover to have hydraulic conductivity ≤1×10⁻⁷ cm/s, matching or exceeding the bottom liner performance.
Question 6: What is the primary environmental concern with uncontrolled landfill gas migration?
- Increased leachate generation rates
- Explosion hazard in confined spaces and greenhouse gas emissions (Correct answer)
- Accelerated liner degradation
- Reduced compaction efficiency
Correct answer: Explosion hazard in confined spaces and greenhouse gas emissions
Methane migrating laterally can accumulate in confined spaces (buildings, utility corridors) creating explosion hazards, and uncontrolled venting contributes to greenhouse gas emissions.
Question 7: During active landfill operations, what is the minimum frequency required for inspecting the run-on and run-off control systems under federal regulations?
- Daily
- Weekly (Correct answer)
- Monthly
- Annually
Correct answer: Weekly
Federal Subtitle D regulations require run-on and run-off control systems to be inspected at least weekly and after storm events exceeding the 25-year storm threshold.
CPWM Landfill Design and Operations 3
What is the purpose of a groundwater monitoring network at a permitted landfill?