87Sr/86Sr Ratios in Shallow and Deep Aquifers and Thermal Water from the Eastern Boundary Fault of the Northern Upper Rhine Graben at the Heidelberg Basin, Germany

Strontium isotopic ratios are important tracers of hydrological processes such as water-rock interaction and the detection of the ascent of deep fluids at hydraulically active faults. The 87Sr/86Sr ratios have been measured in shallow and deep aquifers (n=19) and thermal water at the Eastern Boundary Fault (EBF) of the northern Rhine Graben Rift in Germany. 87Sr/86Sr ratios and Sr contents from Quaternary groundwater conducting sediments of the shallow aquifers range from 0.70896 to 0.70965 and 0.23 to 0.67 mg/l, respectively. Strontium contents of the shallow groundwater are low and highly variable in their 87Sr/86Sr ratios. No unequivocally hints of ascending deep fluids at the EBF fault have been found in the shallow aquifers of the Heidelberg Basin. However, groundwater from production wells of up to 205 m deep aquifers and the 1100 m deep thermal well show radiogenic 87Sr/86Sr ratios and high Sr contents in the range from 0.70984 to 0.71074 and 0.39 to 319 mg/l, respectively. The radiogenic Sr ratios are clear indications for ascending deep fluids into deep aquifers at the EBF fault of the northern upper Rhine Graben.