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Environment
Know the Age of
Thy Groundwater
by VK
Joshi
Water is
essential for life. There is life on the planet Earth, because of water.
Isn't it strange that water falls from the heaven (read sky) and part of
it percolates down to the depths of the earth? This makes the earth one
of the best storage systems for water. The water vaults of the earth are
ancient. The modern science has made it possible to find out how old is
the water stored in the earth? Since this water came on the earth from
the water vapors in the sky or in simpler terms from the clouds, dating
the ancient water helps in establishing when and how much it rained in
the past.
This natural storehouse of water is however, fraught with its own
perils. There are chances of contamination of groundwater through
natural or anthropogenic reasons. The latter's interference is on the
increase and everyday we read about the pollution of groundwater. The
tracers used to find the age of groundwater often help the scientists in
establishing the source of pollution of groundwater.
One ponders to think why should there be so many hullabaloos about the
age of the groundwater! Well for the hydrologists the determination of
age has at least three applications. First, it can be used to ascertain
the rate of degradation of contaminates. They are in plenty now thanks
to industrial development! Second, dating the groundwater can be used to
verify mathematical models of flow of groundwater. These models are
particularly useful while predicting the travel-time of source water to
wells. And third, the vital information gathered from dating the
groundwater is the climate of the past.
Groundwater is stored in the subsurface in various conditions. For
example, in the alluvial terrains it is stored as thin film around the
permeable sands brought by the rivers in spate and deposited as layers
over a period of time. Such sand bodies are known as aquifers. Within
the aquifer this film moves as per the hydraulic gradient. Similarly
water is also stored in not so permeable finer sands called as aquitards.
The movement of water in aquitards is not as free as in the aquifers.
Rainwater
percolates down to subsurface and upon reaching an aquitard remains
there as long as it is not pumped out for consumption. Thus water of the
aquitards is often much older compared to the aquifers.
Several methods are used to estimate the age of ground water. Carbon-14
dating method is used for dating waters up to about 70,000 years old.
Still older groundwater has been dated with recently developed laser
based method to count individual atoms of krypton-81. The new technique
dates the Saharan groundwater as million years old. The hot and arid
sands of Egypt and Libya are thus believed to have covered a verdant
lush green landscape of the past. This new ultra-sensitive technique
could revolutionize the study of continental groundwater flow; long term
glacier movement and seawater circulation at the ocean floor say the
experts.
Water while traveling from the atmosphere to the land surface and from
there to the depths of the earth gathers even the anthropogenic
contaminants. The Chloro-fluo-carbon (CFC) thus acts as anthropogenic
tracers for determining the ages of water that was present on the
surface during the beginning of the industrialized era.
Similarly thanks to nuclear testing since 1952 and the consequent
release of tritium in the atmosphere has enriched the level of tritium
in the groundwater. Thus it is possible for the scientists to
distinguish the 'old' water from the 'new' water. The later has
naturally a higher concentration of tritium. Though tritium dating is
considered to be the most accurate methods, the ban on nuclear testing
has diminished the levels of tritium in the atmosphere. Being highly
unstable the element would disintegrate over a period of time even from
groundwater. Thus the method may not remain an accurate one for long.
The prediction of the movement of contaminates in the groundwater has
assumed quite a significance in developed countries. It is a common
practice to dump the urban waste into depressions on the ground. As per
norms these landfills should be lined to prevent leaching of pollutants
to the sub-surface aquifers. It is not possible every time to line the
landfills to make them impervious. The determination of travel time of
surface water carrying the dissolved contaminants to the depths of
groundwater reservoirs helps in preventing pollution of the precious
commodity.
Lime enriched water droplets in a limestone country percolate into the
caves. These droplets carrying the Oxygen and Carbon isotopes accumulate
to form the stalagmites. Recent scientific studies have shown that these
isotopes preserve the history of past rainfalls. In one such study of
ancient water formed cave deposits, the scientists of the University of
North California are trying to work out the climate patterns of the past
10,000 years in India, China and Nepal.
The age of the groundwater is assuming significance. S.K. Gupta and R.D.
Deshpande of Physical Research Laboratory, Ahmedabad compiled the
information on isotopic measurements available from various parts of
country and came with interesting conclusions. In a research paper
published in the Current Science in 2005 they conclude that on the basis
of delta 18 O isotope distribution in groundwater the country can be
divided in to three contiguous regions.
The region A
is approximately triangular in outline and covers most of Maharashtra
and parts of Karnataka, Andhra Pradesh, Madhya Pradesh, Goa and Gujarat.
This area in geographic terms comprises the northern section of Western
Ghats and the adjoining parts of Deccan Plateau.
Region B is
more or less hex lateral in outline and includes areas of West Bengal,
Orissa, Bihar, Madhya Pradesh, Uttar Pradesh, Delhi and parts of
Rajasthan and Gujarat. In terms of geography it covers the Eastern
Plateau, the Ganga Plains, North and South Central Highlands and Western
Plains.
The region C
covers most of Tamil Nadu, southern Andhra Pradesh, south-eastern
Karnataka and southern part of Kerala. Geographically it includes
southern Western Ghats and East Cost Plains.
The pattern of delta 18 O isotope in ground water of these regions shows
it is more than -2‰ (‰ =parts per thousand) in region A, in the region B
it is less than -4‰ and in the region C it is -3‰ to -4‰ with transition
area in between. The coastal parts of region A groundwater reflects
isotopic characters of rainfall from SW monsoon from the Arabian Sea. On
the other hand the oxygen isotopes from the groundwater of region B show
that rains from SW monsoon from the Bay of Bengal had enriched the
ancient waters. In the region C the isotopes show signatures of the
rainfall from Arabian Sea branch of SW monsoon on the west coast and of
NE winter monsoon on the east coast.
It is amazing to note that in the regional aquifer systems groundwater
recharged over thousands of years ago still retains some of the ancient
characters despite having moved tens of hundreds of kilometers away.
The 'Vats' in the womb of the earth perhaps store the oldest and the
purest waters like those with the blender's of the best wines. Better
know the age of your groundwater before you sip!
March 30, 2008
Image under license with
Gettyimages.com
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