THE DUNBAR-HUNTER EXPEDITION (1804-1805): EARLY ANALYSES OF SPRING WATERS IN THE LOUISIANA PURCHASE

In the years following the Louisiana Purchase Treaty with France in 1803, U.S. President Thomas Jefferson commissioned four expeditions to explore the new territory. Best known of these, of course, was the Lewis and Clark expedition of 1804-1806 across the northern margin of the Purchase. Of some interest in the history of hydrogeology is the second expedition Jefferson commissioned, the Dunbar-Hunter expedition of 1804-1805 up the Ouachita (Washita) River in what is now Louisiana and Arkansas to the hot springs of central Arkansas. On the way upriver, George Hunter sampled and made measurements on saline spring waters approximately 3 km southeast of the present town of Arkadelphia. Both Hunter and William Dunbar detected the bitter taste of calcium and/or magnesium chloride. This is significant because the subsurface hypersaline waters of southern Arkansas are known to be enriched in Ca and Mg.

The explorers reached the hot springs in December 1804. Hunter made qualitative measurements of the chemistry of the hot waters using reagents he had brought with him. His analyses are broadly consistent with what we know today: the waters have a low TDS content, are dominated by Ca-HCO₃, have moderate levels of sulfate, and are low in chloride. They contain trace amounts of Fe and have a neutral pH. The physical model that Hunter and Dunbar invoked in their journals for the origin of the hot springs has its origin in medieval thought on the nature of the hydrologic cycle. It was thought that seawater made its way into the interior of the earth, where it was distilled by local sources of heat. The vapors condensed in caverns in mountains to form spring waters. Dunbar attempted to measure the total discharge rate of the springs by noting the time it took a large spring to fill a kettle with water and extrapolating the results to other springs. His calculated results are equivalent to 240,000 gal/day, which is surprisingly close to modern measurements of 750,000 to 950,000 gal/day. Material from the reports sent Jefferson after the expedition was published in three different editions in 1806. Early public interest in the springs, which was contributed to in great measure by the Dunbar-Hunter reports, led to Federal management of the area in 1832 as the Hot Springs Reservation.