Groundwater Age Dating

By Darrel Dunn, Ph.D., PE, Hydrogeologist

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The age of groundwater ranges from less than a month to more than a million years.  Figure 1 shows some of the methods for dating the age of groundwater and the age for which each method is applicable.

Bar chart of age range for groundwater age dating isotopes.

Figure 1.  Isotope methods for dating old groundwater.  (From International Atomic Energy Agency Publication 1587.)

Argon (Ar) is a dissolved gas, so sample collection without dissolution of the gas involves special equipment used with great care.  The helium (4He) method also requires dealing with a dissolved gas, plus the interpretation of the laboratory results is complicated.  Sample collection for Carbon-14 (radiocarbon) is about as easy as filling a bottle.  However, analysis of laboratory results is complicated by chemical reaction between carbon in the groundwater and carbon in such rocks as limestone.  Post-processing of laboratory results to adjust for the carbon reactions increases uncertainty.

Interpretation of laboratory results for groundwater age is very complex.  One problem is that almost all groundwater samples are mixtures of water of varying age, combining all of the flowlines reaching a well or spring.  Another problem is that the groundwater may contain a very small proportion of connate water diffused from clay or other low permeable material.  If the connate water is extremely old even a very low admixture can cause the water to appear to be much older than it really is.  There are many other problems.

Another way to estimate the age of groundwater is to develop a computer model of the system and use particle tracking.  It might be useful to compare particle tracking results to the results of dating the age of a water sample.