Hydrology of Hot Springs National Park
Learn more about the hydrology of Hot Springs National Park.
Hot Springs Sandstone of the Early Mississippian Period generally ranges from 10 to 250 feet in thickness. The thermal springs found in Hot Springs National Park originate exclusively from fracture zones in this sandstone. While there are other minor springs, the majority of the springs found within the park originate from the northwest limp and axis of the Hot Springs Mountain Anticline. The hot springs flow upward through large, open fractures and are effectively sealed by the overlying Stanley Shale.
When many people think of hot springs, there is often the association of volcanoes, geysers, and perhaps underground chambers of magma. While these features are often found in association with many thermal springs around the world, this is not the case in Hot Springs National Park. In Central Arkansas, the earth is relatively quiet. There is no evidence of magma beneath the surface to heat the thermal water. Instead, many geologists believe that the right combination of rock types and fractures allow for water to travel from the surface to areas deep within the Earth. As water percolates down into the earth, it is then heated by surrounding rock. In Hot Springs National Park, this process takes approximately 4,000 years! That’s right. The water collected and distribute today in our park was once rainwater over 4,000 years ago. Thanks to the highly porous Bigfork Chert and Arkansas Novaculite, rainwater can travel down these conduits to depths between 2,000 and 8,000 feet.
A natural thermal gradient heats the water. As the water moves deeper within the Earth, the hotter the water becomes. At great depths, the heated water meets fractures and faults in the Hot Springs Sandstone, and the water is quickly brought to the surface as one of our many thermal hot springs. On average, the water arriving to the surface is approximately 143 degrees Fahrenheit.
Most springs found within Hot Springs National Park are capped by green collection boxes, which can be found throughout the park. Beneath the lids, one will find stainless steel cylinders connected to the springs. While the springs previously flowed without containment, caps are now essential and necessary to preserve the purity and health of our natural thermal water. The thermal water is then routed to a larger reservoir beneath bathhouse row in Hot Springs National Park to be redistributed for public use.
In addition to capping the most valuable resource, the hydrologic technician for the park monitors 35 of the 47 spring boxes for temperature, conductivity, dissolved oxygen, barometric pressure, and pH. Further mineral testing is conducted through the Buffalo National River, and through the United States Geological Survey. Thanks to these efforts, Hot Springs National Park can safely distribute over 600,000 gallons a day for public use.