USGS - Groundwater Age - 2019

Hydrogeology, Geology, & Environmental Issues / Field Methods & Procedures

Jurgens reported in 2019 that the age of groundwater is key in predicting which contaminants it might contain. There are many tracers and techniques that allow us to estimate the age—or mix of ages—of the groundwater we depend on as a drinking water supply. 

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 also 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. 

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 (here).

[This webpage offers many tools for professional hydrogeological investigations.]