The quality of both groundwaters and surface waters that arise during flooding of abandoned lignite open pits are influenced by regional and local factors. A typical regional factor is due to oxidised sedimentary sulfides. A more local factor is the interaction of shallow water with highly saline groundwater, which is important in Merseburg-Ost (Germany). Investigation of this system is aided by the use of many environmental isotope tracers but special problems can arise. In order to reveal processes in the mine environment (shallow groundwater, lake water) and to characterise mixtures with saline groundwater results are described using the tracers δD, δ¹⁸O, δ¹³C, δ³⁴S, ⁸⁷Sr/⁸⁶Sr, ³H, ¹⁴C,³⁹Ar, and ²²²Rn. Deep highly saline groundwater had a radiocarbon concentration typically below 10 pMC. The values of δ¹³C(DIC) are around−5 ‰. As δ¹³C of the aquifer rock samples (Permian, Zechstein carbonates) was in the range of−6···+5 ‰, residence time corrections based on δ¹³C are questionable. Additional checks with ³⁹Ar, as well as results from the variationof δ¹⁸O (or δD) with respect to the salinity, emphasise a Holocene age; as is also the case for most mineralised groundwaters and also for water having a low δ¹⁸O (and δD).

For saline groundwater residing in the Zechstein aquifer the measured δ³⁴S values of about 12 ‰ are close to those expected from the literature. In contrast, the ⁸⁷Sr/⁸⁶Sr ratio of dissolved strontium is far from the values anticipated for the aquifer rocks despite there being proportionality between the chloride concentration and the strontium concentration. Furthermore, the proportionality is not valid in lower mineralised water. The ⁸⁷Sr/⁸⁶Sr ratio can, therefore, hardly be used as a tracer for the distribution of ascending saline water. The amount of salt-water coming from below into the residual quarry basins is an essential contribution to the lake inventories. Therefore, ²²²Rn was used to assist in determining the renewal of salt-water layers that formed in deep lake locations. In the deep zones ²²²Rn concentrations up to 6 Bq/l were measured but were dominantly in equilibrium with ²²⁶Ra, which was found in all higher mineralised groundwater samples. Excess radon was limited to just a few decimetres above the lake sediment surface but does not appear to be caused by continuous groundwater discharge.

Hydrochemical investigations of groundwater from the Quaternary aquifer were carried out over the last six years before flooding was complete. Apart from a slight downward shift of the average sulfate concentration, other changes showed virtually no trends. An increase of the sulfate concentration was mostly correlated with a decrease of δ³⁴S for individual sites only, but not for the whole ensemble of sampling locations. Sulfate from pyrite oxidation plays an important role but cannot be attributed unequivocally to coal mining. There are hints that the conditions closer to the basin edges may differ from those remoter parts of the flood plain.