Understanding groundwater chemistry using mixing models

For the last 15 a, SKB (the Swedish Nuclear Fuel and Waste Management Company) has been using the Aspoe Hard Rock Laboratory (HRL) as the main test site for the development of suitable tools and methods for the final disposal of spent nuclear fuel. Major achievements have been made in the development of a new groundwater modelling technique. The technique described in this paper is used within the ongoing site investigations of Forsmark and Simpevarp in Sweden. The limitations of existing geochemical models used at many sites and the need to decode complex groundwater information in terms of origin, mixing (transport) and reactions at site scale, necessitated the development of a new modelling tool, M3, (multivariate mixing and mass-balance calculations). In M3 modelling, the assumption is made that the groundwater chemistry is a result of mixing as well as water/rock interactions. The M3 model compares groundwater compositions (measured in terms of major ionic components, stable isotopes and 3H) at a site. The model differs from many other standard geochemical models which primarily use water-rock interactions, rather than mixing, to determine groundwater chemical evolution. The tool is not dependent on thermodynamic databases, potentially uncertain redox and pH data, and can deal with the effects of biological reactions. The results of mixing calculations can be compared or integrated with hydrodynamic models. In this paper, the M3 method has been applied to two large hydrogeochemical databases, for the Aspoe Hard Rock Laboratory in Sweden, the Underground Research Laboratory in Canada (URL) together with data from the ongoing site investigations of Forsmark and Simpevarp in Sweden. Decreasing amounts of precipitation and increasing proportions of saline and brine waters are seen with increasing depth in both areas. Biogenic waters, caused by uptake of CO2 from organics causing additional formation of HCO3 by water-rock interaction, occur at intermediate depths in both areas but more glacial water is detected in the URL area. The origin and evolution of the groundwater at the Aspoe and URL sites have been quantified with the aid of M3. In addition, the conceptual present/post-glacial hydrogeological model has been verified