NSZD and Soil-Gas Transport

Agronomists have long recognized that soils emit carbon dioxide (CO₂ efflux, or flux). Soil emits CO₂ as a result of natural soil respiration processes attributed to microbial and plant activity in the "root zone." Microbial degradation of LNAPL in soils also results in a net contribution to the CO₂ efflux.

Petroleum hydrocarbons can be degraded aerobically (in the presence of oxygen) or anaerobically (without oxygen). Aerobic degradation produces CO₂ and water, while anaerobic degradation produces methane and CO₂ .

Methane anaerobically produced by methanogenesis moves upward through the soil until it encounters downward-diffusing oxygen and is oxidized. Other common anaerobic degradation processes include sulfate and iron reduction. Such processes also result in the production of CO₂ .

The chemical equations below describe degradation processes using octane (C₈H₁₈) as the representative LNAPL compound.

Anaerobic Biodegradation (Methanogenesis):

C₈H₁₈ + 3.5 H₂O → 6.25 CH₄ + 1.75 CO₂

Aerobic Biodegradation:

C₈H₁₈ + 12.5 O₂ → 9 H₂O + 8 CO₂

CH₄ + 2 O₂ → 2 H₂O + CO₂