ORIGINAL PAPER Hydrochemistry and gas geochemistry of the northeastern Algerian geothermal waters M. Belhai 1 & Y. Fujimitsu 2 & J. Nishijima 2 & M. Bersi 3 Received: 16 January 2016 /Accepted: 5 December 2016 # Saudi Society for Geosciences 2016 Abstract This study focuses on the water and gas chemistry of the northeastern Algerian thermal waters. The helium gas was used to detect the origin of the geothermal fluid. In the Guelma Basin, the heat flow map shows an anomaly of 120 ± 20 mW/ m 2 linked to the highly conductive Triassic extrusion. The chemical database reveals the existence of three water types, Ca-SO 4 /Na-Cl, which are related to evaporites and rich in halite and gypsum minerals. The third type is Ca (Na)-HCO 3 , which mostly characterizes the carbonated Tellian sector. The origin of thermal waters using a gas-mixing model indicates a meteoric origin, except for the El Biban hot spring (W10), which shows a He/Ar ratio of 0.213, thus suggesting the presence of batholith. The helium distribution map indicates a lower 3 He/ 4 He ratio between 0 Ra and 0.04 Ra in the W10 and W15 samples, which is compatible with the crustal ratio. Reservoir temperatures es- timated by silica geothermometers give temperatures less than 133 °C. The geothermal conceptual model suggests that a geo- thermal system was developed by the deep penetration of infil- trated cold waters to a depth of 2.5 km and then heated by a conductive heat source (batholith for El Biban case). The ther- mal waters rise up to the surface through the deep-seated fractures. During their ascension, they are mixed with shallow cold groundwater, which increase the Mg content and cause the immature classification of the water samples. Keywords Northeastern Algeria . Geothermal waters . Gas chemistry . Geothermometry . Conceptual model Introduction More than 70 principal hot springs are located in northeastern Algeria, and they have a temperature discharge ranging be- tween 31 and 97 °C. Most of the geothermal waters have been used therapeutically for balneology since the Roman period. The hydrogeochemistry of the geothermal water discharge is strongly controlled by the host lithology (Issaadi 1992; Bouchareb-Haouchine 1993; Kedaid and Mesbah 1996; Saibi 2009). The origin of the geothermal manifestations could be linked to the Mio-Plio-Quaternary magmatic events (Verdeil 1982) and a relatively high geothermal gradient that exists in northeastern Algeria (approximately 4.32 ° C/100 m; Bouchareb-Haouchine 2012). This study focus- es on the water chemistry and key source rocks to under- stand the interactions with the host rock. Gas species (Rezig and Marty 1995) are used to ascertain the origin of the different thermal manifestations using several gas ratios to estimate the geothermal reservoir temperature with gas geothermometry techniques. Finally, the 3 He/ 4 He ratio variation and distribution map are used to examine the eventual intrusion at depth and to develop a geothermal conceptual model for northeastern Algeria (Fig. 1). * M. Belhai mkhader89@gmail.com 1 Department of Earth Resources Engineering, Graduate School of Engineering, Kyushu University, Fukuoka 819-0395, Japan 2 Department of Earth Resources Engineering, Faculty of Engineering, Kyushu University, Fukuoka 819-0395, Japan 3 Department of Geophysics, Faculty of Earth Sciences, Geography and Territorial Planning, Houari Boumediene University of Sciences and Technology, BP 32, Al Alia, 16111 Algiers, Algeria Arab J Geosci (2017) 10:8 DOI 10.1007/s12517-016-2790-2