The age of groundwater is defined as the time that has elapsed since the water first entered the aquifer . For example, some of the rain that falls on an area percolates (trickles) down through soil and rock until it reaches the water table . Once this water reaches the water table, it moves though the aquifer. The time it takes to travel to a given location, known as the groundwater age, can vary from days to thousands of years.
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.
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.
CFCs and SF 6 are useful tools for determining groundwater ages in the range of years to decades, but what do hydrologists use to measure groundwater ages in the range of thousands of years? For very old groundwater, carbon-14 dating often is used. As water from atmospheric precipitation falls on the Earth's surface and percolates through soil and rock on its way to an aquifer, it dissolves carbon. This carbon includes small amounts of radioactive carbon-14, which originally formed in the upper atmosphere by nuclear reactions caused by the impacting of solar and cosmic particles on terrestrial atmospheric gases. Hydrologists know the rate of decay of carbon-14, so by measuring differences in groundwater carbon-14 in an aquifer, they can calculate groundwater ages. Because the half-life of carbon-14 is long (5,730 years), this method is useful for determining the age of groundwater between about 1,000 and 30,000 years old.
A common complication of carbon-14 dating is the introduction of additional, carbon-14–free carbon (carbon containing no carbon-14) from the aquifer. For example, when limestone dissolves, carbon-14–free carbon (or "dead" carbon) is released to groundwater. Hydrologists account for this dilution before calculating carbon-14 groundwater ages.
In one classic example of carbon-14 dating, groundwater ages in the Madison Aquifer in parts of Montana, Wyoming, and South Dakota were shown to range from modern (in this context, since the 1950s) to over 20,000 years. Given that groundwater can be quite old, it becomes apparent that activities that cause groundwater pollution today can result in long-term resource problems—problems that have the potential to last for millennia.
Stephen R. Hinkle
Cook, Peter, and Andrew Herczeg, eds. Environmental Tracers in Subsurface Hydrology. Boston, MA: Kluwer Academic Publishers, 2000.
Plummer, L. Niel et al. "Geochemical Modeling of the Madison Aquifer in Parts of Montana, Wyoming, and South Dakota." Water Resources Research 26 (1990): 1981–2014.