Paradox of groundwater age has implications for hydrology

December 03, 2001

CHAMPAIGN, Ill. -- How old is your groundwater? Chances are, it's much older than you, or many scientists, had thought.

Recent work by two University of Illinois geologists, Craig Bethke and Thomas Johnson, has shown that groundwater in aquifers is generally older than expected on the basis of flow velocity. That means that flow rates along an aquifer are higher than predicted by age measurements.

While of little consequence when taking a sip from the tap, the researchers' findings have important implications when using radiometric methods to estimate the sustainable yield of a water supply, for example, or to predict the rate at which a contaminant will migrate through the ground.

Groundwater tends to flow through aquifers that are constrained by layers of less permeable rock called aquitards. Hydrologists commonly figure that a groundwater's age reflects the time it takes to migrate along the aquifer - conceptually like the time it takes for water molecules to flow through a pipe.

"But water molecules don't see an aquifer as a pipe," Bethke said. "Some water mixes between the aquitards and aquifers, and the water in aquitards is generally very old."

Bethke and Johnson have shown that the effect of aquitards on the age of groundwater depends only upon the ratio of water mass involved, not on the mixing rate. "At low mixing rates, very old water is supplied to the aquifer, but the water in the aquitard remains old," Bethke said. "At high mixing rates, less-old water is supplied to the aquifer, because younger water is moving into the aquitard."

While mixing increases the age in aquifers, it also has the counter-balancing effect of decreasing the age in aquitards. "The two effects exactly cancel," Bethke said. "This is the paradox of groundwater age, and it has broad implications in groundwater hydrology."

Like carbon-14 dating of artifacts, scientists can use radioactive isotopes to age-date groundwater. "If a sample proves to be very old, we might conclude that it has been migrating very slowly through the aquifer," Johnson said. "However, the water may have migrated quickly, and only appears very old because of the effect of the surrounding aquitards."

Such a situation could be particularly troublesome when injecting toxic chemicals in the ground for disposal, as the waste could enter the biosphere much faster than anticipated. Incorrect age/flow determinations also could allow landfill leachate to contaminate water supply wells.

"Because the rate of mixing doesn't matter, the contribution of aquitards to the age of groundwater in aquifers depends only on the ratio of water mass in aquitards to that in aquifers," Bethke said. "You can think of groundwater age as a quantity distributed by diffusion, dispersion and cross-formational flow."
Bethke will discuss the paradox of groundwater age at the American Geophysical Union meeting in San Francisco, Dec. 10-14. The National Science Foundation supported his work.

University of Illinois at Urbana-Champaign

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