B. Wallin | USGS Science

USGS1999

Calcite fracture fillings as indicators of paleohydrology at Laxemar at the Aspo Hard Rock Laboratory, southern Sweden

Isotopic compositions of C (??13C), O (??18O) and Sr (??87Sr) were determined for calcite fracture fillings in the crystalline rock penetrated by a 1.6 km drill hole at Laxemar, near the Aspo Hard Rock Laboratory (AHRL) in southern Sweden. These calcites precipitated from groundwater some time in the past, and their ??13C, ??18O and ??87Sr values reflect those of the source waters. The present-day groundwater system is hydrochemically stratified with highly saline water underlying more shallow brackish and fresh water. The origin of this stratified system is probably related to past glaciations although the ultimate origin of the deep, highly saline water is still problematical. None of the calcite fracture fillings sampled below 900 m could have precipitated from any of the present-day ground waters which in view of the glacial history of the region is not surprising. However, several shallow calcite fracture fillings are formed by precipitation from the present-day groundwater. Coupled variations in ??13C, ??18O and ??87Sr isotopes at depths in excess of 900 m suggest that these isotope systems in calcite are recording a time-dependent evolution of groundwater composition.

Applied Geochemistry↗

USGS1999

Synopsis of strontium isotope variations in groundwater at Aspo, southern Sweden

Strontium isotope ratios are used to identify end-member ground-water compositions at Aspo in southeastern Sweden where the Hard Rock Laboratory (HRL) has been constructed to evaluate the suitability of crystalline rock for the geologic disposal of nuclear waste. The Hard Rock Laboratory is a decline (tunnel) constructed in 1.8 Ga-old granitic rock that forms islands in an archipelago along the Swedish coast. Ground-water samples were obtained for isotopic analyses from boreholes drilled from the surface and from side boreholes drilled within the HRL. Infiltration at Aspo occurs primarily through fractures zones in the granitic bedrock beneath thin soils throughout the area. Because of extremely low Sr concentrations, rain and snow are not important contributors to the Sr isotope budget of the ground-water system. At shallow levels, water percolating downward along fractures and fracture zones acquires a ??87Sr between +9.5 and +10.0??? and maintains this value downward while Sr concentrations increase by two orders of magnitude. Ground-water samples from both boreholes and from in the HRL show the effects of mixing with saline waters containing as much as 59 mg/L Sr and ??87Sr values as large as +13.92%, Baltic Sea water is a potential component of the groundwater system with ??87Sr values only slightly larger than modern marine values (+0.3???) but with much lower concentrations (1.5 mg/L) than ocean water (8 mg/L). However, because of large Sr concentration differences between the saline groundwater (59 mg/L) and Baltic Sea water (1.5 rag/L), ??87Sr values are not particularly sensitive indicators of sea-water intrusion even though their ??87Sr values differ substantially.

Applied Geochemistry↗