The goal of this monograph is to disseminate a fundamental understanding of the physical processes of saltwater contamination of shallow sandy aquifers due to an event like December 2004 tsunami or a storm surge event like Hurricane Katrina in the United States in 2005. The main objectives of this book are, 1. Describe the processes occurring in the unstable phenomena associated with tsunami contamination and its nature 2. Disseminate knowledge on parameters which influence saltwater plume migration, use of field experiments, physical experiments and computational modeling, 3. Provide guidelines for mitigation of such contamination and water use after such incident Summary Contaminants are typically released into the subsurface as constituents of a liquid phase in small concentrations with low contaminating speed. Tsunami is a natural hazard which is very fast and therefore the contamination time is also fast. However, the contamination resides very long as well as it can make huge devastation for the whole environment including groundwater. First chapter will discuss the characteristics of tsunami as a natural hazard and its effects. Tsunamis occurs in coastal areas and therefore the groundwater in coastal regions faces contamination by saltwater during a tsunami event. Before discussing the contamination of the aquifer it is important to know the characteristics of such aquifer systems. The nature of the hydrogeologic environment such as the heterogeneity, permeability, conductivity and porosity will affect the subsequent transport process in the subsurface. Hence, the Chapter 2 will introduce the characteristics and the vulnerability of the coastal aquifers especially sandy aquifers. Chapter 3 addresses several aspects of the effect of tsunami on groundwater, including different contamination modes, cleaning of drinking water after the catastrophic event and the studies carried out with regard to water. Three important contamination modes can be pointed out from few others. Direct infiltration, infiltration through saltwater filled wells and infiltration through topographic depressions are those. Chapter 4 outlines the basic parameters, to be tested in different types of field experiments such as water quality investigations, flow and transport modeling etc. separately. For each of these, characterization methods and tools that can be used, including invasive and non invasive sampling approaches, methods for laboratory analysis will be discussed. Geophysics is an important tool for subsurface characterization especially when saltwater is present as a contaminant. Use of geophysics, challenging data and disadvantages of inadequate characterization of the aquifer will also be discussed in this chapter. Physical experiments are very useful in understanding the behavior of subsurface contaminant flow and affecting factors. However, with spatial scale, time and stratigraphy of the media the results may show differences. Specific sensors can be used to detect the concentrations of saltwater with time thus the types, advantages and disadvantages of the sensors will be included in here, Chapter 5. There are few critical parameters needed in these physical experiments such as porosity, dispersion coefficient etc. Methods of obtaining this information will also be incorporated as a guide. Plume shape and behavior changes mainly with the subsurface characteristics of the media. Overall time taken for refreshing the aquifer to the pre-contamination stage will be focused in this chapter. With the increase of computer power, numerical modeling of groundwater flow and solute transport started focusing on density driven flow for risk assessment of the aquifers. Numerous modeling studies were carried out to examine the propagation of dense leachate plumes and also to understand the fundamental issues of hydrodynamic instability and fingering phenomena in density driven flow and transport problems. During variable density flow modeling, plume instability may arise due to many circumstances. Numerical dispersion (artificial, grid dependant smearing of sharp solute concentration fronts) and oscillation (overshooting and undershooting of true solution) are well known difficulties in simulating advection-dominated solute transport problems. It is very important to distinguish between truncation errors and numerical oscillation because truncation errors can result in a different picture from the real physics of the actual system. Numerical studies of dense contaminant plumes from tsunami or storm surge like events and the aquifer cleansing times has not been performed in great detail and so knowledge of variable density plume behavior remains incomplete and this will be addressed in Chapter 6. Chapter 7 explains the different case studies carried out afterwards the Dec, 2004 tsunami to observe and enhance the knowledge on the effect of tsunami on groundwater. Chapter 8 is dedicated to present the technologies and activities that can serve as possible candidates for remediation the saltwater pollution in the aquifers as well as to provide drinking water to the public. Several different techniques have become increasingly popular after the South Asian tsunami incident and this chapter will present those technologies and methods, their strengths and weaknesses as well as cost considerations. Finally, guidelines for use of wells plus drinking water will be given in here.