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