ORGANIC-CARBON OXIDATION-INDUCED BY LARGE-SCALE SHALLOW-WATER INTRUSION INTO A VERTICAL FRACTURE-ZONE AT THE ASPO HARD-ROCK LABORATORY (SWEDEN)

Entrance tunnel construction at the Aspo Hard Rock Laboratory opened a conductive vertical fracture zone at a depth of 70 m on March 13, 199 1. Three weeks later a sharp dilution front corresponding to 80% shall ow water inflow to the originally saline fracture zone arrived at the entrance tunnel depth. In spite of this large inflow of shallow water, the fracture zone has remained persistently anoxic over a subsequent 2 1/2-year period. Results from gas sampling and C-14 dating of dissol ved organic and inorganic carbon conclusively show that recent organic carbon is being transported into the fracture zone and oxidized to ca rbon dioxide. These results are important when considering possible ch anges of redox status in the deep groundwater environment during const ruction and operation of a repository for spent nuclear fuel. Opening this fracture zone to large-scale surface water inflow adds reducing c apacity in the form of organic carbon. This implies that the soil cove r may provide important protection against input of dissolved oxygen t o fractures being drained during the open phase of the repository.