Environmental isotope studies and tracer testing to determine capture and protection zones in the Zechstein-Karst on the northern verge of the Thuringian Forest

On the northern downfall of the thuringian forest, groundwaters of the permo-triassic sediment series are utilized for water supply of communities and industry.

For the securing and protection of the water withdrawal installations in the thuringian Rinne valley a secure determination of the grade of protection as well as the location of the encatchment and recharge areas is essential.

The application of isotope and tracer techniques enables a distinction of the hydraulic contact between the karst, cleaved and porous aquifers of the permotriassic sediment series on top of the palaeozoic bedrocks.

According to the tritium content (exponentential modell), the mean residence time in the Zechstein springs amounts to between two and ten years.

Short-dated admittance of younger groundwater was not detectable, using either hydrochemical measurements or tritium methods alone.

A significant reduction of the δ-notation in the δ¹⁸O-hydrographs of the springs and the surface water in the Rinne valley in the winter half-year 1993/94 indicates on the other hand, an abrupt and rapid admittance of younger groundwaters after strong precipitation and snow melt, originating from the southern recharge area (Zechstein outcrop).

A marking of the Zechstein and bunter sandstone waters with coloured dyes lead to the demarcation of three hydraulic systems, which each are drained within the permotriassic aquifersystem, through different springs.

The dominant apparent flow velocities amount to 2400 m/d in the Zechstein karst on the northeastern verge of the thuringian forest. The passage of groundwater from the porous aquifer of the lower bunter sandstone into the plate dolomites was confirmed by means of a multi-summited, arched breakthrough curve. The combined application of various tracer methods enabled the distinction of short-termed infiltration of waters from different encatchment areas south of the Zechstein outcrop, as well as their combination with the static and dynamic reserve in the sub-aquifers of the overlying bunter sandstone northwest of the Rinne valley.