LAUSR

Abstract This paper deals with the results of a hydrochemical and geothermal study of thermal waters in northeastern Algeria. Eleven samples were collected during the period between November 2013 and April 2015. To identify the origin of the thermal groundwater and for the evaluation of the reservoir temperature in the geothermal systems, the following data were used: concentrations of major chemical constituents, stable isotope ratios (δD and δ18O), saturation indices and chemical geothermometer temperatures. The physicochemical parameters (temperature, pH, and electric conductivity) were measured in-situ; the temperature of the thermal water samples varied from 38 to 96 °C, the pH value of these springs is slightly acid to neutral, with high electrical conductivities up to 4500 μS/Cm. Piper diagrams highlighted two major hydrochemical facies namely sodium chloride (Na-Cl) and sodium sulfate (Na-SO4). The mineral composition of the thermal waters reflects the geological formations found in the deep origin reservoir and chemical changes in the fluids were highly influenced by water-rock interaction. The thermal waters from the study area are depleted in 18O and D and plot on the global meteoric water line (GMWL), their deep-circulating meteoric origin shows that most thermal waters plot on or near the meteoric water line, with some exceptions due to Mediterranean precipitation, probable water-rock isotope exchange or mixing takes place between the ascending geothermal water and shallow colder groundwater. The subsurface reservoir temperatures were calculated using different solute geothermometers and computation of saturation indices for different solid phases. The highest estimated reservoir temperatures are indicated by the cation composition geothermometer (CCG) and the Na–K–Ca geothermometer, while slightly lower estimates are obtained using silica geothermometers, with local geothermal gradients ranging from 25 to 45 °C/km.