Geoindicators Workshop and Field Trip

FRANCISCA, FRANCO MATÍAS

Córdoba

Taller; Geoindicators Workshop and Field Trip; 2002

Asociación Argentina de Geología Aplicada a la Ingeniería (ASAGAI)

Soil and groundwater quality are geoindicators that assess the effect of human activities on environment. Some potential causes of contamination are industrial and agricultural activities, leakage in municipal landfills, septic tanks, underground storage tanks, ducts and injection wells. Contamination modifies soil properties such as hydraulic conductivity, wettability, dielectric permittivity, swelling potential, crack formation and electric conductivity. These effects can be explained by the analysis of phase interaction, surface effect and double layer behavior. Organic and inorganic contaminants are usually determined from the chemical analysis of the pore fluid of samples obtained in the field. Recent studies show that non-destructive techniques can be employed for this purpose. The influence of contamination on soil electrical properties allows using geophysical exploration (based on electrical properties or electric waves propagation) to detect the presence of contaminants. This work shows the ability of dielectric measurements to detect contaminants in soils. This allows evaluating the potential application of dielectric measurements as a method to evaluate soils and groundwater quality. Inorganic contaminants increase electrical conductivity of soils due the higher ion concentration. Soil contamination with inorganic fluids can be very important in the proximity of abandoned waste sites, and landfills. On the other hand, non-polar organic contaminants (such as petroleum, fuel oil, pesticides, and hydrocarbons) modify the dielectric permittivity of soils. Dielectric permittivity is a complex parameter, k* = k?+ k?, where k? depends on polarizability of the material, and k? depends on soil conductivity at a given frequency. Soils are particulate materials composed by a solid phase (particles) and voids. Additionally, water, air, electrolytes, organic fluids, or mixtures of them, can be present inside soil pores. Hence, dielectric permittivity of soils depends on the present phases, interaction between them, and their dielectric permittivity and relative volumetric content. For example, permittivity of sand saturated with water (e.g. k?35) results higher that permittivity of the same sand partly saturated (e.g. k?15) or contaminated with organic fluids (e.g. k? 8). The higher permittivity of water (k?=78) respect to air (k?=1) and organic fluids (k?2) explains these trends. Hence, when soils are permeated and fluid phase changes, soil dielectric permittivity tends to vary (Francisca, 2001, Ph.D. Thesis, National University of Cordoba, Argentina). This variation allows monitoring the displacement of fluids inside soil pores by means of dielectric measurements. These measurements can be made in laboratory tests by means of Networks and Impedance Analyzers, and in the field with the Time Domain Reflectometer (TRD), Ground Penetrating Radar (GPR), resistivity measurements, and Radio Frequency Modulation (RFM) devices mounted in the static Cone Penetration Test (CPT). It is concluded in this work that dielectric measurements can be employed to detect the presence of contaminants in soils, and to monitoring contaminant plumes. Therefore geophysical methods based on electrical soil properties are very useful tools for evaluating soil and groundwater quality.