In recent years, there is a growing interest in using geo-electrical methods to detect and monitor organic contaminants within the subsurface.
Knowledge regarding the effect of organic contaminants on the electrical properties of porous media is usually empirically based, and is mostly limited to the presence of non-miscible
contaminants in the saturated zone.
The aim of this work is to determine the effect of organic contaminants and their biodegradation by-products on the electrical
properties of saturated and unsaturated porous media by combining laboratory experiments and theoretical modeling.
The results of this work can be categorized into three groups.
In the first group results concerning the effect of adsorbed ionic organic compounds on the SIP signature of porous media are considered.
We show that adsorption of organic
compounds on the soil mineral surface results in a significant decrease in the soil polarization. In addition exchange process between inorganic and organic compounds
resulted in an increase of the bulk conductivity.
Using the Stern layer polarization model we show that due to the low mobility of the organic compounds at the Stern layer,
the contribution of the organic compounds to the polarization is negligible, and that the decrease in polarization can be attributed primarily to the decrease in the surface site
density of inorganic ions.
In the second group, results concerning the effect of free-phase organic contaminants on the SIP signature of porous media are considered.
We show that the presence of free-phase organic contaminants results in a decrease of the soil polarization and relaxation time, but doesn’t affect the bulk conductivity.
We explain these results using the membrane polarization model, and argue that the free-phase contaminant change to pore scale fluid phase distribution, thus affecting membrane polarization.
In the third group, results related to the effect of organic contaminant biodegradation by-product (organic acid) on the soil electrical properties were considered.
We show that the
reduction in pH caused by the biodegradation by-product reduces the soil polarization and the soil bulk conductivity.
The decrease in the bulk conductivity is associated with mineral weathering,
while the reduction in polarization is mostly associated with an exchange process between inorganic cation and hydronium.
Overall, we provide new experimental evidence and a new conceptual model that explain the different mechanisms that govern the electrical properties of contaminated soil.
This work can be used to better interpret electrical data collected at sites contaminated with organic pollutants.