Abstract:
The objective o f the present study was to assess the impact o f the Tsunami that occurred on
26 December 2004 on coastal groundwater salinity in Matara district and to identify its
recovery patterns. Three hundred andforty-four dug wells were selected for the study and
the salinity situation monitored for 33 consecutive months from June, 2005 to February,
2008. The study area is an extremely flat coastal plain; the elevation is o f about 1 to 10m
above mean sea level with some elevated locations reaching a maximum o f 15 m above
mean sea level. Main river basins o f the area are Nilwala and Pohvattumodara river
basins. The Goviyapana river situated in the western boundary o f Matara district carries
has a relatively small catchment. An emphasis was also made to relate groundwater
salinity to the surface and subsurface soil properties. Average saturation permeability
coefficient o f soils were estimated as 2.573 x 1(T3 cm/sec. The porosity had an average o f
55% in the entire soil profile. The geological conditions o f the area were very favorable
for Tsunami water intrusion. The soils o f the area are categorized under non-saline class,
with the electrical conductivity below 4000ps/cm as at July, 2007 and pH o f soils ranged
between 6.1- 8.4. Soils belong to two soil pH classes, mildly alkaline and moderately
alkaline soils. It was found that the elevated electrical conductivity o f groundwater due to
the Tsunami, had recovered by October 2007. Since then, seasonal fluctuation o f salinity
was observed with rainfall fluctuations. Average EC value o f well water after October
2007 were stabilized within 800 to lOOOps/cm level, except in some localities where EC
values were higher than the average value. The study revealed that, this high salinity
patches associate with low groundwater level, closeness to river mouth and proximity to
sea. When the groundwater pH is considered, the entire area had almost neutral (7.0) to
mildly alkaline (7.8) water, which is typical for coastal sandy aquifers. Majority o f well
waters in the affected area had “Sulphate, Chloride, Sodium water” with primary
salinity. However, the well water in the non-affected area was “Calcium Magnesium
bicarbonate water” with secondary alkalinity. Quality o f the well waters were within the
WHO recommended range with respect to electrical conductivity (EC), total dissolved
solids (TDS), pH, Ca+2, Mg+2, i f 1, Na+I C u 2, C (t2, Z n 2 ,M n 2 and Pb+2 ions by October
2007, except for a few wells which had high C a 2, Mg 2 and Ccf2 concentrations.
