HYDROGEOLOGY AND GEOCHEMISTRY OF THE SHALLOW GROUND WATER SYSTEM IN THE RECHARGE AREA OF THE GEOTHERMAL SPRINGS IN HOT SPRINGS, ARKANSAS

A reconnaissance of wells and springs, including collection of water levels, pH, conductance and water temperature data, was made for a 40 square-mile area including the recharge area for the geothermal springs in Hot Springs, Arkansas. The wells and springs are completed in or issue from five major geologic formations in the area: the Hot Springs Sandstone Member of the Stanley Shale, the Stanley Shale, the Arkansas Novaculite, the Bigfork Chert, and the Womble Shale.

A potentiometric map was completed using water levels from 50 wells. Resulting contours closely parallel topographic contours and indicate a broadly connected, unconfined aquifer system (to depths of approximately 400 feet) which drains locally to streams and through springs in the area. Median values for conductance and pH in well water from the combined chert, sandstone and novaculite formations were 32 micro-Siemens per centimeter (µS/cm) and 4.8 µS/cm, respectively, whereas median values for water from wells completed in the shale units were 290 µS/cm and 7.0 µS/cm, respectively. The low conductance and pH values for water in the sand, chert, and novaculite formations reflect the lack of carbonates or other highly soluble minerals in these units, whereas the shale units are highly buffered by carbonate dissolution. Chemical analyses from selected wells substantiate this conclusion, with median calcium concentrations for the shale and sand/chert (collectively) of 48 milligrams per liter (mg/L) and 0.39 mg/L, respectively, and median bicarbonate concentrations of 180 mg/L and 5 mg/L, respectively.

Water chemistry, including isotopic analysis, is currently being analyzed for samples from 15 wells, two cold springs, and 10 hot springs. Results of the sampling will be used to assist in further delineating the recharge area for the geothermal springs, and defining the dominant formations that contribute recharge water to the geothermal springs.