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 ground water 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 be
