Chemical evolution of thermal waters from limestone aquifers of the Southern Upper Rhine Valley

Thermal spas in the Upper Rhine Graben recover their waters mainly from two different limestone aquifers, Hauptrogenstein (Middle Jurassic) and Muschelkalk (Middle Triassic). The thermal waters are heated along anomalous high thermal gradients in the Tertiary rift valley. The highest well head temperature is about 40°C in Hauptrogenstein wells and 60°C in Muschelkalk wells. Mineralization (TDS) is up to 5 g/kg in Hauptrogenstein and as high as 17 g/kg in the Muschelkalk aquifer. About 300 chemical analyses from 13 wells were used in this study. Compositional relationships between major chemical components (Na/Cl, K/Cl, Mg/Cl, SO4/Cl, Cl/Br and Na/Br) suggest that thermal water from the Hauptrogenstein originates from mixing of 3 components: (a) meteoric water, (b) fossil seawater (residual formation water) and (c) a third component that resulted from water–rock reaction. The total amount of dissolved solids and the water type from the deeper Muschelkalk aquifer depends on the depth of the aquifer at the well location. The chemical characteristics of the thermal water indicate that water composition is derived mainly from water–rock interaction.