180
Permeable Reactive Barrier
Gas PRB instrumentation can be installed with minimal effort. Only a limited number of small diameter vertical perforations are needed, and sequential reactive zones can be formed in undisturbed geologic structures. In this
way, the invasive effects of groundwater flow are minimized and the RGBZ
operates as a hydraulically passive technology. The injection and propagation of a gaseous mixed phase in the subsurface is performed using controllable 3D gas flow networks. Reactants are temporarily stored in trapped gas
clusters in the porous matrix adjacent to adsorbed contaminants and biofilms, and the delivery of gaseous reactants into the groundwater flow can be
adjusted by controlling the partial pressures of gas components.
Similar to other in situ technologies, RGBZ are strongly dependent on the
hydrogeological domain, described by the porous rock or sediment, groundwater flow, and migration properties. RGBZ are additionally dependent on
the pneumatic or gas flow characteristics of the subsurface. Thus, the management of a complex heterogeneous multiphase multicomponent flow and
migration domain demands that the engineer who is planning and applying
the RGBZ displays a high level of professionalism.
RGBZ are ideally applied in horizontal multiple-layered sediment formations of nonuniform fine- to coarse-grained sands and fine gravels. Depths
to 50 m below the ground surface are accessible without the use of heavy
drilling techniques. Enclosed finer texture lenses or thin layers do not limit
the application of RGBZ, as they are typically not continuously shaped and
contain weak zones of gas-available threshold pressures. A time scale of 1–3
years is required to complete a stable formation of a gas PRB. The horizontal scale needed for a gas storage domain depends on the geological structure. In the direction of groundwater flow, it is typically in the same order of
magnitude as the saturated thickness of the aquifer. Stimulation of intrinsic
microorganisms can be achieved when a suitable environment is established
(redox, pH) and dominant electron acceptors or donors for the biodegradation of groundwater soluble contaminants are supplied. Variable zones of
redox potential can be induced by sequential reactors (Figure 10.1) or rate
controlled and time-variable gas injections. Products of precipitation reactions (e.g., iron or manganese oxidation or pH-induced instability of carbonate) do not put the long-term operation of the RGBZ at risk. The well-known
effects of bypassing or channelling groundwater flow due to gas clogging
can also be monitored and controlled.
The most common RGBZ application uses atmospheric air and pure oxygen gas or its mixture to supply electron acceptors for aerobic biodegradation. Luckner (2001) reviewed the potentially available gaseous reactants
and their impacts on biodegradation. Noble gases such as He, Ar, Ne, and
SF 6 are used as tracers (Weber, 2007). Electron donor supply due to methane
(Zittwitz and Gerhardt, 2006) and hydrogen gas (Bilek and Wagner, 2009)
injection have been tested for in situ stimulation of cometabolic CHC degradation and autotrophic sulfate reduction. In situ iron removal can also be
forced by oxygen and ammonia gas applications.
Permeable Reactive Barrier
Gas PRB instrumentation can be installed with minimal effort. Only a limited number of small diameter vertical perforations are needed, and sequential reactive zones can be formed in undisturbed geologic structures. In this
way, the invasive effects of groundwater flow are minimized and the RGBZ
operates as a hydraulically passive technology. The injection and propagation of a gaseous mixed phase in the subsurface is performed using controllable 3D gas flow networks. Reactants are temporarily stored in trapped gas
clusters in the porous matrix adjacent to adsorbed contaminants and biofilms, and the delivery of gaseous reactants into the groundwater flow can be
adjusted by controlling the partial pressures of gas components.
Similar to other in situ technologies, RGBZ are strongly dependent on the
hydrogeological domain, described by the porous rock or sediment, groundwater flow, and migration properties. RGBZ are additionally dependent on
the pneumatic or gas flow characteristics of the subsurface. Thus, the management of a complex heterogeneous multiphase multicomponent flow and
migration domain demands that the engineer who is planning and applying
the RGBZ displays a high level of professionalism.
RGBZ are ideally applied in horizontal multiple-layered sediment formations of nonuniform fine- to coarse-grained sands and fine gravels. Depths
to 50 m below the ground surface are accessible without the use of heavy
drilling techniques. Enclosed finer texture lenses or thin layers do not limit
the application of RGBZ, as they are typically not continuously shaped and
contain weak zones of gas-available threshold pressures. A time scale of 1–3
years is required to complete a stable formation of a gas PRB. The horizontal scale needed for a gas storage domain depends on the geological structure. In the direction of groundwater flow, it is typically in the same order of
magnitude as the saturated thickness of the aquifer. Stimulation of intrinsic
microorganisms can be achieved when a suitable environment is established
(redox, pH) and dominant electron acceptors or donors for the biodegradation of groundwater soluble contaminants are supplied. Variable zones of
redox potential can be induced by sequential reactors (Figure 10.1) or rate
controlled and time-variable gas injections. Products of precipitation reactions (e.g., iron or manganese oxidation or pH-induced instability of carbonate) do not put the long-term operation of the RGBZ at risk. The well-known
effects of bypassing or channelling groundwater flow due to gas clogging
can also be monitored and controlled.
The most common RGBZ application uses atmospheric air and pure oxygen gas or its mixture to supply electron acceptors for aerobic biodegradation. Luckner (2001) reviewed the potentially available gaseous reactants
and their impacts on biodegradation. Noble gases such as He, Ar, Ne, and
SF 6 are used as tracers (Weber, 2007). Electron donor supply due to methane
(Zittwitz and Gerhardt, 2006) and hydrogen gas (Bilek and Wagner, 2009)
injection have been tested for in situ stimulation of cometabolic CHC degradation and autotrophic sulfate reduction. In situ iron removal can also be
forced by oxygen and ammonia gas applications.