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of groundwater. Multiphase water and gas flow interactions were studied
using parallel flow, counterflow, and cross-flow (horizontal, vertical). A
phase balancing system runs under system pressure conditions. A 2D tank
test device was used for imaging and the bulk estimation of trapped gas
lenses in layered sediment formations. The influence of gas trapping to a
stationary groundwater flow was evaluated using in situ sensor arrays and
noninvasive geophysical gas monitoring systems (e.g., geoelectric-induced
polarization) as discussed in Boerner et al. (1996). Bulk parameters for the
field-scale NDI application were preliminary estimated by pilot scale testing.
Typical gas injection rates for the bench to pilot scale testing of the NDI
were 10
−3
-10
−1
m³/h STP, and the flow sections ranged from 10
−2
to 10
−1
m².
Field-scale
research test sites (up to 500 m² of treatment area and 5-50 m
in depths) were operated by the Berlin Water Company (Figure 10.3) and
the Helmholtz-Centre for Environmental Research UFZ. Several field-scale
test applications were run with DGI technology (NDI and HDI methods)
in sediment environments. There was also an application of NDI in fis-
sured sandstone bedrock (Schinke, 2008). The field sites were equipped
with conventional and state-of-the-art injection and monitoring techniques,
and high-resolution site investigations were performed. From these field-
scale tests, best available technologies and strategies for site characteriza-
tion, injection, and monitoring system operation, and control of RGBZ were
derived (Ehbrecht and Luckner, 2004; Beckmann et al., 2007). An integral
balancing algorithm for gas injection and biodegradation and a tracer test
method using noble gases was also developed.
Typical gas injection rates for the field-scale testing of NDI were
1 × 10
−1
- 2.5 × 10
0
m³/h STP and the flow sections ranged from 10
1
to 10
2
m².
10.2.3 Gas Injection and Gas-Water Displacement
There is a difference in the gas-water-displacement effects of low pressure
(NDI) and high pressure (HDI) DGI methods. In particular, the effects of
interest are the injection pressure gradient, gas injection rate, and apparent
gas propagation velocity.
In the NDI system, the placement or mixing of a low amount of immobile
(gaseous) reactants in a natural groundwater flow and their dissolution are
typical of full-section PRBs. Following this, the desired in situ reactions
occur in downstream aquifer regions. In addition, a stationary gas chan-
nel network is typically formed. The same gas flow paths are used mul-
tiple times, even when a pulsed injection is applied. The density of a gas
channel network and the volume of gas clusters are functions of texture
in homogeneous sediment regions. The coarser the material, the lower the
gas network density; however, the mean dimension of moving gas clus-
ters is higher. Typical cluster diameters in the order of <2 mm in fine- and
nonuniform-grained sands and >20 mm in coarser sands bubbly clusters
have been reported (Weber, 2007).
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