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The particle-tracking method has several advantages over finite difference or finite element approaches in terms of performance and accuracy, particularly for heterogeneous, advectively-dominated systems, such as avoiding numerical dispersion and improved efficiency. In practice, particle-tracking models are mostly used to study the advective and dispersive behavior of solute transport in groundwater. Although there is existing work that couples multispecies geochemical reactions with particle-tracking models, the approach adapted by these studies is to simplify the reaction terms into a first-order decay process because of the challenge of simulating high order chemical reactions by particle movements. In this study, we integrated a particle-tracking model with a multispecies biogeochemical reaction system of nitrogen transport and transformation in aquifers. Instead of using first-order decay process, the biodegradation processes within the system are modeled by multiple Monod equations and the multispecies reactions are represented by a system of chemical reaction equations. In our coupled system, the reactive multispecies transport process is operator split into two steps: (1) the physical movement of the particles including the attachment/detachment to solid surfaces, which is modeled by a Lagrangian random walk algorithm; and (2) multispecies reactions including biodegradation, which is solved by an ordinary differential equation solver. After step (1), the particles are converted to grid-based concentrations based on the mass and position of the particles, and after step (2) the newly calculated concentration values are mapped back to particles. The coupled system is capable of simulating subsurface nitrogen transport and transformation in a three-dimensional domain with variably saturated conditions. Implementation details, performance of the computer code, verification against other numerical and one-dimensional analytical models will be presented. The challenges we are facing with this approach will be discussed as well.
