Effect of dissolved oxygen manipulation on diffusive emissions from NAPL-impacted low permeability soil layers

Area of Science:

• Environmental Science
• Geochemistry
• Hydrogeology

Background:

• Nonaqueous phase liquids (NAPLs) in low-permeability layers can continuously release contaminants into adjacent groundwater systems.
• Understanding diffusive emissions and their reduction is crucial for effective groundwater remediation strategies.

Purpose of the Study:

• To investigate the efficacy of aerobic biotreatment at the interface of low- and high-permeability layers for reducing NAPL diffusive emissions.
• To assess the persistence of emission reductions after the cessation of aerobic treatment.

Main Methods:

• Physical aquifer model experiments using two tanks with distinct sand and crushed granite layers saturated with NAPL (indane and benzene).
• Controlled horizontal groundwater flow with stepwise increases and decreases in dissolved oxygen (DO) levels over extended periods (≥45 days per condition).
• Monitoring of contaminant emissions and concentration distributions under varying DO conditions.

Main Results:

• Aerobic biotreatment significantly reduced benzene emissions by up to 90% at high dissolved oxygen levels (17-20 mg/L).
• Post-treatment, emission reductions were substantially lower (29% and 6% in two tanks) after 120 days.
• A 1-2 month lag was observed between the end of DO delivery and the rebound of emissions to near-steady levels.

Conclusions:

• Aerobic biotreatment can temporarily mitigate diffusive contaminant emissions from low-permeability zones.
• The long-term effectiveness of this method in achieving persistent emission reduction is limited.
• The observed lag in emission rebound has significant implications for designing post-treatment monitoring strategies.