|M.Sc Student||Idit Shefer|
|Subject||Identifying Contaminants in Soils Using Spectral|
|Department||Department of Civil and Environmental Engineering||Supervisor||Professor Furman Alex|
|Full Thesis text|
As soil and groundwater resources become scarcer and more vulnerable there is a pressing need to manage their usage. Therefore, a great need exists to develop tools and approaches for monitoring and characterizing the variety of processes in the subsurface, preferably in a useful noninvasive manner. Geophysical methods can fulfill that need, specifically the spectral induced polarization (SIP) method. SIP measures the frequency dependent electrical conductivity and soil's polarization by applying an AC field. Its response is a complex function of pore solution volume and chemistry, microgeometry and surface chemical properties. Hence, it is greatly affected from the presence of pollutants. The interactions of pollutants with the soil phases influence the soil's electrical properties which are reflected in the SIP signature.
This study's main goal is to examine SIP as a tool for identifying and quantifying the presence of organic and inorganic pollutants in the soil. Several experiments were performed. First the influence of a free-phase organic pollutant on the SIP signature was examined on an unsaturated 'Hamra' soil. Next, we used Loess soil to investigate the SIP effect of several different organic pollutants and their mixtures, in order to examine the ability to distinguish them by the SIP method. Third, the soil solution and adsorbed phase inorganic composition influence on the SIP signature was examined.
Adding free-phase NAPL (decane) to the soil caused a decrease of the imaginary part of the complex conductivity as well as the relaxation frequency. We suggest membrane polarization, which is related to the diffuse layer and pore space, as the main polarization mechanism responsible for these results. We suggest that altering the geometry of narrow water passages controls the SIP response when a free-phase compound is added to the system.
The same trend of decreasing polarization was observed when calcium-rich soil was contaminated with other organic compounds on top of decane. However, the real part of the conductivity had a clear decrease when decane was added, as opposed to no significant change in previous experiments. The calcium rich environment had apparently contributed, in our perspective, to the formation of different surface interactions of the organic compounds (polar or charged with different functional groups) in the presence of decane. Additionally, we present an artificial neural network classification with preliminary satisfying ability to indicate the existence of a specific contaminant.
A clear influence is observed for the inorganic chemical composition of the soil solution and the adsorbed phase has on the soil's electrical signature. Coherent changes exist in the relaxation time and chargeability when the chemical composition of the soil was changed. It seems that divalent cations had a unique influence on the electrical signature: addition of divalent cation to the porous media causes an instantaneous shift in the relaxation frequency, while the polarization magnitude is affected in a more gradual way. Three types of data driven models to potentially predict inorganic species are introduced. Dominant species were fairly well predicted.