Groundwater Age

Water that infiltrates the landscape moves downward to the water table as recharge to the aquifer system. As additional recharge continues to enter the aquifer, older recharge is pushed deeper by the newer recharge, resulting in a trend of increasing groundwater age with depth. Groundwater moves slowly—a flow rate of 1 foot per day is fast for groundwater, and flow rates can be as low as 1 foot per year or 1 foot per decade. It can take tens, hundreds, or even thousands of years for groundwater to travel through an aquifer.

Why does groundwater age matter? Young groundwater is more likely than old groundwater to have contaminants from recent manmade sources, such as pesticides, nitrate, and solvents, because those chemicals were applied to or released on the landscape when the young groundwater recharged the aquifer. For example, water that entered the aquifer after 1950 is more likely than older water to contain the herbicide atrazine, whose use has increased since that time. On the other hand, old groundwater is more likely than young groundwater to have contaminants from natural sources, such as metals and radionuclides, because old groundwater can spend thousands of years in contact with and reacting with aquifer rocks and minerals that might contain these elements. The geochemical processes that frequently occur in old water, such as redox reactions, can profoundly affect groundwater quality.

Groundwater usually is young—often only a few decades old—in shallow, unconfined aquifers with high rates of recharge.  This recharge can be driven by precipitation, like in the eastern U.S., or by human applications of water for irrigation, like in parts of the western US. Groundwater can be thousands of years old in aquifers where recharge rates are low (arid regions), where the aquifer is very thick, or where aquifers are separated by confining units.

Try the Tools

Within the same aquifer, groundwater that is shallow and near the recharge area is younger than groundwater that is deep or that has moved far from the area where recharge occurs. Because wells are typically screened across long segments of aquifer, water from wells is often a mixture of many different ages.  The tools below can aid in learning more about groundwater that is a mixture of ages.

• Groundwater Age Mixtures and Contaminant Trends Tool: Use the GAMCTT tool to explore how basic aquifer properties and well configurations affect groundwater age mixtures in groundwater discharge and on contaminant trends from nonpoint-source contaminant input scenarios. 
• TracerLPM: Teasing out the distribution of groundwater ages in a single groundwater sample is a thorny task, but has been made easier by the development of the tool TracerLPM, an Excel workbook for interpreting groundwater age distributions from environmental tracer data.
• Dissolved Gas Modeling and Environmental Tracer Analysis (DGMETA) Tool: Use DGMETA to compute recharge conditions from dissolved gases and environmental tracer concentrations commonly used for determining groundwater ages and reaction rates.

Determining Groundwater Age

Groundwater age is determined from the measurement of age “tracers”, chemical or isotopic constituents dissolved in the groundwater. These tracers include naturally occurring isotopes, which decay at a known rate; isotopes that were introduced into the atmosphere at known times relating to nuclear tests; and manufactured gases whose concentration in the atmosphere over time is known.

Young groundwater is commonly defined as water that entered the aquifer since about 1950 because several chemical and isotopic substances related to human activities were released into the atmosphere since that time.  The presence of these substances in groundwater tell us that the water is young. These substances include tritium (3H), which was released into the atmosphere by detonation of nuclear bombs in the 1950s and early 1960s, chlorofluorocarbons (CFCs), which were released into the atmosphere from refrigeration and other uses from the 1930s through the 1980s, and sulfur hexafluoride (SF₆), which is used primarily in electrical equipment and manufacturing semiconductors and whose use has been increasing steadily since about 1965. These age-dating tracers can help water-resource managers to develop management strategies for shallow groundwater systems that contain mostly young groundwater.

Old groundwater is defined as water that entered the aquifer before 1950 and more commonly refers to water older than 1,000 years. Many common and rare isotopes are produced naturally in the Earth’s atmosphere from the bombardment of cosmic rays or solar radiation, and their presence in groundwater can help determine the groundwater age. These isotopes are adsorbed by rainfall and can enter the aquifer with recharge. Argon-39 can be used to identify water that recharged between 50 and 1,000 years ago. Carbon-14 or radiocarbon is the most common method used to determine groundwater ages between 1,000 and 30,000 years. Groundwater older than 30,000 years can be determined using isotopes like helium-4, which is produced from the decay of uranium and thorium in aquifer solids, or by chlorine-36 and krypton-81, which decay over extremely long timescales and thus are useful for determining the age of ancient groundwater—hundreds of thousands of years old or more.

Resources on age dating groundwater can be found at the USGS Reston Groundwater Dating Laboratory web page.

Go to the Publications tab to read about USGS research that uses groundwater age dating.