  The typical temporal resolution used in modern simulations significantly
exceeds characteristic time scales at which the system is driven. This is
especially so when systems are simulated over time-scales that are much longer
than the typical temporal scales of forcing factors. We investigate the impact
of space-time upscaling on reactive transport in porous media driven by
time-dependent boundary conditions whose characteristic time scale is much
smaller than that at which transport is studied or observed at the macroscopic
level. The focus is on transport of a reactive solute undergoing diffusion,
advection and heterogeneous reaction on the solid grain boundaries. We first
introduce the concept of spatiotemporal upscaling in the context of
homogenization by multiple-scale expansions, and demonstrate the impact of
time-dependent forcings and boundary conditions on macroscopic reactive
transport. We then derive the macroscopic equation as well as the corresponding
applicability conditions based on the order of magnitude of the P\'{e}clet and
Damk\"{o}hler dimensionless numbers. Finally, we demonstrate that the dynamics
at the continuum scale is strongly influenced by the interplay between signal
frequency at the boundary and transport processes at the pore level.
