Answer: Traces Of Mercury

If you were to think of ways to retrace the path that the famed Lewis and Clark expedition took across the American landscape in their bold early 19th century quest to be the first Americans to travel to the Pacific Ocean, surely you’d think rather pragmatically. Maybe they took such good measurements and kept such good logs, you could simply walk the route they described (they certainly kept good notes, but that wouldn’t help us locate the exact places members of their party stood on today). Maybe they marked trees every few miles and branded a tree every time they struck camp for the evening. One thing that probably didn’t cross your mind was that maybe they used incredibly potent mercury laxatives that indefinitely contaminated the soil they dug up to use as temporary latrines.

Yet that’s exactly the kind of hard environmental evidence modern scientists can use to pinpoint exactly where on the described trail the Lewis and Clark expedition physically were. Among the many medicines they packed for the expedition—including zinc sulfate to treat eye conditions, magnesia in case of poisoning or illness, and laudanum, an opiate based pain killer—they also packed 600 of “Dr. Rush’s Bilious Pills”. The good doctor—a prominent American physician and Declaration of Independence signatory—crafted the pills out of calomel (mercurous chloride, six parts mercury to one part chlorine) and jalap (a plant root of Mexican origin historically used in medicine).

Now, why would anyone take such a pill? The pills were extremely potent laxatives. Elemental mercury is an intense laxative as is jalap, and the combination of the two could defeat exactly the kind of constipation a group of men living off hardtack biscuits and wild game meat would expect to encounter. Mercury doesn’t break down in the environment and the soil they passed over, as it were, is to this day contaminated with the mercury from their laxatives.