Environmental Engineering Reference
In-Depth Information
mass transfer coefficient and its scale dependency (Luckner and Schestakov,
1991). Estimation of active gas-water interface areas and water diffusion
lengths are also needed. Estimates of hydraulic conductivity changes due
to residual gas storage cannot easily be derived from well-known functions
of vadose zone modeling due to the nature of gas-water-displacement near
saturation (Giese, 2012).
A lot of experimental and modeling work to determine the mass transfer
coefficients at the pore to bench scale has previously been reported; an over-
view of this work is presented in Geistlinger et al. (2005). Best practice scalable
mass transfer calculations take into account the dimensionless numbers: the
Peclet number (Pe: relates water flow velocity to diffusion), Sherwood num-
ber (Sh: relates mass transfer to Pe), and Damkoehler number (Da: relates
hydraulic resistance to mass transfer times). State-of-the-art modeling tech-
niques were tested and further developed, and field-scale modeling capabili-
ties of multiphase multicomponent reactive transports were demonstrated
for operation control of RGBZ (Horner et al., 2009, Geistlinger, 2010, Weber
et al., 2013) using adapted codes of PHT3D, TOUGH2, and MIN3P. It has been
reported that for the practical purposes of RGBZ control, first-order transfer
functions can be applied to residual gas dissolution.
Balanced experimental data sets (Geistlinger et al., 2006; Weber, 2007;
Ehbrecht and Luckner, 2004), and field-scale balance and sensing estimates
(Engelmann, 2010; Ehbrecht and Luckner, 2004, Beckmann et al., 2007) are
available for pure oxygen gas dissolution. Residual oxygen gas saturations
of 2%-4% in sandy sediments can completely dissolve when 2-3 pore vol-
umes of gas-free groundwater have passed. This measure is used in practice
to periodically reload storage zones of the PRB BIOXWAND. Mass transfer
rates decrease when inert gases are present (e.g., during air injection or in
presence of high-dissolved nitrogen concentrations in natural groundwater).
Degassing
in conjunction with DGI is defined as the reduction of gas
caused by gas stripping and/or diffusive degassing from groundwater.
Stripping occurs as a bulk gas escape (buoyancy and convection driven)
of mobile gas clusters reaching the phreatic groundwater surface and cap-
illary fringe. A multicomponent gas volume is injected into the coherent
gas phase of the vadose zone, and the entire mass of the gas mixture is
transferred. Mixing in the soil gas is only limited by gas diffusivity and the
partial pressure gradients of the gaseous components. Stripping may also
be generated when a gas flow network connects to unsealed technical or
natural macropores (e.g., boreholes, wells, and other observation installa-
tions), or natural fissures in overlaying gas barriers. Partitioning of volatile
compounds such as volatile organic compounds (VOCs) and short-chained
alkanes to the gas flow and their escape to the soil gas is of concern due to
safety implications.
Diffusive degassing of dissolved compounds from groundwater is a sub-
stance-specific mass transfer through the capillary fringe, and is driven by
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