Soil Gas Transport

Natural Source Zone Depletion

 
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About NSZD     Soil Gas Transport    NSZD Monitoring


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 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

Aerobic Biodegradation:

C₈H₁₈ + 12.5 O₂  → 9 H₂O + 8 CO₂

CH₄ + 2 O₂  → 2 H₂O + CO₂

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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.

 Ma, J., Z.‐Y. Wang, B. A. Stevenson, X.‐J. Zheng, and Y. Li (2013), An inorganic CO2diffusion and dissolution process explains negative CO2 fluxes in saline/alkaline soils,  Sci. Rep. , 3, 1–7, doi: 10.1038/srep02025 .

Ma, J., Z.‐Y. Wang, B. A. Stevenson, X.‐J. Zheng, and Y. Li (2013), An inorganic CO2diffusion and dissolution process explains negative CO2 fluxes in saline/alkaline soils, Sci. Rep., 3, 1–7, doi:10.1038/srep02025.

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