Application of mass-balance and flow simulation calculations to interpretation of mixing at Aspo, Sweden

The hydrogeology of a vertical fracture zone at 70 m depth at the access tu nnel to the Aspo Hard Rock Laboratory was monitored over 3 a for hydrochemi cal changes that could be effected by construction of a deep repository for high-level nuclear waste. Tunnel construction dramatically disturbed the h ydrogeological system, but this provided an opportunity to integrate hydrog eochemical and hydrological evaluation of the zone. The objective of this s tudy was to evaluate hydrogeochemical evolution, groundwater flow and surfa ce water intrusion during the experiment using an integrated approach of ge ochemical mass-balance calculations and numerical how simulations. The dilution of major ions was the dominant hydrochemical trend. However, H CO3 and SO4 showed significant enrichment. Increasing activity of C-14 sugg ested that oxidation of organic C was the likely source of HCO3. Any minera l source dissolving during the experiment seemed insufficient to account fo r changes in SO4 and current intrusion of sea water was excluded according to the data. Cation exchange as well as minor calcite reactions in fracture s were assumed probable in such temporary chemical conditions. Conservative two end-member mixing models with shallow groundwater in the zone and init ial groundwater at tunnel level also assumed remarkable mass transfer (seve ral mmol/l). Therefore a third SO4-rich end-member, a regional shallow grou ndwater type which may mix by lateral flow in the system, was tested. This was also expected from hydraulic measurements and preliminary flow simulati ons assuming homogeneity. Three end-member mixing calculations using Cl and SO4 as conservative trace rs give a constant proportion of lateral water in all boreholes after 300 d ays, which is consistent with the steady state character of the flow field in the late part of the experiment. To predict reactions on plausible level s needs significant adjustments of initial and final waters, indicating unc ertainties in the hydrochemical information of the fracture zone. In the fl ow simulations the transmissivities were selected so that the chemical mixi ng proportions would match simulated portions of flow as closely as possibl e. The simulated total recoveries (drawdowns) differ from the measurements mainly due to overly simple parametrisation of the transmissivity in the fr acture zone. However, integrating hydrochemistry in flow modelling is consi dered encouraging in producing additional information of the heterogeneity of a flow structure. (C) 1999 Elsevier Science Ltd. All rights reserved.