CEA Remediation Technologies 5 ā Questions and Answers
Question 1: Phytostabilization is distinguished from phytoextraction in that phytostabilization:
- Accumulates contaminants in above-ground biomass for harvest and disposal
- Immobilizes contaminants in the root zone, preventing uptake or leaching, without harvesting the plant (Correct answer)
- Degrades organic contaminants using plant-root exudates and associated microbes
- Draws down contaminated groundwater through high transpiration rates
Correct answer: Immobilizes contaminants in the root zone, preventing uptake or leaching, without harvesting the plant
Phytostabilization uses tolerant plants to immobilize contaminants (particularly metals) through root-zone sorption and precipitation, reducing bioavailability without accumulating them in harvestable tissue.
Question 2: The 'rebound effect' in groundwater remediation occurs when:
- Contaminant concentrations continue to decrease after pump shutdown
- Contaminant concentrations rise again after active remediation is paused or terminated (Correct answer)
- The water table rebounds to pre-pumping levels, improving hydraulic containment
- Biodegradation rates accelerate after removal of the primary substrate
Correct answer: Contaminant concentrations rise again after active remediation is paused or terminated
Rebound occurs when residual contamination (sorbed to soil, trapped in low-permeability zones) slowly re-dissolves into groundwater after pumping or active treatment stops, causing concentration increases.
Question 3: High-temperature thermal treatment of hazardous waste in a rotary kiln incinerator requires a minimum destruction and removal efficiency (DRE) of:
- 99% for all hazardous organic constituents
- 99.99% (four nines) for principal organic hazardous constituents (POHCs) (Correct answer)
- 99.9999% (six nines) for dioxins and furans only
- 90% for all listed hazardous wastes under RCRA
Correct answer: 99.99% (four nines) for principal organic hazardous constituents (POHCs)
RCRA regulations require hazardous waste incinerators to achieve at least 99.99% DRE for each POHC, meaning only 0.01% or less of the input POHC mass may exit the stack.
Question 4: An aerobic bioventing system differs from soil vapor extraction (SVE) primarily in that bioventing:
- Operates at higher vacuum pressures to maximize VOC extraction rates
- Uses low air flow rates designed to deliver oxygen for in situ biodegradation rather than to volatilize contaminants (Correct answer)
- Injects steam along with air to enhance desorption of semi-volatile compounds
- Is only applicable to saturated zone petroleum hydrocarbon contamination
Correct answer: Uses low air flow rates designed to deliver oxygen for in situ biodegradation rather than to volatilize contaminants
Bioventing uses low air flow rates to supply oxygen to indigenous microbes for in situ aerobic biodegradation of petroleum hydrocarbons, while SVE uses higher flows to physically extract VOCs to the surface.
Question 5: When selecting between ex situ and in situ remediation approaches, a key advantage of in situ treatment is:
- Lower uncertainty in contaminant mass destruction verification
- Avoidance of excavation and material handling, reducing worker exposure and site disturbance (Correct answer)
- Faster treatment time due to controlled reactor conditions
- Simpler regulatory approval because no off-site disposal is required
Correct answer: Avoidance of excavation and material handling, reducing worker exposure and site disturbance
In situ treatment treats contaminants where they are, eliminating excavation risks, dust generation, transport costs, and worker exposure associated with ex situ approaches.
Question 6: Under CERCLA, the 'source control' element of a remedy focuses on:
- Identifying potentially responsible parties (PRPs) financially liable for cleanup
- Eliminating or reducing the release of contaminants from the primary source area (Correct answer)
- Treating contaminated groundwater extracted from the dissolved-phase plume
- Monitoring natural attenuation in downgradient portions of the aquifer
Correct answer: Eliminating or reducing the release of contaminants from the primary source area
Source control measures address the primary contaminant source (e.g., soil hot spots, NAPLs, contaminated structures) to stop or significantly reduce ongoing releases that feed the dissolved-phase plume.
Question 7: Activated carbon amendment injected into groundwater as a colloidal suspension functions primarily as:
- An electron donor to stimulate reductive dechlorination
- A sorbent to immobilize hydrophobic organic contaminants and reduce their bioavailability (Correct answer)
- A pH buffer to neutralize acidic groundwater from acid mine drainage
- A reducing agent to chemically transform hexavalent chromium to trivalent chromium
Correct answer: A sorbent to immobilize hydrophobic organic contaminants and reduce their bioavailability
Colloidal activated carbon (e.g., PlumeStop) adsorbs dissolved hydrophobic organics (chlorinated solvents, PAHs) from groundwater, dramatically reducing aqueous-phase concentrations and bioavailability.
CEA Remediation Technologies 5
Phytostabilization is distinguished from phytoextraction in that phytostabilization: