Although we often refer to dating of groundwater, we are actually dating a chemical substance that is dissolved in the groundwater, not the water itself. Rather than referring to groundwater dating, it would probably be more correct to say we are interpreting temporal aspects of chemical and isotopic substances in groundwater. Isotopes can be separated into environmental isotopes, which are found in the groundwater, and isotopes that are introduced into the ground as a part of a groundwater study.
Environmental isotopes can be either radioactive or stable. They can be used to determine the locations of groundwater recharge areas, circulation patterns in aquifers, sources of dissolved solids in groundwater, and the age of groundwater- the length of time it has been out of contact with the atmosphere.
Various environmental isotopes and tracers are used to determine the age of groundwater. Carbon-14 is used to date groundwater older than 1000 years. Chlorofluorocarbons (Freon) and tritium techniques are used to date groundwater that is less than 50 years old.
Theoretically, ages can be estimated by (1) the travel time of groundwater from the point of recharge to the subsurface point of interest as calculated by Darcy's law combined with an equation of continuity, (2) the decay of radionuclides which have entered the water from contact with the atmosphere, (3) the accumulation of products of radioactive reactions in the subsurface, (4) the degree of disequilibrium between radionuclides and their radioactive daughter products, (5) the time-dependent changes in the molecular structure of compounds dissolved in water, (6) the presence of man-made materials in groundwater, (7) the correlation of paleoclimatic indicators in the water with the known chronology of past climates, and (8) the presence or absence of ions which can be related to past geologic events that have been previously dated. Owing to uncertainties in each of the methods, as many methods as possible should be used in every field situation.
The residence time of water underground has always been a topic of considerable speculation. But with the advent of radioisotopes, determination of the age of groundwater has become possible.
Hydrologists employ a variety of techniques to measure groundwater age. For relatively young groundwater, chlorofluorocarbons (CFCs) often are used. CFCs are human-made compounds that are stable in the environment. Atmospheric CFC concentrations increased from the time of their development in the 1930s until the 1990s, and hydrologists now know how atmospheric CFC concentrations have changed over time.
CFCs can be used to determine groundwater age because water that is in contact with the atmosphere picks up CFCs from the atmosphere. Thus, CFCs are incorporated in the water before it enters an aquifer. Once water enters an aquifer, it becomes isolated from the atmosphere, and it carries a CFC signature (a distinctive chemical composition) as it travels through the aquifer. This signature reflects the atmospheric concentration when the water was at the surface. By measuring the CFC concentration in groundwater, hydrologists know how long ago the water entered the aquifer.
In the United States and other developed countries, CFCs are being phased out of use because they contribute to atmospheric ozone depletion. As a consequence, atmospheric CFC concentrations have begun to decrease. Atmospheric concentrations of CFCs are not expected to decrease quickly, so CFC dating will continue to work for most young groundwater for many years to come. However, for very young groundwater (groundwater entering aquifers after the late 1990s), CFC dating soon will yield ambiguous results.
Sulfur hexafluoride (SF 6 )-
Hydrologists recently have developed another dating technique that may ultimately replace CFC dating. The new technique uses sulfur hexafluoride (SF 6 ) concentrations in groundwater to determine groundwater age. SF 6 is another stable, human-made compound that has exhibited increasing concentrations in the atmosphere. Unlike CFC concentrations, atmospheric SF 6 concentrations are expected to increase for the foreseeable future. The method, although relatively new, shows promise.
Tritium, 3H, is an unstable isotope of hydrogen with a half life of 12.4 years. It 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 the water entered the aquifer, λ is the decay constant, and “t” is the age of the water.
Tritium in the atmosphere is typically in the form of the molecule H3 HO and enters the groundwater as recharging precipitation. Prior to 1953, rainwater had less than 10 tritium units (TU). Starting in 1953, the manufacturing and testing of nuclear weapons have increased the amount of tritium in the atmosphere, with a resulting increasing in tritium in the groundwater. As a result 3H can be used in a qualitative manner to date groundwater in the sense that groundwater with less than 2 to 4 TU is dated prior to 1953 and if the amount is significantly greater than 10 to 20 TU it has been in the contact with the atmosphere since 1953.
Tritium has been used to trace the seepage of contaminated groundwater from low-level nuclear waste disposal areas.
Radiocarbon dating of groundwater (Carbon-14)-
Radiocarbon dating methods can be applied to obtain the age of groundwater. Carbon exists in several naturally occurring isotopes, 12 C, 13 C and 14 C.
Like tritium, carbon 14 is produced in the atmosphere by interaction of cosmic rays with nitrogen, and was introduced in large amounts by nuclear weapons testing. Unlike tritium 14 C has a half life of 5730 years, making it a useful tool for dating water as old as 50,000 years. The 14 C generated in the atmosphere is carried down to the earth’s surface by precipitation, and becomes incorporated into the biomass or transported into water bodies such as lakes, the ocean and groundwater. 14 C undergoes radioactive decay (to 14 N), so that once isolated from the atmosphere, the amount of 14 C decreases with time according to the equation
(14 C)t = (14C)0e –Kt
Where (14 C)t is the amount present at time “t”, (14C)0 is the amount present at t = 0, and K is the decay constant, which is related to the half-life T1/2 by the equation
T1/2 = In2/k
To determine the time since a water was last in contact with atmosphere, it is necessary to know (14 C)0. This is determined by tree rings for the most recent 7000 years; there is no accurate way to determine it prior to 7000 years, so it is generally assumed arbitrarily to have been constant. This gives rise to a time scale in “14 C years”, which may be different from astronomical years.
There are some complications in the behavior of 14 C during recharge, so that the “absolute” age of a groundwater cannot be determined reliably. However, if the 14 C concentration is measured at several points along a flow line within an aquifer, the differences in age between the points and hence the flow velocity can be determined. One complication is that dissolution of carbonate minerals or oxidation of organic matter within an aquifer may add “old” or “dead” (no detectable 14 C) carbon to the water and give an erroneously old age. The contribution of carbon from these sources can sometimes be estimated from 13 C/ 12 C measurements and chemical arguments, so that corrections can be made. Another complication is mixing. A low 14 C concentration may mean that we are looking at relatively “old” water, or it may mean that we are looking at a mixture of relatively “young” water and “dead” water. 14 C measurements can be interpreted as ages only when mixing is insignificant.
Measurements of water samples taken from deep wells in deserts of the United Arab Republic and Saudi Arabia indicate ages of 20,000 to 30,000 years. This period is compatible with the Wisconsin Ice Age, when these desert areas last had high rainfall capable of recharging the underlying major aquifers.
Drever, J.I.(1982). The Geochemistry of Natural Waters. Prentice-Hall, Englewood cliffs, NJ.
Thatcher, L. et.al. (1961). Dating desert groundwater. Science 134(3472): 105-106.
Todd, D.K. (1995). Groundwater Hydrology. Wiley, Toronto, pp. 24-25.
Wigley, T.M.L. (1975). Carbon-14 dating of groundwater from closed and open systems. Water Resour. Res. 11: 324-328.
Wigley, T.M.L. (1976). Effect of mineral precipitation on isotopic composition and 14C dating of groundwater. Nature 263: 219-221.