Groundwater characterisation and modelling

Abstract: For the last 10 years, the Äspö Hard RockLaboratory (HRL) in Sweden has been the main test site for thedevelopment of suitable methods for the final disposal of spentnuclear fuel. Major achievements have been made in thedevelopment of new groundwater sampling and modellingtechniques.The natural condition of the groundwater is easily disturbedby drilling and sampling. The effects from borehole activitieswhich may bias the real character of the groundwater have beenidentified. The development of new sampling techniques hasimproved the representativeness of the groundwater samples. Inaddition, methods to judge the representativeness better havebeen developed.For modelling of the Äspö site, standardgroundwater modelling codes based on thermodynamic laws havebeen applied. The many limitations of existing geochemicalmodels used at the Äspö site and the need to decodethe complex groundwater information in terms of origin, mixingand reactions at site scale necessitated the development of anew modelling tool. This new modelling concept was named M3. InM3 modelling the assumption is that the groundwater chemistryis a result of mixing as well as water/rock reactions. The M3model compares the groundwater compositions from a site. Thesimilarities and differences of the groundwater compositionsare used to quantify the contribution from mixing and reactionson the measured data. In order to construct a reliable modelthe major components, stable isotopes and tritium are used.Initially, the method quantifies the contribution from the flowsystem. Subsequently, contributions from reactions arecalculated. The model differs from many other standard modelswhich primarily use reactions rather than mixing to determinethe groundwater evolution. The M3 code has been used for thefollowing type of modelling: calculate the mixing portions atÄspö, quantify the contribution from inorganic andorganic reactions such as biogenic decomposition and sulphatereduction, comparison of groundwaters at different sites inSweden and natural analogue modelling in Africa. Thegroundwaters at the Äspö site have been calculated toconsist of a complex mixture of meteoric water, Baltic Seawater, glacial meltwater and brine water. This reflects presentand known historical groundwater flow situations which haveaffected the site.The major conclusions are that the groundwaters record thepresent and paleo effects but this signal can be easilyweakened by borehole activities such as drilling, boreholetesting and sampling. The complex groundwater information hasbeen decoded and quantified by means of new modellingtechniques.