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 s olid salts, brines, groundwater, and surface water in the vicinity of a Per mian salt dome in Northern Germany (diapir of Gorleben-Rambow) being consid ered for disposal of high level radioactive waste. The site is situated in the NW European Basin that is dominated by intensive diapirism of Permian s alt and overprinted by glacial erosion/sedimentation. The local hydrogeolog y comprises an upper aquifer system of heterogeneous Pleistocene sediments and a lower aquifer system of Tertiary elastics tectonically disturbed by d iapirism. There is a clear chemical and isotopic evolution with TDS, that g enerally increases with depth up to 328 g . l(-1). The chemistry and isotop e ratios of the saline groundwater can be explained by a two end-member mix ing system. The Na-Cl brine end-member is derived from evaporite dissolutio n as indicated by major ion chemistry, Cl/Br ratios, Sr and B isotopes. It is mixed with fresh-brackish groundwater dominated by water-silicate intera ction. This low salinity end-member TDS from 0.3-10 g . l(-1)) shows radiog enic 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 sampl es 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 p arts 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 w aters and are within the range of halite. The delta B-11 values of the brin es vary between 25.1 parts per thousand to 33.5 parts per thousand, which i s close to the range of solid salt but well below those of evaporating pres ent-day sea water. B isotope composition is thus compatible with salt disso lution rather than with residual evaporite brines. The chemistry and isotop e characteristics of the brine end-member derived from halite dissolution a re not significantly modified by subsequent interaction with the siliciclas tic aquifer material. Most highly saline groundwaters show cold climate O a nd H isotopic signatures with delta O-18 values ranging from -10.5 to -9 pa rts 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 ongoin g salt dissolution. (C) Copyright 2001 Elsevier Science Ltd.