Numerical modelling on three-dimensional subsurface flow, heat transfer, and fate and transport of chemicals and microbes

A three-dimensional model to simulate the subsurface flow, the heat transfer, the microbial growth and degradation, and the transport and biodegradation of chemicals in groundwater environments is developed. This model, 3DFATMIC, is designed to evaluate the fate and transport of microbes and chemicals in saturated-unsaturated porous media under either isothermal or non-isothermal conditions. The system may include transport of seven components (one substrate, two electron acceptors, one nutrient, and three microbial populations), heat movement, and subsurface flow through saturated-unsaturated media. A numerical scheme, LEZOOMPC (adaptive local zooming and peak/valley capturing), is developed and employed to accurately solve two-and three-dimensional transport problems. With the implementation of LEZOOMPC, advective transport can be accurately approximated using large grid and time-step sizes.;Microbiological processes can transform organic and inorganic chemicals existing in the subsurface. Many physical factors can decrease or increase the rate of microbial growth. Temperature is one of the most important ones and has a large effect on the growth rate of microbes. It is also recognized that the density and dynamic viscosity of fluids vary with temperature. Therefore, the consideration of heat transfer is prerequisite to simulating the fate and transport of microbes and chemicals accurately. Since the advection of chemicals and microbes is determined by subsurface flow, which changes with the temperature-and constituent-dependent density, the modeling of subsurface flow is coupled in the model.;The model has been verified with 17 examples; five of which are specifically presented here. One demonstrated application in a three-dimensional regime has also been made. The satisfactory results show the model's ability to produce the expected solutions for flow, heat transfer, transport, and coupled flow and transport simulations. The implementation of LEZOOMPC has been proven by comparing simulation results and analytical solutions to exactly capture the peak and greatly reduce numerical dispersion. From the model calibration in this study, a correct flow field and a good well discretization are extremely important. The inclusion of heterogeneity, anisotropy, unsaturated zone, and mixed element shapes enhances this three-dimensional model's ability to deal with many real-world problems.