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One of the criteria that make a Scotch a Scotch is that it's made with Scottish water. That means that a clever connoisseur should be able to tell whether her drink is authentic by tracing the source of its H2O molecules. But how? After it's collected and filtered of impurities, water is water, right?
Not exactly. The elements hydrogen and oxygen appear in nature in various forms, called isotopes, which differ in neutron number. By measuring the relative abundance of these isotopes in a water sample using a technique called mass spectrometry, scientists can determine its "isotope ratio." And as it turns out, the isotope ratio of water varies dramatically from place to place.
By the 1990s, the U.S. Geological Survey had amassed a collection of nearly 5,000 isotope measurements of river water and precipitation from around the country. When hydrologist Carol Kendall and her colleague Tyler Coplen analyzed these data, they confirmed a clear pattern: The proportion of heavier water isotopes decreased at higher latitudes, higher elevations, and further from the coasts. In 2001, they created the first detailed map of this variation, called an isoscape, filling in the spaces between…
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