Perchlorate is widespread in U.S. Southwest.

that disrupts thyroid function by inhibiting the uptake of iodide, is best known for its use in rocket fuel but also occurs naturally in the environment. Since the beginning of the last century, Chilean nitrate fertilizers that contained naturally produced perchlorate were applied to U.S. agricultural fields. Today, perchlorate hot spots occur, some of which seem to extend farther than what is predicted from known contamination sources. Researchers have recently turned to isotopic analysis to help pinpoint the origins of perchlorate, because manufactured and atmospherically deposited perchlorate have very different isotope signatures. Andrew Jackson of Texas Tech University and colleagues took a different tack to examine atmospherically derived perchlorate in the Southwest’s dry unsaturated or vadose zones. They collected 10–20 grams of soil to get enough perchlorate from 11 sites across seven ecosystems. Isotopic analyses would have required tons of soil. Previous research led by coauthor Michelle Walvoord of the U.S. Geological Survey (USGS) examined many of the same sites for