The two methods could prove useful for gauging how rising sea levels -- one of the possible effects of global climate change -- might cause salt water to infiltrate drinking water along coastal areas in the future.
Anne Carey, assistant professor of geological sciences at Ohio State, likened Baldwin County in southwestern Alabama -- where the study was conducted -- to the Chesapeake Bay area, where rising seawater has already covered some islands and ruined agriculture on others.
"Sea level is rising in places where coastal development is rapid," Carey said. "Some wells have been abandoned in Baldwin County due to salt water intrusion. Increased water usage and sea-level rise are likely to exacerbate the problem."
To map water usage, geologists often measure the age of water taken from different sites around a region. The age suggests how quickly rainwater renews the water supply, and how quickly seawater could potentially enter the system.
The Alabama site was ideal for Carolyn Dowling, a post-doctoral investigator with Ohio State's Byrd Polar Research Center, to compare two different methods of water dating for her doctoral dissertation. One was the well-known radiocarbon dating, which measures the presence of the isotope carbon-14, while the other was a lesser-known method that measured the isotope helium-4.
Though scientists long thought the two methods were incompatible, Carey and Dowling successfully used both together to determine that ages in different wells ranged from 50 years to 7,500 years. The results appear in a recent issue of the Geological Society of America journal Geology.
Carbon dating placed the ages of water from different wells in a range of approximately 375 to 7,000 years old, and the helium method suggested a similar range -- 50 to 7,500 years old.
To Carey, the ages themselves are not particularly surprising.
"This isn't really, really old water -- it's all from the Holocene, the period since the last ice age," she said. "In northern Ohio, there are places where people are pumping Pleistocene water [more than 10,000 years old]."
"It was startling to the well operators that their waters were that old, but it isn't a startling story geologically," she continued. "The important part of the research is that we could show the nice correspondence between the two methods, which has never been done before."
Any prediction of how quickly global climate change could cause salt water to infiltrate Baldwin County would require further study, Carey said. But this early work shows that scientists can use both dating methods simultaneously to get a more reliable view of water usage.
"Any time you can measure something with two different methods, you can be more confident in the results," she said.
Water usage in Baldwin County surges during the spring and summer tourist seasons, when turf grass farms also draw on freshwater supplies for irrigation. More than 20 percent of the county is water, and its extensive lowlands would make it particularly susceptible to flooding, should water levels rise in the Gulf of Mexico.
That's why the Department of Energy's National Institute for Global Environmental Change was interested in the region, and commissioned the study.
Carey, Dowling, and colleague Robert Poreda at the University of Rochester tested the water from 12 wells around Baldwin County using both methods.
Carbon dating measures how much carbon-14 is left in the water since the last time the water contacted carbon dioxide in the air. Helium dating measures how much helium has dissolved into the water from surrounding rock as it lay underground. Both give scientists a measure of how long water has been in a particular well or aquifer.
The results also suggested that the Baldwin County well operators are doing a good job of drawing fresh water from the right places at the right time to keep salt water from entering the system for now.
Saltwater infiltration could become a bigger issue in the future with global climate change.
Over the last 100 years, sea level worldwide has risen an average of 2 millimeters per year. The melting of tropical glaciers and polar ice caps, which scientists have documented in recent years, could increase that rate. A warmer climate would also heat the oceans, causing them to expand -- and sea level to rise further.
Written by Pam Frost Gorder, 614-292-9475; Gorder.firstname.lastname@example.org.