Know the Age of Thy Groundwater

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!           


More by :  V. K. Joshi (Bijji)

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