Soil Gas Transport
Natural Source Zone Depletion
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 aerobic 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
C₈H₁₈ + 12.5 O₂ → 9 H₂O + 8 CO₂
CH₄ + 2 O₂ → 2 H₂O + CO₂
CO₂, the final LNAPL degradation product, moves upward through the soil until it is eventually released at the ground surface. LNAPL degradation rates can therefore be quantified by measuring the CO₂ flux at grade.
The magnitude of this soil CO₂ flux changes throughout the day following fluctuations in ambient temperature, soil moisture, and ambient pressure. Therefore, a one-time CO₂ flux measurement is not adequate to estimate a representative NSZD rate. Taking long-term measurements of CO₂ fluxes at contaminated sites under a range of conditions (summer through winter) produces a time-integrated result, which provides a realistic yearly estimation of actual NSZD rates.
The graphs on the right (modified from Ma et al., 2013; used with permission) demonstrate this principle. CO₂ fluxes were monitored continuously at multiple locations over a 24-hour period. The produced sine curves show how total CO₂ fluxes change throughout the day as ambient pressure changes pump gases into and out of the soil.