Geology and Hydrogeology of the Cover Sediments above the Gorleben Salt Dome

摘要

The Gorleben salt dome is being investigated for its suitability as a repository for radioactive wastes. The cover deposits above the Gorleben salt dome were investigated within the framework of a comprehensive geological and hydrogeological site programme from 1979 to 1999. The Gorleben salt intrusion attained the diapir stage during the Gretaceous. An almost complete sequence of principally clayey-silty marine sediments from the Upper Paleocene onwards is preserved. The strata are thinner over the diapir and thicker in the surrounding subsidence areas. In the early Tertiary the facies became brackish to continental, with the profile containing quartz and mica sands, dark clayey silts and occasional lignite seams. The strata sequence ends in the Lower Miocene. Neogene strata are not present above the diapir New sedimentation conditions began in the Quaternary. Apart from Menapian to Cromerian deposits above the western part of the diapir, the strata deposited since the Elsterian glacial stage are dominated by sand, till and glacial lake sediments. The maximum thickness of Quaternary deposits is >300 m in the Gorleben Channel (Gorlebener Rinne), which was formed during the Elsterian. In the deepest part of the channel, the late Tertiary sediments have been eroded completely. After the channel was filled during the Holsteinian Interglacial it was covered on three occasions by inland ice sheets, but with less erosion and sedimentation than during the Elsterian. Hydrogeologically the Quaternary and Tertiary sediments can be divided into a lower aquifer of Tertiary sands and Quaternary gravel deposits and an upper aquifer of Saalian and Weichselian sediments separated by aquitards. The area of the Gorleben channel is of particular interest for the long-term safety of the planned repository. In the central part of the channel, the lower aquifer directly overlies the cap rock or is in contact with the salt itself Brines are found in these areas. On the basis of the present distribution of salt concentration in this particular area, two flow paths of saline water have been identified: a) Lateral transport of brines into the rim syncline to the northwest of the salt dome. This salt water collects at the base of the aquifer system due to its higher density. b) Vertical flow of saline water into the upper aquifer as far as the surface due to locally elevated vertical permeabilities of the aquitard overlying the lower channel aquifer.