The purpose of this work is to investigate the extents of the local thermal equilibrium (LTE) assumption at the fluid-porous medium boundary (i.e., in a heterogeneous region). The analysis is performed in terms of the one-domain approach. Therefore, we posed and solved the associated closure problems in order to compute the spatial dependence of the effective coefficients at the fluid-porous medium boundary. Steady-state comparisons with direct numerical simulations evidence that the LTE is, in general, justifiable everywhere in the system, i.e., in both homogeneous and heterogeneous regions.