Geological Survey Professional Paper 1044—C The Waters of Hot Springs National Park, Arkansas—Their Nature and Origin

The 47 hot springs of Hot Springs National Park, Ark., issue from the plunging crestline of a large overturned anticline, along the southern margin of the Ouachita anticlinorium, in the Zigzag Mountains. Rocks in the vicinity of the hot springs range in age from Ordovician to Mississippian. The rocks—cherts, novaculites, sandstones, and shales—are well indurated, folded, faulted, and jointed. The springs emerge from the Hot Springs Sandstone Member of the Stanley Shale near the anticlinal axis, between the traces of two thrust faults that are parallel to the axis of the anticline.

The combined flow of the hot springs ranges from 750,000 to 950,000 gallons per day (3.29x10^-2 to 4.16x10^-2 cubic meters per second). The flow of the springs is highest in the winter and spring and is lowest in the summer and fall. The temperature of the combined hot-springs waters is about 62 degrees Celsius.

The radioactivity and chemical composition of the hot-water springs are similar to that of the cold-water springs and wells in the area. The dissolved-solids concentrations of the waters in the area generally range from 175 to 200 milligrams per liter. The main differences in the quality of the hot water, compared with nearby cold ground waters, are the higher temperatures and the higher silica concentrations of the hot springs. Cold waters in the area generally range from 15.0 to 26.8 degrees Celsius. The silica concentrations of cold ground waters range from 2.6 to 13.0 milligrams per liter, whereas the silica concentration of the hot springs is about 42 milligrams per liter. The high silica concentration of the hot springs is due to the increased solubility of silica in hot water. The silica concentration of the hot springs indicates that the maximum temperature reached by the hot-springs water is no more than a few degrees higher than the temperature at which the springs emerge.

The tritium and carbon-14 analyses of the water indicate that the water is a mixture of a very small amount of water less than 20 years old and a preponderance of water about 4,400 years old. The deuterium and oxygen-18 concentrations of the hot-springs waters are not significantly different from those of the cold ground waters.

The presence of radium and radon in the hot-springs waters has been established by analyses. Recent (1973) analysis showed the radium concentration to be 2.1 picocuries (10^-12 curies) per liter. Analyses made in 1953 of the radon gas, a radioactive decay product of radium, ranged from 0.14 to 30.5 nanocuries (10^-9 curies) per liter.

Mathematical models were employed to test various conceptual models of the hot-springs flow system. The geochemical data, flow measurements, and geologic structure of the region support the concept that virtually all the hot-springs water is of local, meteoric origin. Recharge to the hot-springs artesian-flow system is by infiltration of rainfall in the outcrop areas of the Bigfork Chert and the Arkansas Novaculite. The water moves slowly to depth where it is heated by contact with rocks of high temperature. Highly permeable zones, related to jointing or faulting, collect the heated water in the aquifer and provide avenues for the water to travel rapidly to the surface.