Temperature and radon-222 as tracer of groundwater flow: application to El Hamma geothermal aquifer system, southeastern Tunisia

Hydrogeological and hydrochemical investigations were applied to explain geothermal groundwater flow factors controlling groundwater mineralization, and 222Rn concentrations were controlled using RAD7-H2O in El Hamma aquifer system, southeastern Tunisia. Nineteen geothermal water samples were collected from different locations (thermal baths and deep wells). Physical parameters (EC, pH, TDS, T) were measured, and major ions were analyzed. Hydrogeochemical modeling and thermal parameters reveal that groundwater from El Hamma aquifer system has several origins. The first type was characterized by a hot temperature >55 °C with a chemical facies Na-Mg-Ca-SO4-Cl. This type characterizes lower cretaceous deep water (continental intercalary aquifer). The second water type was dominated by Na-Ca-Cl-SO4 facies with a temperature <45 °C and represents the superficial aquifer. However, Ca-SO4-Na-Cl-Mg is the third water type which is characterized by mixed water from deep and shallow water with a temperature ranging from 45 to 55 °C. Shallow aquifer displays higher 222Rn concentration (677 to 2903 Bq/m3). However, deep water from continental intercalary displays relatively a low radon concentration (203 to 1100 Bq/m3). Compared to Rn-222 activity in some countries, radon concentration in El Hamma thermal aquifer system has a medium value controlled by structures, porosity, and permeability of aquifers. Variographic analysis and ordinary kriging techniques were used to map temperature spatial variability in the study area. Geostatiscal techniques in addition to geological data and radon activities confirm the existence of vertical communication between continental intercalary aquifer and superficial Senonian aquifer through vertical faults system.