Water Provides Clues to Suspect Whereabouts
University of Utah scientists have developed a new crime-fighting tool by showing that human hair reveals the general location where a person drank water, helping police track past movements of criminal suspects or unidentified murder victims.
"You are what you eat and drink – and that is recorded in your hair," says geochemist Thure Cerling, who led the research effort with ecologist Jim Ehleringer.
Their findings were published online Feb. 25 in the journal Proceedings of the National Academy of Sciences. The new hair analysis method also may prove useful to anthropologists, archaeologists, and medical doctors.
"We have found significant variations in hydrogen and oxygen isotopes in hair and water that relate to where a person lives in the United States," Ehleringer says. "Police are already using this to reconstruct the possible origins of unidentified murder victims."
Isotopes are forms of the same chemical element with different atomic weights. Stable isotopes are those that do not decay radioactively.
Ehleringer is a distinguished professor of biology at the University of Utah, and Cerling is a distinguished professor of geology and geophysics, and of biology. Three years ago, they co-founded IsoForensics, Inc., a company that is using stable isotope analysis of forensic substances to find slight variations in a chemical element's various isotopes.
Ehleringer previously developed a method now used by the U.S. Drug Enforcement Administration to help learn where cocaine or heroin were produced based on local variations in carbon, nitrogen, oxygen and hydrogen isotopes absorbed into coca and poppy plants from soil and water. He also analyzed hydrogen and oxygen isotopes to help track counterfeit $100 bills, based on the water used to grow the cotton with which the bills were made.
The new technique analyzes stable isotopes of hydrogen (rare hydrogen-2 and common hydrogen-1) and oxygen (rare oxygen-18 and common oxygen-16) incorporated in growing hair from water and food that a person consumes and from air they breathe.
The ratio of oxygen-18 to oxygen-16 in the air is the same everywhere, and Americans tend to get similar oxygen and hydrogen isotope ratios from food because the U.S. diet is becoming homogenized.
The study found a strong correlation between hydrogen and oxygen isotope levels in hair and drinking water; 85 percent of the variation in isotope levels in a person's hair was explained by variations in drinking water isotope levels in areas where they spent time.
So a single hair can help determine a person's location during recent weeks to years, depending on the length of the hair sample and thus how much time it took to grow.
The scientists used the method to produce color-coded maps showing how ratios of hydrogen and oxygen isotopes in scalp hair vary in different areas of the United States.
The maps cannot pinpoint a person's exact locations in the past, but identifies general geographic areas where they stayed and drank local water.
"You can tell the difference between Utah and Texas," Ehleringer says. But, Cerling adds, "You may not be able to distinguish between Chicago and Kansas City."
The maps were based on isotope analyses of hair and water samples collected from barbershops and tap water in 65 cities in 18 states across the United States. Each city had 100,000 or fewer people to ensure that hair collected from barbershops most likely came from local residents than from tourists visiting a big city.
Why do isotopes in water – made of hydrogen and oxygen – vary with geography?
As clouds move off the ocean and onto the continent, rain water with oxygen-18 and hydrogen-2 tends to fall first because it is heavier. That should make ratios of oxygen-18 to oxygen-16 and hydrogen-2 to hydrogen-1 higher near the coast and lower farther inland.
But other factors also are important, including cloud temperatures, the season during which rain falls, and the amount of water that evaporates from soil and plants.
These factors explain why oxygen-18 and hydrogen-2 levels in drinking water decrease rapidly moving inland from the West Coast (where winter storms are cold) and remain high inland from the Gulf and southern Atlantic coasts (where clouds are warmer).
Isotope concentrations vary seasonally in rain and snow, but drinking water in reservoirs or groundwater represents a region's average precipitation over time and space.
Drinking water from any area has an isotope signature that is incorporated into growing hair. That signature is not complicated by other beverages because "a significant fraction of beer, soft drinks and milk is local in its origin," Ehleringer says.
The study found that areas where drinking water and hair have the lowest concentrations of hydrogen-2 and oxygen-18 include northern and western Montana, north-central Idaho and northwest Wyoming. The heavy isotopes drop out of clouds before they reach those inland areas because of both the cold and the distance from the Pacific Ocean.
Regions with the highest amounts of hydrogen-2 and oxygen-18 in drinking water and hair were southern Oklahoma, north-central Texas, Florida, south Georgia and southern South Carolina. Heavy isotope levels are high there because most water falls as summer rain and some evaporates from lakes, leaving more of the heavier isotopes behind in drinking water.