Pseudokinetics arising from the upscaling of geochemical equilibrium

Multicomponent contaminant transport models in groundwater are typically based on assumptions of local geochemical equilibrium on the grid scale. However, in heterogenous systems there may be significant coupling between transport processes and geochemical equilibrium at smaller than grid block scale. Here various pore- and field-scale examples are considered to illustrate the impact of transport processes on assumptions of geochemical equilibrium. In each example the flow length scales required to reach equilibrium are calculated. It is shown that these can be as large as many meters at the pore scale and kilometers at field scales. The influence of heterogeneity in the distribution of the reactive zones is assessed for the pore-scale example, and it is shown that patchiness of reactive zones within a pore increases equilibration length, with the length and density of reactive zones, pore radius, and diffusion coefficient all playing a role in the equilibration length. When constructing models of field-scale problems it may not be reasonable to apply geochemical equilibrium, and it may be necessary to explicitly couple pore-scale and field-scale models in a multiscale simulation. A field-scale example is also shown to illustrate that the upscaling of geochemical equilibrium poses a significant practical problem because we usually do not know the spatial location and distribution of geochemically active sites, and this information is essential input to geochemical transport models.