Blog post: swansont: I Know What You Did Last Summer, Based on Isotopic Ratios in Your Hair and Fingernails

[L]et's follow a raindrop from the Pacific Ocean to Saltair Sally's strand of hair. When water evaporates from the ocean, the heavier molecules containing deuterium and tritium and oxygen-18 do not evaporate as readily and are left behind in greater numbers. As droplets gather into clouds and eventually fall as rain, the heavier molecules fall first. This means rainwater in regions closer to oceans and large lakes is isotopically enriched compared with regions farther inland. Isotope levels are again fractionated in drinking water depending on whether it is drawn from wells or reservoirs (lighter isotopes more easily evaporate from the surface of reservoirs). Currently, scientists are busily creating maps of various isotopes' distributions to assist investigators.

If Saltair Sally had been in Salt Lake City in the weeks preceding her death, the hair closest to her scalp would reflect the isotopic signature of Salt Lake City's local water supply. If she had been in, say, Seattle instead, her hair's isotopic composition would be different, giving investigators a valuable clue.