A NON DESTRUCTIVE METHOD TO CHARACTERISE DIESEL CONTAMINATED SOILS

 

 

Hydrocarbon contamination of soils represents a serious geo-environmental problem not only for its negative effects on chemical, physical and mechanical properties of the contaminated soil but also for the health risks caused to humans and live species. In order to achieve exploitation or rehabilitation of these soils, identification and a rigorous characterisation of the contaminated site is required. Among the current methods of soil characterisation, the geophysical methods are non-destructive and can be applied both on sites and on laboratory cells. These methods exploit the effect caused by the contaminant on some physical properties of the soil such as magnetic, electric or elastic properties (Pitchford et al.1989). The Time Domain Reflectometry (TDR) is a geophysical method based on the exploitation of the dielectric properties of the soil (O'Connor and Dowding 1999). This technique is widely used for the following advantages: non-destructive, accurate, and simple to use (DeRyck et al. 1993). The TDR is a powerful method in measuring the water content of clean saturated and unsaturated soils (Topp et al. 1980). It has also shown a good efficiency in determining migration pathways of DNAPL and LNAPL contaminants (Redman et al. 1994). However, we must note that the application of this technique to hydrocarbon contaminated soils has been generally restricted to fluid saturated soils (Benson et al.1999).

The main objective of the present study is to identify the effect of diesel presence and its concentration on the TDR signal reflected by an unsaturated diesel contaminated soil. It is a direct approach of the problem which objective is to try to build a method based on the exploitation of TDR signal, which should permit the presence detection and the concentration estimation of the contaminant. However, the presence of air complicates this study in reason of its inhomogeneous distribution in the soil samples and the closeness of the dielectric constants values of air, diesel, and soil respectively about 1, 2.88 and 4.

The experimental device used is comprised of an excitation unit and an acquisition unit. The excitation unit is essentially constituted by TDR signal generator that emits an electromagnetic step pulse through a two rods TDR probe inserted in the soil sample. The reflected signals by the soil samples are collected by a PC, which constitutes the acquisition and processing unit. This latter controls also, by the mean of a command interface, the excitation unit. The conception of the device offers the advantage to be entirely automated and minimise human intervention.

A set of four soil samples at constant water content of 15 % is prepared. One of these samples is kept clean, when the others are contaminated with diesel at a diesel content in fluid of 10 %, 20 % and 30 % corresponding to diesel concentrations in soil of 16675 ppm, 37500 ppm and 64275 ppm respectively. For each contamination rate, several samples have been prepared separately and two TDR tests have been carried out for each sample to assure the repeatability and reduce errors during sample preparation. The two TDR tests consist of emission of TDR signals to the sample and acquisition of the reflected signals, for two perpendicular positions of insertion of the TDR probe.

The precision offered by the experimental device allowed the characterisation of the temporal delay between the TDR signals reflected by the contaminated soil sample and by the reference signal reflected by the clean soil samples. Both the temporal delay and the reflection coefficient have been found to increase when the diesel concentration increases. These findings are in disagreement with what has been found in previous studies when dealing with diesel contaminated soils that are fluid [water and diesel] saturated (Redman et al. 1992). Furthermore, this study allowed the identification of some error sources related to the experimental setup and to samples preparation conditions, likely to deteriorate the TDR signals.

Authors: Djaouida Chenaf* and Nabil Amara

GeoEngineering Centre Queen's-RMC, Civil Engineering Department
Royal Military College of Canada, P.O. Box 17000, Station Forces, Kingston, Ontario, K7M 7B4. Tel: (613) 541-6000 ext. 6603, Fax: (613) 545- 8336, E-mail: Chenaf-d@rmc.ca