Abstract:
Coastal aquifers are highly vulnerable to seawater intrusion due to natural and
anthropogenic factors such as over-extraction, sea-level rise, and geological conditions.
The shallow coastal aquifer along Sri Lanka’s eastern coast faces salinization risks due
to overexploitation. This study focused on a 50 km stretch from Kallady to Karaitivu,
where 47 groundwater samples were collected from deep tube wells (25) and dug wells
(21) along ten transects spaced at 5 km intervals. Each transect included at least five
wells, with samples taken 300–500 meters apart. Groundwater chemistry was analyzed
using pH, electrical conductivity (EC), total dissolved solids (TDS), salinity, and major
ion concentrations, alongside hydrochemical classification methods such as Piper
diagrams and saltwater intrusion indicators. pH values ranged from 6.37 to 7.44, with
most samples exhibiting mildly acidic to neutral conditions. EC values varied
significantly, with 72.34% of the samples surpassing the freshwater threshold, indicating
potential seawater intrusion. TDS levels further confirmed this, with some samples
exceeding the WHO drinking water limit of 1,000 mg/l. The dominant ions detected were
sodium (Na⁺), calcium (Ca²⁺), and bicarbonates (HCO₃⁻), reflecting geological influences
and ion exchange processes. Piper diagram analysis classified 82.98% of the samples as
Ca-Mg-HCO₃ type, characteristic of freshwater influenced by rock weathering and
mineral dissolution. Saltwater intrusion indicators, including the chloride-bicarbonate
ratio, sodium-chloride ratio, and base exchange indices (BEX), revealed that 91.49% of
contamination was due to seawater intrusion, while 8.51% was linked to anthropogenic
activities. BEX values showed that 87.23% of samples were undergoing a freshening
process, although localized salinization persisted. Isolated saline intrusion zones were
identified at 250 m, 300 m, and 500–600 m from the coastline in the highly populated
areas of Koddaikallar South, Karaitivu, and Kalmunai respectively. The study
underscores the need for continuous monitoring and groundwater management strategies
to mitigate saltwater intrusion and ensure long-term water sustainability.
