Design and performance of subgrade biogeochemical reactors

Area of Science:

• Environmental Engineering
• Geochemistry
• Microbiology

Background:

• Subgrade biogeochemical reactors (SBGRs), or in situ bioreactors, are specialized for treating contaminant source zones and groundwater plumes.
• SBGRs are frequently configured for enhanced reductive dechlorination (ERD) of chlorinated solvents but can be adapted for other contaminants.

Purpose of the Study:

• To review the design considerations and performance of SBGRs for contaminant remediation.
• To synthesize literature findings and field data from multiple SBGR applications.

Main Methods:

• Excavation or augering of contaminated soil, followed by backfilling with amended materials (gravel, organic carbon sources).
• Installation of recirculation piping and low-flow pumping systems to circulate groundwater through the reactor.
• Tailoring treatment media and design to specific contaminant types and site constraints.

Main Results:

• SBGRs achieved 85-90% molar reduction of chlorinated volatile organic compounds (CVOCs) near the reactor.
• Rapid cleanup of dissolved contaminant source areas adjacent to SBGRs was observed.
• Key design factors include optimizing hydraulic residence time (10-60 days), balancing organic mulch and carbon sources, utilizing native bacteria, and incorporating iron sulfides/pyrite for polishing.

Conclusions:

• SBGRs are a versatile and effective in situ technology for groundwater and soil remediation, particularly for chlorinated solvents.
• Careful site-specific design, including hydraulic residence time and material selection, is crucial for optimal SBGR performance.
• The combination of biological treatment and abiotic polishing (iron sulfides/pyrite) enhances remediation efficiency for chlorinated solvent sites.