Environmental Engineering Reference
In-Depth Information
Due to insufficient gas storage capabilities, fissured rock domains and
unconfined aquifers with a saturated thickness less than 3-5 m are less suit-
able for gas PRB applications. In addition to the geological domain, the type
and complexity of the limiting reactants for in situ transformations, and the
ability to deliver them by gas flow can also impose restrictions. A stand-
alone RGBZ is unable to provide vital nutrients (e.g., available phosphorous
or trace metals) where they may be deficient. A gas injection-based method
to support the natural buffering capability of a subsurface domain against
high proton production is still needed.
Care must be exercised when transformation of high volatility migrants
(e.g., chlorinated ethenes or short-chained aliphatics) is intended. These sub-
stances can be enriched and stored in gas clusters, and even in cases where
gases are not allowed to be stripped from the groundwater zone, the sub-
stances may become less accessible to biofilms. The presence of nonaqueous
phase liquids (NAPLs) will lower the gas storage capability, because residual
NAPL blobs occupy the same pore space portions; additional impacts are
changes in the wettability or emulsifications. Toxic concentrations of con-
taminants, but also unfavorable environmental conditions (e.g., sulfide or
pH) in the vicinity of NAPL are frequently reported.
Furthermore, the availability of sufficient time and space to achieve the
given protection or remediation goals can limit the application of RGBZ.
10.2 Gas-Water-Dynamics in the Groundwater Environment
10.2.1 Basic Phenomena
Gas flow transport phenomena, capillary gas storage, or entrapment and
mass transfer between the water and gas phases have been evaluated at both
pore and field scales. Gas-water displacement and mass transfer due to gas
injection in a water-saturated subsurface domains occur in a different man-
ner to that in the unsaturated soil zone (Figure 10.2).
The transport of a nonwetting gas phase in groundwater environments is
mainly driven by pneumatic pressure, capillary, and buoyancy forces. The
pneumatic pressure gradient has to overcome the hydraulic pressure head at
the injection point, an additional capillary entry pressure required to open a
gas channel network, a pneumatic flow resistance that is formed by friction at
nonrigid moving gas-water interfaces, and pressure-dependent gas viscosity
(Geistlinger et al., 2006). With increasing distance from the injection point, gas
volume portions become disconnected due to a decrease of pneumatic pres-
sure, and pore trapping forms incoherent gas bubbles and clusters. Due to
the heterogeneous layered nature of sediment domains, bubbly flow in gravel
structures or channelling flow in fine-grained media cannot hold for larger
distances (Brooks et al., 1999). The propagation of gas clusters is commonly
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