PNNL

Abstract

A Lagrangian multi-scale particle model based on smoothed particle hydrodynamics (SPH) was used to simulate pore-scale flow, reactive transport and biomass growth in fractured porous media. The biomass growth was controlled by double Monod kinetics. The deformation and attachment and detachment of biomass was modeled through a combination of pair-wise short range repulsive and medium range attractive forces and hydrodynamics forces resulting from the fluid flow. The multi-scale model was compared with a cellular automata model that explicitly assumed that biomass can grow only in directions with shear stresses smaller than a critical value. For the set of parameters used in the simulations both cellular automata and multi-scale models predicted that: 1) biomass grows in the shape of bridges connecting soil grains and oriented in the direction of flow so as to minimize resistance to the fluid flow; 2) the solution containing electron donors and acceptors gets rapidly depleted as it enters the fractured porous domain; 3) and the biomass growth occurs mainly at the entrance into the fracture. The multi-scale model predicted that biomass would spread uniformly along the walls of the fracture while the cellular automata model predicted that the most of the biomass would concentrate at the entrance of the fracture. The proposed multi-scale model shows that biomass can spread along preferential flow paths and seal off a porous matrix creating biogeochemical barriers to contaminant migration and that attachment/detachment of biomass is an important mechanism in biomass spreading and can not be disregarded in computational models.