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A significant increase of technogenous load on the environment caused by aerosol fallout of heavy metals consisting of industrial atmospheric injections and automobile exhaust gases, wide use of pesticides and fertilizers in agriculture, as well as the threat of radioactive contamination as a result of nuclear weapons tests and danger of nuclear war have led to apprehensions of the possible contamination of shallow groundwater that occurs over wide areas and intensively used for the water supply of villages, small settlements and farms. [ABSTRACT FROM AUTHOR]

Landfill sites commonly use the space available in disused quarries or special purpose-built structures but not all past landfill operations were adequately controlled or documented such that the site boundaries, and the type and volume of fill are unknown in some old covered landfill sites. Even in controlled sites, the final form and depth extent of the landfill may not conform to those indicated in the original plan submitted to the regulatory authorities during the application for a site license. Thus, a significant amount of work is required in order to accurately define the relevant parameters of a covered landfill site. Our hydrogeophysical interest in landfill sites lies in assessing the pollution threat they pose since they may contain hazardous substances. [ABSTRACT FROM AUTHOR]

Engineered barriers for pollutant containment and remediation present unique opportunities and challenges for geophysical investigation. Three fundamental issues pertain to all engineered barriers placed in the subsurface: (1) is the structure located in the ground according to the engineer design specifications? (2) does the barrier effectively perform its intended function? (3) how does design performance degrade over time? Issues 2 and 3 also pertain to engineered barriers placed on the ground surface. [ABSTRACT FROM AUTHOR]

The self-potential method consists in the passive measurement of the distribution of the electrical potential at the ground surface of the Earth and in boreholes. The purpose of this method is to map the electrical potential to reveal one or several polarization mechanisms at play in the ground. In some cases, the self-potential signals are monitored with a network of non-polarisable electrodes, which provides both a better signal-to-noise ratio and the possibility to discriminate between various sources. The two main contributions to the self-potential signals are (1) the streaming potential or hydroelectric coupling (Fournier, 1989; Birch, 1993, 1998; Aubert and Yéné Atangana, 1996; Revil and Leroy, 2001) and (2) electro-chemical processes (membrane or diffusion potentials) associated with gradients of the chemical potentials of ionic species in the pore water (e.g., Sen, 1991; Naudet et al., 2003, 2004; Revil and Leroy, 2005). In the former case, the self-potential signal correponds to the electrical field associated with the flow of ground water in a porous medium and more precisely with the drag of the excess of charge generally contained in the so-called diffuse layer in the vicinity of the mineral surface (e.g. Revil and Leroy, 2004). If the chemical potential concerns the electrons (redox potential), the transfer of electrons through an electronic conductor also generates self-potential signals in the surrounding conductive medium as discussed by Sato and Mooney (1960). [ABSTRACT FROM AUTHOR]

In many countries all around the world, the most densely populated regions are located in coastal areas since they normally provide the best conditions for both economical development and quality of life. On the other hand, coastal zones more than other regions suffer from natural disasters such as flooding and tsunami as well as from scarcity of fresh groundwater due to seawater intrusion. The latter problem is usually caused by a violation of a delicate hydrogeological balance that exists between freshwater and seawater in coastal aquifers. In most cases, the balance is disturbed by groundwater pumping and other human activities (e.g. land drainage) that lower groundwater levels and ultimately cause seawater encroachment into coastal aquifers. Although this problem is fairly important in all coastal aquifers, it becomes crucial in arid and semi-arid regions, where precipitation is limited and groundwater thus represents the main, if not only, source of fresh water. [ABSTRACT FROM AUTHOR]

The use of geophysical methods to characterize distribution of continuous and discontinuous permafrost has been widely applied (e.g. Arcone et al., 1998; Sullivan et al., 2000; Hauck et al., 2001; Hinkel et al., 2001; Moorman et al., 2003) including ground penetrating radar (GPR), electrical resistivity (ER) and electromagnetic (EM) sounding. Different methods have been employed to study water movement in frozen soils: time domain reflectrometry (TDR), electrical resistivity (ER), EM soundings and self potential (SP). In this chapter the main focus is on the use of geophysical methods to solve hydrological problems in areas with frozen conditions, but also includes examples of characterisation of structures in the permafrost which have an importance for hydrogeological processes in these regions. Frozen and thawed structures in the subsurface may cause water and contaminant pathways which are not intuitively based on traditional soil stratification procedures. Because these structures are temperature dependent there may also be a need to monitor spatio-temporal changes in these structures in order to assess how they may affect flow paths in the ground. Time-lapse measurements may hence be required. To study changes in the soilwater system under frozen or partially frozen conditions can be a challenge; soil water sampling equipment may freeze and destructive methods such as conventional soil sampling is impossible under frozen conditions. The objective of conducting various geophysical methods in a hydrological perspective is to describe flow paths and to improve models for water and solute transport under partially frozen conditions. [ABSTRACT FROM AUTHOR]

The geologic record suggests the presence of microbes on Earth as early as the Precambrian (Hall-Stoodley et al., 2004). Microbes are involved in practically every aspect of earth evolution. The term microbe is a general descriptor for tiny organisms that individually are too small to be seen with the unaided eye. Microbes may include bacteria, archaea, fungi, and protists. Viruses are also included as a major type of microbe, although there is some debate whether viruses can be classified as living organisms. The role microbes play in altering environmental systems is well documented in many biogeochemical studies. Notable is the role of microbes in water-rock interactions (Chapelle and Bradley, 1997). Field observations and laboratory experiments suggest that bacteria can accelerate silicate weathering either by direct contact with minerals or by producing organic and inorganic acids that enhance the dissolution of silicates (Heibert and Bennett, 1992). Thus, microbes are able to directly alter mineral surface chemistry and pore water chemistry over short to geologic time scales. Microbial induced changes in water-rock-regolith environments over variable time scales cause changes in the physical properties of these environments that may be detected and measured using geophysical methodologies. [ABSTRACT FROM AUTHOR]

Soils and aquifers are major compartments of the subsurface environment, which together control the terrestrial hydrological cycle. This subsurface is important for water resources and also as repository for municipal, industrial, and government waste. Aquifers are typically recharged by natural rainfall entering the soil profile and leaching into deeper soil layers. Due to intensive agricultural or industrial activities the leachate leaving the soil profile and entering the aquifer may contain concentrations of toxic substances such as agrochemicals, heavy metals, and polycyclic aromatic hydrocarbons. At contaminated industrial sites light and dense non-aqueous phase liquids (LNAPLs, DNAPLs) may be transported as a separate phase to the underlying aquifer systems. Once any of these chemicals have entered the aquifer they can be transported over large horizontal distances thus contaminating large parts of the aquifer and threateningwater supplywells. Remediation of highly contaminated aquifer systems is commonly a long-term and expensive proposition. As safe and effective use of the subsurface environment is a major challenge facing our society, there is a great need to improve our understanding of the shallow subsurface and the groundwater systems. This particularly includes the understanding of transport processes, which are responsible for the fate of contaminants. [ABSTRACT FROM AUTHOR]

The vadose zone, i.e., the part of subsurface above thewater table, is home to a number of key processes that control the mass and energy exchanges between the subsurface and the atmosphere. Vadose zone hydrology provides boundary conditions for both atmospheric processes, including micro-meteorology and climatic changes, and subsurface water migration, with strong implications in water resources management. The rates, timing, and patterns of aquifer recharge are controlled by percolation through the vadose zone. Contaminants released near the ground surface can be altered, retarded or wholly removed by biological, chemical and physical processes in the vadose zone before reaching underlying aquifers. Unsaturated processes control also the availability of water for agriculture, and are the driving mechanisms in slope stability, floods and other major engineering geology problems. Fewhydrological problems of practical interest can neglect the importance of the complex, non linear dynamics of vadose zone processes. However, in practice, the impact of the vadose zone on hydrologic problems is often ignored or treated using highly simplified approximations, mainly because of limited available data in this region. A proper characterization of the vadose zone should also account for the natural variability of the soil properties at different scales. [ABSTRACT FROM AUTHOR]

A dc resistivity survey is an inexpensive and widely used technique for investigation of near surface resistivity anomalies. It recently has become popular for the investigation of subsurface pollution problems (NRC, 2000). In principle, it measures the electric potential field generated by a transmission of dc electric current between electrodes implanted at the ground surface. Then, an apparent (bulk or effective) electrical resistivity for a particular set of measurement electrodes is calculated using formulas that assume homogeneous earth. Many pairs of current transmission and electric potential measurements are used to "map" subsurface electrical resistivity anomalies. [ABSTRACT FROM AUTHOR]

The potential benefits of including geophysical data in hydrogeological site characterization have been stated numerous times (e.g. Ezzedine et al., 1999; Hubbard et al., 1999; Chen et al., 2001; Hubbard and Rubin, 2005). The principle reason for the growing interest in using geophysical methods in hydrogeological studies is that geophysics may provide spatially distributed models of physical properties in regions that are difficult to sample using conventional hydrological wellbore methods (e.g. Butler, 2005). The geophysical models often reveal more details compared with hydrogeological models derived from hydrogeological data, such as pump tests and observations of hydraulic heads. Furthermore, geophysical methods are less invasive compared with hydrogeological methods and they are comparatively cheap. Therefore, geophysical surveys can improve hydrogeological characterization if we could relate the geophysical and hydrogeological properties in an appropriate way. The added value of including geophysics in hydrogeological characterization has become increasingly accepted and several published case studies clearly show the worth of including geophysics for different applications and data types (e.g. see reviews by Hyndman and Tronicke, 2005; Goldman et al., 2005; Daniels et al., 2005). However, the success of a given hydrogeophysical case-study is dependent on many different factors and it is often difficult to develop an opinion a priori about the applicability of a method at another site or for another application. Here, we discuss some of the choices that need to be considered in a characterization effort and point out similarities and fundamental differences between different hydrogeophysical parameter estimation approaches presented in the literature. [ABSTRACT FROM AUTHOR]