Deep Groundwater Recharge Mechanism in the Sedimentary and Crystalline
Terrains of Sri Lanka: A Study Based on Environmental Isotopic and
Chemical Signatures of Spring Water
Abstract
In many instances, dynamic, potential status and geochemical
characteristics of groundwater discharging through natural springs are
not well known. Present study has assessed the deep groundwater in the
form of thermal and non-thermal spring in artesian condition in the
selected zones in Sri Lanka, using isotope and geochemical
characteristics. The results revealed that evaporation-fractional
crystallization and cation-exchange in the sedimentary aquifers while
rock-water interaction in crystalline deep aquifers, are the significant
mechanism that control the groundwater chemistry. All the deep
groundwater recharged from meteoric water at different elevations and
further influenced by either evaporation or rock-water interaction
during the subsurface flow. Artesian aquifers in the sedimentary terrain
in the north-western coastal zones showed the recharging elevation as
from 100 to 200 m amsl. They are not mixed with sea water and slightly
impacted by the locally evaporated surface waters. Almost all these
waters are comparatively old; indicating slow movement along the
regional flow paths. Considering the recharge and discharge conditions
of artesian non-thermal waters in the Southern lowlands of crystalline
terrain can be classified as non-mixed, non-evaporated and young
groundwater with higher elevation recharge. The artesian non-thermal
waters in the East North Central lowlands, have shown the same
characteristics but with evaporated conditions. All artesian thermal
waters are tritium free, hence they are older and deep percolated.
Intensive rock-water interaction and higher altitude origin were
observed in some thermal springs. Some spring clusters in the weathered
overburden have shown significant mixing with recent local rains.
Non-mixed, non-evaporated and less rock-water interacted nature is a
significant in two thermal springs that emerges through (chemically
inert) quartzite bed rock. Both thermal and non-thermal water with
artesian condition have clearly indicated that they are originated from
a common recharge source but with different flow paths in different
penetration depths and travel distances, resulting different chemical
characteristics. Fresh water springs are mostly young and recharged from
local rains followed with shallow percolation.