Chris Stantis, assistant professor in the Department of Anthropology, helped lead a recent survey and analysis of tap water from 30 cities across the U.S. to compare the variability of oxygen-stable isotopes. The work will help forensic investigators in identification cases. (Photo by Russell Bailey)
November 19, 2024
SIU researcher analyzes water isotopes, refining a method to solving mysteries
CARBONDALE, Ill. – While the sleuths on popular forensic crime shows make solving a case with science look easier than it actually is, a researcher at Southern Illinois University Carbondale is refining those methods using chemical analysis of local tap waters.
Chris Stantis, assistant professor in the Department of Anthropology, helped lead a recent survey and analysis of tap water from 30 cities across the U.S. The resulting paper, which compares the variability of oxygen-stable isotopes in the various samples, recently was published in the journal PLOS One.
Because the body’s oxygen isotopes signify the water the person drinks when forming tissues, those telltale signs have long been to study migration patterns in archeological studies and increasingly with success in forensic identification investigations. But researchers wanted to examine how much variation exists in such isotopic values in any given place, which could significantly impact the accuracy of such tests.
Analyzing more than 4,000 samples and bringing her expertise in statistical analysis to bear, Stantis examined how variable oxygen-stable isotope values were in tap water samples from diverse locations. Nailing down those variations could in turn help investigators identify the body of a missing person by indicating where that person grew up.
A tell-tale signature
Three stable isotopes occur naturally in all waters. Of those three, two are abundant and easily measured using mass spectrometry methods.
Scientists for years have used these oxygen isotopes, along with two hydrogen ones that also occur in water, to study basic hydrological cycles and other phenomena, including groundwater recharge, the origin of precipitation and runoff dynamics.
The isotopes in water, however, also provide important clues about where people and animals originate and live. Forensic investigators more recently have used these “tags” to help identify missing persons, as well as solve crimes.
While many researchers have looked at the average and mean values of isotopes found in hair and other tissue, Stantis’ work aims to take this approach a few steps further.
“We knew there was a lot of complexity underlying this method that remained to be unexplored,” she said. “As we dug deeper, we saw huge variability from city to city.”
That issue was potentially troubling, Stantis said.
“If there's huge variability in a certain city regarding tap water oxygen stable isotope values, we’re not sure we can confidently say that someone comes from that city based on the values found in their hair,” she said. “Two people from the same city could have really different values.”
Important differences
The researchers found that forensic investigators need to be aware of the variations, as several factors can result in multiple isotope values within the same city.
In Atlanta, for example, tap water mostly originates from one river system. The city’s treatment centers and reservoirs, however, alter the isotope values in water in different ways, creating variability.
“In Portland, there isn’t much variation, so we can reach conclusions with high confidence,” Stantis said. “But in places such as San Francisco and Atlanta, there is high variability, while Los Angeles is really complicated, due to its increasingly complex water-sourcing system.
“So, trying to use this method in those cities might be much more difficult, and investigators should consider those factors.”
Major undertaking
The project has roots stretching back to 2013, when some of the first samples were originally collected for other studies. The research team eventually amassed 4,068 samples across dozens of cities, with Stantis starting her analysis in 2022 while at the University of Utah before coming to SIU.
“At first, we wanted to know what tap water data was even out there,” she said. “But as this data came together and questions formed, I took over the data cleaning and analysis, both of which proved to be enormous tasks.”
On top of the various isotope factors, the researchers also took into account potential climatic, environmental and socioeconomic variables that might impact the findings. Along with her expertise in statistical analysis and coding, Stantis also brought the perspective of an anthropologist to the effort.
“As an anthropologist, I also tend to think about all of the different factors that affect human environments,” she said. “I look at the cultural-economic, of course, but also physical factors like elevation and available water. Pulling together tons of different datasets from the U.S. Geological Survey, the U.S. Census, and ecological and hydrological papers meant shifting gears a lot of times for how different researchers view and analyze the world.”
Stantis said bringing order and analysis to the chaotic variability was a significant challenge.
“It was possibly the biggest headache I've ever encountered as a researcher,” she said, with the custom computer analysis requiring her to write what ended up as almost 2,000 lines of code, which, in turn, took many days’ worth of computer-chewing to produce results.
The resulting insights will pay dividends, Stantis said, and point to the large amount of work still to be done.
“There's still a lot of underlying causes of variability to explore across the country,” she said. “We can also now see just how much data are missing from big cities like Chicago and New York City, which were so underrepresented in terms of tap water samples that we couldn't even examine these places where millions of people live.”