Genetic Classification of Water 𝐨𝐫 Origin of Groundwater

Almost all groundwater can be thought of as a part of the hydrologic cycle,including surface and atmospheric (meteoric) waters. Relatively minor amounts of groundwater may enter this cycle from other origins. 

1. Connate water - Also known as Fossil water or Paleowater.

    Water that were trapped in the pores of sedimentary rocks for a long period of time. This fossil water is often saline. 

2. Magmatic Water - Also known as Juvenile water. 

    Water that exist within and in equilibrium with a magma or water rich volatiles fluid that 

    are derived from magma. it has high mineral content.

3. Meteoric water - It is a water deriveed from prcipitation (snow and rain). this include
    water from lake, river and icemelts which all originate precipitation directly. This water is in circulation. 

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Except this there are some terminologies used in hydrogeology related to water as below.

• Vadose water - Denoting underground water above he main water table.
• Metamorphic water - Water that is or has been associated with rocks during their metamorphism.

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Residence time of Groundwater- 

The residense time of water underground has always been a topic of considerable speculations. But with the help of radioisotopes, determination of the age of groundwater have become possible.

Hydrogen 3 (3H) Tritium and carbon 14 (¹⁴C) are the two isotopes that have proved most useful.

1) Tritium with half life of 12.33 years is produced in the upper atmosphere by cosmic radiation; carried to earth by rainfall and hence underground, this natural level of tritium begins to decay as a function of time, such that 

A = A₀ e^-λt

Where 

A is the observed radioactivity,
A₀ is the activity at the time water entered the aquifer,
λ is the decay constant  and 
t is the age of water.

Tritium is applicable for estimating groundwater residence times of upto 50 years.

2) Carbon 14 has a half life of 5730 years only and is also produced at an established constant level in the atmosphere. this isotope is present in groundwater as dissolved bicorbonate originating from the biologically active layers of the soil where CO2 is generated by root respiration and the decay of humus.

carbon 14 spans the age bracket of a several hundred to abount 50,000 years.