Environmental Engineering Reference
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specific diffusivity and fugacity according to Raoult's law. Flux limitations
typically arise from dispersivity and fluctuations in groundwater flow.
Stripping is probably the dominant degassing effect during DGI applica-
tion; however it is difficult to quantify diffusive degassing due to natural
soil gas fluctuations. Until recently, sensors for the direct measurement of
degassing fluxes have not been available. Stripping needs to be limited by
gas injection control, and should be monitored by soil gas monitoring. Best
practice for flux estimation includes stationary model-based balancing of
the gas injection mass, and gas tracer testing (Weber, 2007). Some light gas
escape in the range of 10%-30% of the injected mass often can be tolerated
to ensure a sufficient efficiency of reactant supply to the upper (near-fringe)
groundwater flow region. If oxygen gas is used, aerobization of the vadose
zone can be a desired additional treatment effect of immobile soil water and
of leaches from the topsoil. A low-cost soil venting technique can effectively
support soil gas mixing and minimize safety implications.
10.3 Techniques and Devices for RGBZ Formation
10.3.1 Set of Available Technical Tools
The first step in the technical implementation of RGBZ is a detailed gas-hydro-
geological site investigation. In addition, biogeochemical and contamination
information need to be obtained as part of the investigation. The best practice
depth-oriented soil core sampling includes low-diameter drilling with liner
sampling or percussion core probing, and direct push methods including
CPT, pneumatic percussion, and Sonic
®
vibration sounding (e.g., Geoprobe
®
).
A conceptual gas-hydrogeological site model is required, and can be devel-
oped using sample analysis and geophysical and hydraulic survey data. Such
a model is presented in Figure 10.5. Borehole logging can include gamma
(γ-γ), neutron (n-n), and electric conductivity logging and thermal and per-
meability flow metering. Hydraulic and immission pumping and infiltration
tests can support the establishment of treatment region dimensions.
RGBZ require specialized gas injection and monitoring methods and
devices; techniques to efficiently control gas dynamics and their impact on in
situ transformation processes are also required. The formation and control
of a homogeneous gas distribution and flow-oriented gas dissolution must
be enabled, and excessive gas emission from the groundwater zone must be
avoided.
A gas injection system consists of four main components: injection lances,
a gas delivery and mixing station, an injection control system (pressure, flux,
and time control), and a warning and safety system adapted to the expected
field gas compositions.
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