In this paper, an efficient modified two-grid block-centered finite
difference algorithm with second-order backward difference formula is
presented for coupled nonlinear reaction-diffusion equations of the
solutes in the groundwater flow. The main steps of the modified two-grid
algorithm include dealing with a small-scale nonlinear problem on the
coarse grid with a large time step size and then dealing with a
large-scale linear problem on the fine grid with a smaller time step
size base on the second-order backward difference formula. Moreover, the
theoretical analysis illustrates that the modified two-grid algorithm is
able to maintain second convergence order in space-time domain. Several
numerical experiments including a practical example are carried out to
support the theoretical results and verify that the modified two-grid
block-centered finite difference with second-order backward difference
formula algorithm can achieve high efficiency while maintaining the
second-order accuracy, meanwhile, has a certain applicable value in
actual problem.