Environmental Engineering Reference
In-Depth Information
peristaltic (at shallow groundwater levels) or double-valve pneumatic pumps
(MDP) and shuttle systems (BAT) give point information due to small sam-
pling volumes and short filters. MDP and BAT can be used for a pressurized
groundwater sampling without degassing losses.
Modified RIL-type B and DIL-type A lances can be used to install 25-mm
groundwater filters. Local-scale integrated samples can be obtained using
packers and either multiple MDP or button valve pumps (Uhlig, 2010 and
Zittwitz et al., 2012). When using a Sonic
®
-system, 50-mm direct push fil-
ters can be installed. In addition to these pumps, mobile bailing systems
(e.g., BAT) can be applied. Hydraulic and immission pump tests are then
performed to obtain volume integrated groundwater information. In a gas
injection zone, it is necessary to cover wells with a gas-tight cap.
The sampling of dissolved gases can be performed by pressure conserving
bailing devices and the use of trace gases (Ehbrecht and Luckner, 2004). A
headspace gas phase can be brought into equilibrium with the water sample
and pre- and postsampling can be undertaken using gas chromatography.
Inert gas flushing, volume and mass balancing, and multiple pressure con-
trols are needed in order for confidence to be placed in the results obtained.
10.3.3.2 Gas Monitoring
It is advisable to install an array of in situ gas sensors in the gas injection
zone of an RGBZ. The distribution of gases and ROI dimensions can be
obtained, along with an estimation of injection gas propagation and the dis-
solution of the gas phase. Combinations of sensors are placed and grouted
to the main gas-permeable layers using the direct push method. They can
also be installed in small-diameter observation wells if packers and an auto-
mated pumping system are used. A shuttle-sensing tool MIDZ (Figure 10.9)
can detect high concentrations of dissolved gases. MIDZ uses a pressurized
flow chamber with integrated sensors, and is installed with CPT technology.
The interpretation of gas sensor signals is based on the gas-hydrogeolog-
ical model. Currently, the best available sensors for oxygen gas are in situ
redox electrodes and oxygen optodes (Engelmann, 2010). Carbon dioxide
optodes are recently developed too. Flow-through monitoring systems (e.g.,
MIDZ or packer-sealed filters) can provide meaningful information about in
situ pH and electrical conductivity conditions.
Starting a gas injection, initial gas sensor values are typically widespread.
However, after matrix preoxidation and homogenization by water flow,
sensor signals become meaningful. The signals can serve as a measure of
the change in the heterogeneity of the reactive zone during operation of an
RGBZ.
In situ redox sensors and oxygen optodes can be used to estimate gas prop-
agation and dissolution due to their short reaction time. The travel times for
coherent gases and clusters are measured as the time required for the break-
through reactions of each sensor, and gas flow paths can be elucidated. The
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