Impact of the 26-12-04 tsunami

Introduction
Conceptual models of salt water intrusion in coastal aquifers

Concept 1: evolution of a freshwater lens after flooding by sea water
Concept 2: fingering processes in the subsoil
Concept 3: salinisation due to flow caused by density differences (free convection)
Concept 4: freshwater lens in a coastal aquifer with a brackish lagoon

Effect of shoreline retreat on fresh groundwater resources in coastal aquifers
Impact on fresh groundwater of a coral island during a tsunami
Annex (pdf)

The 26-12-04 tsunami has affected groundwater systems in the low-lying coastal zones of the stricken areas. Figure 1 gives a sketch of possible impacts on freshwater resources in coastal aquifers. Questions can be asked about how serious the impacts of the floods on fresh groundwater resources are: about how harmful they are from a drinking water point of view, and about how long it takes before the contaminated freshwater resources are clean again for consumption.

Figure 1: Schematic representation of the possible effects of the 26-12-04 tsunami on coastal groundwater systems: upconing of brackish groundwater under abstraction wells, intrusion of brackish or saline water from ponds, fingering of brackish water from pools, reduction in freshwater volume due to shoreline retreat, etc...

In this note, we summarize some possible mechanisms that might have occurred in several affected areas. Unfortunately, the complexity of the phenomenon of salt water intrusion does not allow us to extent our conclusions for these specific cases to a global/overall groundwater impact assessment of the tsunami. Local circumstances, such as hydrogeology and soil characteristics, vary too much to come up with only one impact assessment. An important aspect of this complexity is for instance the rate of infiltration of sea water on top of the land. Aquifers that are overlain by clayey and/or loamy low-permeable aquitards will probably be less intruded than aquifers that consist of coarse sand. In addition, the shorter the floods occur, the less infiltration is taking place. Therefore, large differences in impacts by the tsunami of fresh groundwater resources can be expected.

Conceptual models of salt water intrusion in coastal aquifers

By analyzing several possible situations in the subsoil that may have occurred, IGRAC has tried to describe the relevant processes of salt water intrusion in the coastal aquifers by means of conceptual models (figure 2 and 3). Numerical models have been used to support our analysis (see Annex). The main hypothesis is that sea water that flooded the land may have intruded into the subsoil, causing density driven flow through salt water fingers.

Figure 2: Numerical models are used to analyse the impact of the tsunami on groundwater resources (click on figure to start animation of the models)

Factors of importance are:

• Disturbance and reduction of the freshwater lens by the subsurface pressure wave. The extent of this effect cannot be assessed without field observations, but it is expected that mixing of the freshwater with saline water has taken place.
• Local geometry of the inundated areas. If sea water remains some weeks to months in local depressions, lakes and pools, intrusion can easily occur. The topography of the coastal zone needs to be known in detail to quantify the degree of salinisation of the subsoil.
• Duration of sea water standing on the land. The longer salt water remains on the land, the more intrusion into the subsoil can happen.
• Leaching of salts from the soil. The longer it takes to flush out the salts, the longer it will have a negative effect on the salinity of the groundwater.
• Local weather conditions during the coming months. The higher the natural recharge rate, the quicker salt is flushed out from the groundwater resource. As such, the intensity of the coming monsoon seasons also determines the degree of salinisation.

The conceptual models do not take these factors into account; therefore actual conditions of the freshwater volume may be worse than represented in the simulations. This will mean that poor groundwater quality conditions will remain longer than simulated.

Watch animation

Concept 4: freshwater lens in a coastal aquifer with a brackish lagoon

Figure 3: Numerical model of a coastal aquifer

It can be stated that shallow groundwater in some coastal areas may remain unsuitable as drinking water for about one to two years. This may be concluded from density dependent groundwater flow simulations of the effect on a freshwater lens of the flooding by sea water (concept 1 and concept 4) during and after the 26-12-04 tsunami. However to what extent the tsunami will affect the fresh groundwater resources in specific coastal areas will depend on various factors.

For instance, information from Sri Lanka provides provides reports of saline shallow wells, which remained brackish/saline after cleaning by pumping. Locally it is believed, that it is very likely that after the start of the monsoon, the old situation can be restored rather quickly. However, the pace at which the increased concentrations are removed, will depend on the local geometrical and hydrogeological conditions.

Effect of shoreline retreat on fresh groundwater resources in coastal aquifers.

Shoreline retreat will also affect fresh groundwater resources in coastal aquifers (see figure 4).

 Figure 4: Possible effects on the freshwater resource in coastal aquifers due to shoreline retreat:

Case a: freshwater lens under a small island

Case b: salt water wedge in a coastal aquifer with freshwater

Case c: freshwater lens in a coastal aquifer with saline water

Case a.

A reduction in width of the freshwater lens will diminish the volume of fresh groundwater in the freshwater lens. As a rule of thumb, the decrease of volume is proportional to the decrease in width: e.g. 5% smaller island results in a 5% reduction of fresh groundwater volume. Note that the depth of the freshwater lens is proportional to the square root of the natural groundwater recharge and inversely proportional to the square root of the hydraulic conductivity (see Annex).

Case b.

In most coastal aquifers fresh groundwater is replenished by water from recharge areas at the hinterland. A so-called salt water wedge is present in these systems. The length of wedge highly depends on the outflow of fresh groundwater from the hinterland. On average, viz. in natural circumstances, the length of the wedge is not spectacular, viz. often in the order of tens to hundreds of meters. Under normal circumstances, activities as groundwater extractions in the coastal zone and fresh groundwater outflow reductions, e.g. by reduced recharge due to touristic or agricultural developments, have often caused an inland shift of the salt water wedge The shift of the wedge on itself is proportional to the shift in shoreline retreat. However, after shoreline retreat, saline to brackish groundwater may be closer by groundwater extraction schemes, so upconing of saline groundwater could occur more easily. Note that the length is proportional to the hydraulic conductivity and proportional to the aquifer thickness to the power two (see Annex).

Case c.

A special case is occurring when the shoreline retreat causes sea water to intrude into a protected brackish to fresh groundwater reservoir in the hinterland. Then, salt water intrusion might easily occur rather quickly (see concept 4).

Impact on fresh groundwater of a coral island during a tsunami

The sequence of events which can take place when a tsunami hits a coral island is demonstrated in six steps (see also the figure below):

1. Before the tsunami: a freshwater lens is recharged by infiltration of rainfall (and also by infiltration of waste water) and is pumped from shallow wells. The abstraction causes an upconing of brackish and saline water, which in case of overpumping results in high salinity levels in the pumped water.
2. Just before the tsunami: the waterlevel in the sea and in the lagoon is lowered. This is of a short duration and has negligible impact on the fresh groundwater.
3. Arrival of the tsunami: a subsurface pressure wave precedes the surface wave (because the surface wave quickly looses speed when reaching the island) and causes an upward movement of the freshwater lens. Water levels in wells rise. Previously fresh parts of the aquifer turn brackish. Hydrogeological properties (permeability, storage) may change, especially in locations where outflow of groundwater is easy (e.g. at wells or rock fractures).
4. During the tsunami: the island is completely flooded and saline water infiltrates through the unsaturated zone especially in areas with permeable soils. Salt water fills wells and enters the aquifer. Other pollutants present on the surface are spread with the water and will also contaminate the groundwater.
5. Shortly after the tsunami: the floodwater recedes and saline water remains in pools and puddles, increasing the duration of the infiltration. The saline water mixes with the fresh groundwater and intrudes the freshwater lens in brackish fingers. Pumping of wells will remove the saline water in the infiltrated well, but care should be taken not to attract brackish water by over pumping.
6. After the tsunami: rainfall will recharge the freshwater and slowly brackish groundwater will move down to the freshwater/saltwater mixing zone. Gradually the situation before the tsunami is restored. However conditions may have changed (local increase of permeability affects the upcoming and the position of the mixing zone) and wells with previously fresh water may now be brackish.

Figure 5: Impact on fresh groundwater of a coral island during a tsunami

The erosion of beaches on the shoreline will cause a landward movement of the saltwater/freshwater mixing zone resulting in a smaller freshwater lens. This effect is not demonstrated in the figure.