Daily fluctuations of NSZD rate measurements
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.
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