A Brief Colonial History Of Ceylon(SriLanka)
Sri Lanka: One Island Two Nations
A Brief Colonial History Of Ceylon(SriLanka)
Sri Lanka: One Island Two Nations
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Thiranjala Weerasinghe sj.- One Island Two Nations
?????????????????????????????????????????????????Sunday, August 13, 2017
Desalinised Water For Jaffna Peninsula – Is It Necessary?
Expensive,
unnecessary and environmentally damaging desalination projects for
Jaffna can be totally avoided by a simple study to measure groundwater
availability and optimised, evidence based water use management
methods.
There have been many articles in the media about the necessity for a desalinisation project to supply potable water for Jaffna.
The reason often quoted is that there is freshwater shortage due to
over-usage of groundwater resulting in salinity and pollution. This
reasoning will be correct only if it could be proven quantitatively by
actual measurements, that the fresh water is insufficient for
agriculture and domestic use. So far, no one seems to have made any
systematic studies in estimating the actual quantity of available fresh
water for human use throughout the dry season. This should be given the
utmost priority and done immediately instead paying only lip-service. It
can be only be proved by measuring salinity of water at different
depths because fresh water is underlain by saline water. In addition,
this information can be used to predict to what depth the wells should
be dug to keep the water free of salinity always.
Computer
modelling to quantify fresh water is not likely to be applicable to
such limestone aquifers. Due to randomness of dissolution channels and
pores, modelling inputs such as transmissibility and specific yield are
widely variable at different locations, and it will not be possible to
get reasonable results.
Illusion or fact
It
has been proven beyond any doubt that the average rainfall during
October to January recharges the aquifer with fresh water to much more
than its maximum holding capacity ( Arumugam 1971, Wijesinghe 1973,
Joshua et.al.2013). Only
in occasional drought-years is maximum recharge not attained. By
April-May, almost sixty percent of this overall storage of groundwater
seeps to the sea/lagoon by subsurface outflow. This rapid depletion is a
consequence of the increased outward pressure caused by the high
watertable at the end of the rainy season. Normally, the stored water
remaining after May is similar every year and has been sufficient for
intensive agriculture and domestic use even for a population of 700,000
in the 1970s. So, is this water shortage and salinity, an illusion or a
fact?
Groundwater formation and properties
Percolated
rainwater into soil forms a fresh water layer, in the shape of a convex
lens over the denser seawater in the aquifer. The lens will be thick at
the centre and thins out towards the coasts. In all probability, Vadamaradchi, Thenmaradhi, Valikamam and the Islands have separate lenses of fresh water. In
reality however, the lenses have fresh water only from the watertable
down to certain depths, while the lower part of the lenses consist of a
transition zone of increasing salinity. The transition zone is at
variable depths at different location and probably results from the
diffusion of salts from the underlying seawater into the lens. The
existence of the transition zone has been proved by actual measurements
(Arumugam-1971, Balendra 1969 and SMEC 2006). The thickness of the lens
keeps on decreasing during the dry season with human usage and slow
seepage till it is replenished by rainfall. For every unit drop in the
watertable, the lens-seawater interface at the bottom of lens rises by
about forty units due to density differences.
Depth of wells and salinity