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 Musche lkalk (Middle Triassic). The thermal waters are heated along anomalous high thermal gradients in the Tertiary rift valley. The highest well head tempe rature is about 40 degrees C in Hauptrogenstein wells and 60 degrees C in M uschelkalk wells. Mineralization (TDS) is up to 5 g/kg in Hauptrogenstein a nd as high as 17 g/kg in the Muschelkalk aquifer. About 300 chemical analys es 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 Hauptr ogenstein originates from mixing of 3 components: (a) meteoric water, (b) f ossil seawater (residual formation water) and (c) a third component that re sulted from water-rock reaction. The total amount of dissolved solids and the water type from the deeper Mus chelkalk aquifer depends on the depth of the aquifer at the well location. The chemical characteristics of the thermal water indicate that water compo sition is derived mainly from water-rock interaction. (C) 1999 Elsevier Sci ence Ltd. All rights reserved.