Soil contamination is a growing concern worldwide. Not only is contaminated soil a problem in itself, but many contaminants released in the soil interact with the environment they pass through.
“The chemical reactions that ensue can modify the soil and groundwater chemistry,” PhD candidate Perrine Marguerite Fernandez says.
“These chemical reactions that occurs, quite literally, below our feet, affect the water we drink, the food we eat and the foundation of the houses we live in.”
Perrine has been examining groundwater contamination of propylene glycol. Propylene glycol is widely used for aircraft de-icing, and has been studied for decades at Oslo international airport, Norway.
“Even though its toxicity is well known, propylene glycol is often used in cold climate airports, as it is as effective and less toxic than other alternatives,” Perrine says.
The contaminant itself is less problematic to the environment than the degradation products it may cause.
“For example, studies near Oslo airport have shown that under certain conditions, redox reactions induced by propylene glycol release iron and manganese into the ground and groundwater.”
High manganese concentrations can be harmful for aquatic life and people, and the iron may cause interference with water log pumps. In the long term, this degradation leads to methane formation, which is toxic and flammable in contact with oxygen.
The problem and the solution
Perrine has explored three non-intrusive geophysical techniques to monitor and understand the degradation of propylene glycol. She has also examined a potential remediation solution, an electron bridge. In this case, the electron bridge was in the form of an iron bar.
“The iron bar helps transmit electrons between the electron donor, propylene glycol, and oxygen, to the soil below.”
Perrine has then examined how the chemical reactions affect the electrical properties of the soil. These are also known as redox reactions (for more information see side bar).
Iron bar prevents metal and gas releases
Perrine’s results shows that when the propylene glycol is degrading, the release of iron and manganese decrease the resistivity of the soil and increase its imaginary conductivity. She also found that gas release on the contrary increase the resistivity.
“Both the metal and gas release could be avoided by linking the contamination zone to the surface by an electron bridge - in this case the iron bar.”
The activity of this electron bridge can be visualized on the surface with only two electrodes and a voltmeter, thus eliminating the need for disturbing the soil. Perrine’s results indicate that the electron bridge has a good potential for remediation as it prevented the release of manganese, iron, and, most likely, methane in the water.
Looking without digging
Part of Perrine’s research has been to find non-intrusive ways of exploring underground.
“My research is about finding information about the underground without digging it.”
Digging is both expensive and unpractical, as it requires time and effort.
“It is not always possible either,” she continues.
“If the area is protected in some way, for example if it contains extensive infrastructure, like an airport, or some form of cultural heritage, digging may not be an option at all.”
She also points out that digging as a method limits the information to only one spot. Non-intrusive methods can yield multiple results faster.
“In addition, I do not run the risk of destroying the information I need by the mere act of digging.”
Successful non-invasive monitoring
Overall, the three tested geophysical methods appeared to be efficient to locate and monitor redox influenced degradation under anaerobic conditions, as well as the activity of the electron bridge. The latter successfully protected the groundwater in the presented experiment.