Impact of the 26-12-04 tsunami

on groundwater systems
on groundwater based water supplies

The flooding of the coastal areas by the tsunami has caused enormous damage and immeasurable human suffering.

The flooding with sea water during and after the tsunami may have had as effects on the groundwater:

• Salinization of groundwater by infiltration of saline water
  Infiltration of saline water is expected in areas which remained flooded after the tsunami:
  
  • The flooding during the tsunami was of a short duration and is expected to have caused infiltration of saline water,
  • The water which remained in pools, lakes, depressions, etc. after the tsunami is expected to have resulted in infiltration of saline of brackish water.
• Salinization of groundwater by salts leaching from the unsaturated zone
  The flooding during the tsunami has caused a deposition of salts on the soils; infiltration of the seawater and rainfall after the tsunami may have caused penetration of the salts into the unsaturated zone, which in time will leach to the groundwater or which will rise to the surface by capillary action;
• Increased sea water intrusion by landward shift of the coast line
  The destructive force of the tsunami has removed coastal sediments resulting in a landward shift of the coast line in some areas; the intrusion of sea water in the coastal aquifers is expected to shift landward over a similar distance, which may affect nearby groundwater production wells.
• Disturbance of the freshwater lens
  The tsunami wave caused an underground pressure wave, which may have disturbed the freshwater/saltwater equilibrium. The pressure of the wave may have caused mixing of the fresh groundwater with saline water from below. This could result both in a reduction of the volume and an increase in the salinity of the freshwater lens. Water abstracted from wells near the shore will have higher salinities. These will only drop during the wet season when the groundwater is recharged by infiltration of rainfall.
• Pollution of groundwater by infiltration of chemicals
  The damage of storage facilities at factories, petrol stations, storage depots, village shops, etc. may have caused spills of chemicals, fuels, pesticides, medicines (?), etc., which may have infiltrated into the soil and to the groundwater. This will have a significant effect on the groundwater quality and may render water table aquifers unusable for many years.
• Pollution of groundwater by decomposing bodies
  Dead bodies are not a source of epidemic diseases because pathologic organisms die quickly after a person or animal dies. But decomposing bodies do contaminate drinking water and can be a source of infection for persons handling the bodies. Standard precautions should be observed, as well as proper disposal of bodies, to prevent potential contamination of groundwater. (see also Infection Control Issues)
• Pollution of groundwater from shallow dug wells and topographic depressions (both manmade and natural) filled with anthropogenic debris, hazardous solid or liquid waste, etc.
  Shallow dugwells in coastal villages flooded by the tsunami waves will likely be destroyed and filled up of all kinds debris from destroyed buildings and in a worst case also with hazardous solid or liquid waste. Manmade or natural depressions may also have formed traps for debris and waste products.

In many areas the water supply infrastructure has been destroyed, but groundwater sources were affected only when situated near the shore. It is expected that this concerns mainly (open) shallow wells, often used by private individuals, which depending on the force of the waves either were either destroyed or filled with sand and saline water. Pollution of wells by infiltration of saline water or from other pollution sources has rendered water sources unfit for human consumption in many areas.

In some areas wells which survived the tsunami flood are pumped at a higher rate to replace destroyed wells. At such locations risk of a deterioration of the water quality may exist if saline groundwater is attracted by upconing.

An example of the destruction of the shallow wells is shown in the picture below from Sri Lanka. The flood has removed about 0.5 - 1 meter from the soil, leaving the concrete casing of the dug well exposed. The coast line has retreated about 50 meter inland at this location

Photo: Timmo Gaasbeek, ZOA Refugee Care