Concept 1. Evolution of a freshwater lens after flooding by sea water

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The concept of intrusion of saline water in a freshwater lens is demonstrated using data from the Maldives. The intrusion shows as a brackish front moving through the freshwater lens (animation). The lens itself remains intact, but a front of brackish groundwater passes through the lens. An important feature of this concept is that after the tsunami passes the islands, a certain amount of sea water remains on the land. That salty water intrudes into the ground, and will pass through the aquifer the moment rainwater infiltrates into the subsoil. In this concept, we assume that during this dry season of 2005 the amount of rainfall is 10% of an average wet season and that dilution of sea water with rainwater results in a concentration of infiltrated water of 12000 mg Cl-/l. Hydrodynamic dispersion causes the mixing of fresh, brackish and saline groundwater. Figure 5 shows the results of one out of several simulations.

Figure 5: Example of brackish groundwater moving through the edges of a freshwater lens.

Figure 6: Chloride concentration as a function of time at various depths and at various positions in the freshwater lens. Flooding with seawater occurs at year 0. Seasonal variation in concentration is mainly caused by seasonal variation in recharge.

The positions A-F, indicated in figure 5, represent wells at various locations and different depths. The salinity of the groundwater in a well close to the surface (e.g. well A at -0.35 meter below mean sea level) increases in the first weeks after the intrusion and returns to the original value within a few years. With depth, the increase in salinity is retarded, as well as the return to the original value. How fast exactly depends on factors as the magnitude of the hydraulic conductivity as well as the rate of recharge of freshwater during the monsoon. After the tsunami, the shallower the well, the larger the increase in concentration.

Several numerical simulations with different sets of parameters were prepared. The parameters varied were: hydraulic conductivity 10 and 40 m/day, tsunami duration 2 and 4 hours and flood depth 3 and 5 meter, island width 400 m, 1000 m and 2000 m. Also the concept with no sea water remaining on the land has been considered. A lower hydraulic conductivity will reduce the intrusion of sea water into the aquifer.

Based on the simulations, it can be deduced that the duration of the inundation was probably long enough to contaminate the freshwater lens with sea water in some way. This may result in contaminated parts of the freshwater resources during a period of at least several years (depending on the hydrogeological conditions and not taking into account other negative factors). A front of brackish groundwater passes through the freshwater lens and chloride concentrations go up significantly, especially at the upper part of the freshwater lens. The increase in chloride concentration will probably reach values higher than WHO-limits for drinking water.

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