Using MODFLOW and RT3D to simulate diffusion and reaction without discretizing low permeability zones

Low permeability zones (LPZs) are major sources of groundwater contamination after active remediation to remove pollutants in adjacent high (HPZs). Slow back diffusion from LPZs HPZs can extend management polluted sites by decades. Numerical models often used simulate diffusion, estimate cleanup times, and develop site strategies. Sharp concentration gradients present at the interface between LPZs, hence accurate simulation requires fine grid sizes resulting computational burden. Since MODFLOW family codes is widely practice, we a new approach for modeling pollutant using MODFLOW/RT3D that eliminates need discretization LPZ. Instead, LPZ treated as an impermeable region MODFLOW, while RT3D conceptualized series immobile coupled with mobile zone HPZ/LPZ interface. Finite volume reaction within then modeled mass transfer among several species. This results domain significantly fewer cells compared required if all discretized, providing potential improved efficiency. Cases, including layer HPZ over LPZ, thin/thick lens embedded HPZ, multiple tested tracer reactive scenarios.