GROUND PENETRATING RADAR AS A POWERFUL TOOL FOR THE STUDY AND THE MONITORING OF LNAPL-CONTAMINATION IN THE SUBSOIL

MONITORING OF LNAPL-CONTAMINATION IN THE SUBSOIL L. Capozzoli, V. Giampaolo, E. Rizzo, M. Votta, P. Leonardo Cucci, M. De Biase and S. Straface 1 Istituto di Metodologie per le Analisi Ambientali, Consiglio Nazionale delle Ricerche, Tito (PZ), Italy 2 Universita degli Studi della Calabria, Facolta di Ingegneria, Dipartimento Difesa del Suolo, Cosenza (CS), Italy Introduction. The problem of pollution and contamination of soil and groundwater related to human activities, industrial spills, landfills leakage, is a serious risk to the health of the environment. For this reason is of crucial importance the environmental assessment of contaminated areas to minimize the risk to soil and groundwater health. Generally, contaminants can be distinguished between organic contaminants and inorganic contaminants. The first ones are called non-aqueous phase liquids (NAPLs) and are the most common and important organic contaminants. NAPLs can be further subdivided into those that are denser than water (DNAPLs) and those that are lighter than water (LNAPLs). Because of the low solubility and toxicity of NAPLs, they have the potential to contaminate large volumes of groundwater if they are not immediately removed from the subsurface. A DNAPL spilled in to the subsurface will move downwards through vadose and saturated zone until it meets any impermeable horizon, there the contaminant will form DNAPL pools and may move away from the area of dispersion making blobs and ganglia in the pore space of subsoil matrix (Longino et al., 1999). NAPL’s lighter than water, such as gasoline and jet fuel, are referred to as light nonaqueous-phase liquids (LNAPLs). Contamination caused by LNAPL is significantly different from DNAPL contamination being lighter than water. An LNAPL leaked into the subsurface from a surface spill, will move downwards through the unsaturated zone until it encounters an impermeable horizon or a water saturated zone such as the water table or the capillary fringe. Hydrocarbons do not disperse in homogeneous way as hydrogeologic studies (Freeze and Cherry, 1979; Domenico and Schwartz, 1990) have shown but that they move through the unsaturated zone as discrete accumulations of the contaminants. When the DNAPL arrive on the table of water tend to move towards the bottom, unlike the LNAPL that will arrange themselves on the upper area of the saturated zone causing the formation of a distinct smear zone (Sauck, 2000; Lee et al., 2001). For this motivation is more difficult to study and monitor a LNAPL contamination (Redman et al., 1994) Geophysical methods have been widely employed for many years as an aid to hydrogeological characterization (Daniel et al., 2005) and in the last years techniques have been developed to investigate and monitor spatial and temporal distribution of the contaminants and the related phenomena of dispersion of pollutants. Geophysics provides powerful tools including ground penetrating radar, geoelectrical investigations, self-potential method, for the study of phenomena of contamination that occur in the subsurface. These methodologies allow detection of the physical properties variation of the the subsoil significantly influenced by presence of pollutants in the pores and by chemical properties of the pollutant itself. Therefore, through the integration of geophysical techniques operating on large scale with accurate analysis of geochemical is possible to obtain quantitative data on the levels of contamination. Focus of the research is to evaluate the ability of GPR to identify spatial distribution of subsurface contaminants. Ground Pemetrating Raadar (GPR) is a technique that has been widely used to map surface stratigraphy and fractures that affect the transport of contaminants (Davis and Annan, 1989; Benson, 1995; Martinez et al., 1999) and to assist in the characterization of contaminated sites to locate buried characteristics of interest (Daniels et al., 1998; Guy et al., 1999; Radzevicius et al., 2000; Kim et al., 2000). Although GPR has been shown to have potential in certain field conditions to delineate the extent LNAPL hydrocarbon (Olhoeft, 1986; Daniels et al., 1995; Grumman and Daniels, 1995), further research is experimental focuses on strategies for detection in additional situations is necessary, as is currently the potential usefulness of GPR before a survey is difficult to assess. The objective of this article is to provide a potential detection strategy which is demonstrated to be effective in controlled conditions and the basis to develop a relationship between the saturation