Practice question

Bioventing has emerged over past three decades as leading cost-effective in situ technology for vadose zone soils contaminated with mid-range and light petroleum hydrocarbons such as diesel, jet fuel, gasoline and BTEX compounds frequently leaked from underground storage tanks. Unsaturated soils typically harbor indigenous populations of hydrocarbonoclastic genera Alcanivorax, Pseudomonas, Rhodococcus and Mycobacterium carrying genes alkB alkane monooxygenase, CYP153 cytochrome P450, and catechol dioxygenases requiring molecular oxygen both as terminal electron acceptor and as cosubstrate for initial activation via insertion of one oxygen atom into alkane forming alcohol and aromatic ring forming catechol. Low-flow air injection typically 1 to 2 cubic feet per minute per well provides just enough oxygen to maintain aerobic respiration rates measured by oxygen utilization 0.1 to 1 percent per hour while minimizing stripping and need for off-gas treatment, contrasting with soil vapor extraction designed primarily for volatilization. Nutrients such as ammonium nitrate and phosphate salts and moisture optimization to 40 to 70 percent field capacity further accelerate rates. Heavy metals, radionuclides and chlorinated plastics lacking oxidizable C-H bonds not amenable to oxidative bioventing and require alternative strategies.