33.3 Methods for Nzvi Tracing
Methods suitable for nZVI fate and migration monitoring can be divided into two main
groups. Indirect methods, such as pH, ORP, H 2 , and dissolved Fe or tracer concentration measurement, provide a fast and reliable insight into the current situation at the
site and enable to monitor rapid changes in groundwater chemistry. However, they are
lacking in the ability to reliably distinguish between the changes caused by injected
nZVI and the natural fluctuations of the measured parameters or divergences caused
by other factors. By contrast, direct methods, such as spectroscopic, diffraction, and
microscopic techniques, serve for a confident identification of nZVI within the
subsurface. These methods are usually more demanding and costly.
33.3.1 Indirect Methods
33.3.1.1 pH
Concentration of OH
À ions in groundwater is influenced by nZVI due to its reaction
with water. Therefore, a rapid increase in pH can be observed in the area of direct
contact of nZVI with groundwater. Depending on the buffering capacity of a
particular aquifer, the pH values can raise by 1–3 units in the vicinity of nZVI
injection. Moreover, pH tends to drop down slowly to the original values when all
the active nZVI particles are fully oxidized or their surface is passivated. pH should
be measured in a dynamic sampling mode, requiring pumping of elevated amount of
groundwater prior to the measurement.
33.3.1.2 ORP
Similarly to pH, decrease in redox potential, as a response to the reducing conditions
caused by nZVI introduction, is typically fast. Immediately after the nZVI application, ORP can decrease by 100–400 mV, even in the distant monitoring wells.
Nevertheless, ORP tends to increase back to the original value after partial nZVI
consumption. Similarly to the case of pH, ORP measurements require dynamic
sampling and the values should not be taken into account before their complete
stabilization.
33.3.1.3 Magnetic Susceptibility
Magnetic susceptibility probes proved to be suitable for indirect nZVI tracing at a
larger scale (Shi et al. 2015). Measurements can be performed by gradually measuring a single borehole (Fig. 33.4) or by placing the field probes into the ground
(Fig. 33.5). The subsurface sensors allow monitoring the changes in magnetic
properties in the vicinity of the susceptibility probe in a continuous mode. This
33 Tool IV: Monitoring of nZVI Migration and Fate in the Groundwater Conditions
637
Methods suitable for nZVI fate and migration monitoring can be divided into two main
groups. Indirect methods, such as pH, ORP, H 2 , and dissolved Fe or tracer concentration measurement, provide a fast and reliable insight into the current situation at the
site and enable to monitor rapid changes in groundwater chemistry. However, they are
lacking in the ability to reliably distinguish between the changes caused by injected
nZVI and the natural fluctuations of the measured parameters or divergences caused
by other factors. By contrast, direct methods, such as spectroscopic, diffraction, and
microscopic techniques, serve for a confident identification of nZVI within the
subsurface. These methods are usually more demanding and costly.
33.3.1 Indirect Methods
33.3.1.1 pH
Concentration of OH
À ions in groundwater is influenced by nZVI due to its reaction
with water. Therefore, a rapid increase in pH can be observed in the area of direct
contact of nZVI with groundwater. Depending on the buffering capacity of a
particular aquifer, the pH values can raise by 1–3 units in the vicinity of nZVI
injection. Moreover, pH tends to drop down slowly to the original values when all
the active nZVI particles are fully oxidized or their surface is passivated. pH should
be measured in a dynamic sampling mode, requiring pumping of elevated amount of
groundwater prior to the measurement.
33.3.1.2 ORP
Similarly to pH, decrease in redox potential, as a response to the reducing conditions
caused by nZVI introduction, is typically fast. Immediately after the nZVI application, ORP can decrease by 100–400 mV, even in the distant monitoring wells.
Nevertheless, ORP tends to increase back to the original value after partial nZVI
consumption. Similarly to the case of pH, ORP measurements require dynamic
sampling and the values should not be taken into account before their complete
stabilization.
33.3.1.3 Magnetic Susceptibility
Magnetic susceptibility probes proved to be suitable for indirect nZVI tracing at a
larger scale (Shi et al. 2015). Measurements can be performed by gradually measuring a single borehole (Fig. 33.4) or by placing the field probes into the ground
(Fig. 33.5). The subsurface sensors allow monitoring the changes in magnetic
properties in the vicinity of the susceptibility probe in a continuous mode. This
33 Tool IV: Monitoring of nZVI Migration and Fate in the Groundwater Conditions
637