Halite dissolution derived brines in the vicinity of a Permian salt dome (N German Basin).: Evidence from boron, strontium, oxygen, and hydrogen isotopes

The paper presents the isotopic (B, Sr, O, H) and chemical composition of solid salts, brines, groundwater, and surface water in the vicinity of a Permian salt dome in Northern Germany (diapir of Gorleben-Rambow) being considered for disposal of high level radioactive waste. The site is situated in the NW European Basin that is dominated by intensive diapirism of Permian salt and overprinted by glacial erosion/sedimentation. The local hydrogeology comprises an upper aquifer system of heterogeneous Pleistocene sediments and a lower aquifer system of Tertiary elastics tectonically disturbed by diapirism. There is a clear chemical and isotopic evolution with TDS, that generally increases with depth up to 328 g . l(-1). The chemistry and isotope ratios of the saline groundwater can be explained by a two end-member mixing system. The Na-Cl brine end-member is derived from evaporite dissolution as indicated by major ion chemistry, Cl/Br ratios, Sr and B isotopes. It is mixed with fresh-brackish groundwater dominated by water-silicate interaction. This low salinity end-member TDS from 0.3-10 g . l(-1)) shows radiogenic Sr-87/Sr-86 ratios ranging from 0.70910 to 0.71030 and low delta B-11 values between 7.4 and 16.5 parts per thousand vs. NBS951. Two halite samples and a borate sample from the unaltered parts of the diapir show Sr-87/Sr-86 ratios from 0.70696 to 0.70868 and delta B-11 values of 20.8 and 27.7 parts per thousand. Brine (TDS from 100-328 g/l) Sr isotope ratios, ranging from 0.70758 to 0.70788, lie in the uppermost range of late Permian ocean waters and are within the range of halite. The delta B-11 values of the brines vary between 25.1 parts per thousand to 33.5 parts per thousand, which is close to the range of solid salt but well below those of evaporating present-day sea water. B isotope composition is thus compatible with salt dissolution rather than with residual evaporite brines. The chemistry and isotope characteristics of the brine end-member derived from halite dissolution are not significantly modified by subsequent interaction with the siliciclastic aquifer material. Most highly saline groundwaters show cold climate O and H isotopic signatures with delta O-18 values ranging from -10.5 to -9 parts per thousand. A limited group of four rather deep-seated brines (> 200 m) plots in the range of Holocene signatures with delta O-18 > -9 parts per thousand close to those of the shallow groundwater which is interpreted as an indication of rapid groundwater circulation from the surface and ongoing salt dissolution. (C) Copyright 2001 Elsevier Science Ltd.