S- and O-isotopic character of dissolved sulphate in the cover rock aquifers of a Zechstein salt dome

Sulfur and O isotope analyses of dissolved SO4 Were used to constrain a hydrogeological model for the area overlying the Gorleben-Rambow Salt Structure, Northern Germany. Samples were collected from 80 wells screened at different depth-intervals. The study area consists of a set of two vertically stacked aquifer systems. Generally, the isotope data show a good spatial correlation, outlining well-defined groundwater zones containing SO4 of characteristic isotopic composition. Highly saline waters from deeper parts of the lower aquifer system are characterized by rather constant SO4 isotopic compositions, which are typical of Permian Zechstein evaporites (delta(34)S = 9.6-11.9parts per thousand; delta(18)O = 9.5-12.1parts per thousand). Above this is a transition zone containing ground waters of intermediate salinity and slightly higher isotopic values (average delta(14)S 16.6parts per thousand; delta(18)O = 15.3parts per thousand). The confined groundwater horizon on the top of the lower aquifer system below the low permeable Hamburg Clays is low in total dissolved solids and is characterized by an extreme S-34 enrichment (average delta(34)S = 39.1parts per thousand; delta(18)O = 18.4parts per thousand), suggesting that bacterially mediated SO4 reduction is a dominant geochemical process in this zone. Two areas of distinct isotopic composition can be identified in the shallow ground water horizons of the upper hydrogeological system. Sulfate in groundwaters adjacent to the river Elbe and Locknitz has a typical meteoric isotopic signature (delta(14)S= 5.2parts per thousand; delta(18)O = 8.2parts per thousand), whereas the central part of the area is characterized by more elevated isotopic ratios (delta(34)S = 12.7parts per thousand; delta(18)O = 15.6parts per thousand). The two major SO4 Pools in the area are represented by Permian seawater SO4 and a SO4 of meteoric origin that has been mixed with SO4 resulting from the oxidation of pyrite. It is suggested that the S-isotope compositions observed reflect the nature of the SO4 source that have been modified to various extent by bacterial SO4 reduction. Groundwaters with transitional salinity have resulted from mixing between brines and low-mineralized waters affected by bacterial SO4 reduction. (C) 2002 Elsevier Science Ltd. All rights reserved.