XVIII International Conference on Water Resources CMWR 2010 J. Carrera (Ed) CIMNE, Barcelona 2010 A LAGRANGIAN APPROACH TO SOLVE MULTICOMPONENT REACTIVE TRANSPORT IN COMPLEX GEOLOGICAL ENVIRONMENTS Paolo Trinchero*, Jorge Molinero Huguet*, Joaquin Salas Navarro* and Àngels Piqué * * Amphos XXI Consulting S.L. Passeig de Garcia i Faria, 49-51, 1º-1ª - E08019 Barcelona, Spain e-mail: [email protected], web page: http://www.amphos21.com/ Summary. When dealing with reactive transport problems in complex geological systems modelers have usually to adopt a compromise either choosing to describe in detail the geochemical reactions at the expense of a rigorous treatment of the hydrogeological processes or carrying out complex mechanistic descriptions of subsurface flow while linearizing geochemical processes with simplified Kd-based simulations (the blanket leaves the shoulders uncovered). We present a streamline-based framework where the heterogeneity of the flow velocity field is captured using a Lagrangian approach. Multicomponent reactions are then explicitly solved along one single 1D streamline for the case of geochemical homogeneity (LaSAR-PHREEQC, Malmstrom et al., 2009 ) or the entire set of streamlines for the case of geochemical heterogeneity (FASTREACT), thus avoiding the computational burden of solving hundreds of thousands speciation calculations in complex 3d grids. An application of the framework is presented where we evaluate the effects of an hypothetical escape of radionuclides from a deep geological repository. The probability density functions (pdf ) of the concentration of the selected set of radionuclides (Sr, Cs, U and Ra) at the outlet of each streamline allow to explicitly account for the uncertainty due to flow variability as well as to infer effective Kd parameters.