14C in bicarbonate and dissolved organics—a useful tracer?

Abstract

The tunnel excavation at the Äspö Hard Rock Laboratory opened several fracture zones at various depths in the crystalline bedrock. One of these zones is the `Redox zone', a vertical fracture zone penetrated at 70 m depth. Except for the tunnel intersection, several boreholes were drilled to intersect the zone at various depths (ranging from 5 to 70 m) and distances from the tunnel. The response in groundwater chemistry to the opening of the zone has been monitored in these boreholes during 3 a, starting in 1991 and for the boreholes at 70 m depth the monitoring is still ongoing. The water chemistry during this monitoring can be largely explained by mixing between fresh water and native saline groundwater (4900 ppm Cl −). An increase in HCO − 3 was recorded, which was interpreted as due to anaerobic respiration. This was supported by 14C-contents in dissolved organic Carbon and HCO − 3, indicating that recent organic C is transported into the zone and oxidised to CO 2. This study exemplifies the use of 14C-analyses of HCO − 3 in order to trace different C sources contributing to the HCO − 3 in the groundwater. Three sources were identified: (1) dissolved CO 2, dominantly soil-CO 2 possibly with some contribution of atmospheric CO 2; (2) dissolution of calcite, with low 14C content, which dominantly occurs in the near-surface recharge area; and (3) oxidation of organic material through anaerobic respiration. Corrections for 14C and HCO − 3 in the native saline water made it possible to determine 2 different fresh water components corresponding to different flow paths. The C isotope data are in accordance with the results from the tracer test and the groundwater flow model, and support that the extensive build up of HCO 3 − does not mainly takes place locally within the zone but is transported into the zone by dominantly lateral flow. The results from the monitoring showed that new hydrochemical stability is established, which also comprises the interaction between the organic and inorganic C cycles.