Occurrence and origin of thallium in mineral and thermal waters from the northern Upper Rhine Graben and adjacent regions, Germany

Abstract Although thallium is a widespread and highly toxic element, it is rarely investigated in environmental chemistry due to its typically low concentrations in water. Consequently, little is known about the occurrence of thallium in groundwater and its origin. Here we show that high thallium concentrations of 5 up to 99 µg/l occur in Hesse exclusively in Na-Cl-waters and are locally clustered within the Taunus Border Zone. These variable concentrations seem to result from different thallium sources, varying mixing ratios of brine components, and varying degrees of thallium incorporation in hydrothermal minerals. We present evidence for a thallium origin from Zechstein (Permian) and Oligocene evaporites. A third possible source are basement brines from the Rhenish Massif. As the evaporite deposits are located tens of kilometers away from thallium-bearing wells, high thallium concentrations appear to be a good indicator of deep and regionally circulating brines.