Physical, Chemical and Biological Controls on Surface-gas Fluxes Quantified With High-resolution Monitoring of Multiple Tracers.

In the subsurface, water content, gas solubility, adsorption on minerals and chemical reactions control gas fluxes between soil and the atmosphere. Because these processes vary in intensity both in time and space, it is very challenging to quantify emissions, specifically when flux measurements are used for detection, identification or monitoring of a subsurface gas source. An experimental setup for gas percolation though soil column experiments under well-controlled conditions was developed and validated at the ECOTRON IleDeFrance research center. Its design included the effect of: i) watering/evaporation cycles, ii) barometric pressure, iii) injection pressure, iv) tracer behaviors and v) plant metabolism. To better understand subsurface processes controlling gas fluxes, we studied transport of multiple tracers across soil columns using long-term and high-resolution monitoring thanks to online low-flow mass-spectrometry. We injected tracer gases into columns containing different porous media, pure sillica sand and zeolite. This set-up allowed us to evaluate the relative contribution of diffusion, solubility and adsorption on various trace gases (SF₆, noble gas including Xe). All the experimental data are discussed in conjonction with simulations using the NUFT unsaturated flow and transport code.