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

The tunnel excavation at the Aspo Hard Rock Laboratory opened several fract ure 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. Exc ept for the tunnel intersection, several boreholes were drilled to intersec t the zone at various depths (ranging from 5 to 70 m) and distances from th e 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 chem istry during this monitoring can be largely explained by mixing between fre sh water and native saline groundwater (4900 ppm Cl-). An increase in HCO3- was recorded, which was interpreted as due to anaerobic respiration. This was supported by C-14-contents in dissolved organic Carbon and HCO3-, indic ating that recent organic C is transported into the zone and oxidised to CO 2. This study exemplifies the use of C-14-analyses of HCO3- in order to tra ce different C sources contributing to the HCO3- in the groundwater. Three sources were identified: (1) dissolved CO2, dominantly soil-CO2 possibly wi th some contribution of atmospheric CO2; (2) dissolution of calcite, with l ow C-14 content, which dominantly occurs in the near-surface recharge area; and (3) oxidation of organic material through anaerobic respiration. Corre ctions for C-14 and HCO3- in the native saline water made it possible to de termine 2 different fresh water components corresponding to different flow paths. The C isotope data are in accordance with the results from the trace r test and the groundwater how model, and support that the extensive build up of HCO3- does not mainly takes place locally within the zone but is tran sported into the zone by dominantly lateral flow. The results from the moni toring showed that new hydrochemical stability is established, which also c omprises the interaction between the organic and inorganic C cycles. (C) 19 99 Elsevier Science Ltd. All rights reserved.