Application of the M3 code for modelling groundwater chemistry
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$SSOLFDWLRQRIWKH0FRGHIRUPRGHOOLQJJURXQGZDWHUFKHPLVWU\ Marcus Laaksoharju1 Mel Gascoyne2 and Ioana Gurban3 1
Geopoint AB, Fridshyddev. 15, SE-19136, Stockholm, Sweden, E-mail: [email protected] 2 GeoProjects Inc. Box 141, Pinawa, MB R0E 1L0, Canada 3 3D-Terra, 3583 Durocher#1, Montreal, Quebec, H2X 2E7, Canada $%675$&7 The need to decode the complex groundwater information in terms of origin, mixing (transport) and reactions at site scale, necessitated the development of a new modelling tool. This new modelling concept was named M3 (Multivariate Mixing and Mass-balance calculations). Initially, the method quantifies the contribution from the flow system. Subsequently, contributions from reactions are calculated. The M3 code has been used for the following types of modelling: to calculate the mixing portions at different sites and to quantify the contribution from inorganic and organic reactions. For instance, the groundwaters at many sites have been calculated to consist of complex mixtures of different water types such as meteoric water, biogenetically modified water, modern and ancient sea water, glacial meltwater and brine water. Examples from modelling of data from Äspö in Sweden, Oklo in Gabon and the URL in Canada are given. ,1752'8&7,21 A major task within the site investigation work of SKB (Swedish Nuclear Fuel and Waste Management Company) was to develop a modelling technique which could be used to better describe the complex mixing and reaction behaviour of natural groundwaters. The M3 method [4,5] has been tested, evaluated, compared with standard methods and modified over several years within domestic and international research programmes. The main test and application site for the model has been the Äspö Hard Rock Laboratory (HRL) [5]. Mixing seems to play an important role in many groundwater systems dominated by fracture flow where M3 calculations have been applied such as in different Swedish sites [6], Canada [7,8], Oklo in Gabon [2,3] and Palmottu in Finland [4]. In this paper, modelling examples from Äspö, Oklo and URL are discussed. 00(7+2')25*(2&+(0,&$/02'(//,1* In M3 modelling the assumption is that the groundwater composition is always a result of mixing and reactions. Initially, the method evaluates the contribution from the flow system i.e. mixing. Subsequently, contributions from reactions are considered. The aim is to classify and quantify the amounts of, for instance, rain water and sea water contributing to the obtained groundwater composition and to trace the possible reactions. The M3 model consists of 4 steps: standard principal component analysis (PCA), selection of reference waters, calculations of mixing proportions, and mass balance calculations. The steps
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are described in detail by Laaksoharju et al. [5] and Laaksoharju [6]. The model uncertainties have been handled in M3 by calculating an uncertainty of 0.1 mixing units (with a confidence interval of 90%) and stating that a mixing portion
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