A reactive transport modeling approach to simulate biogeochemical processes in pore structures with pore-scale heterogeneities

Abstract

Redox processes, including degradation of organic contaminants, are often controlled by microorganisms residing in natural porous media like soils or aquifers. These environments are characterized by heterogeneities at various scales which influence the transport of chemical species and the spatial distribution of microorganisms. As a result, the accessibility of the chemical species by the resident microbial populations may be limited, altering the efficiency of the biodegradation process. Hence, the biodegradation rate of contaminants at large scales does not only depend on the degradation capacity of the indigenous microbial population but also on the heterogeneities of the hosting media at smaller scales. It is thus important to establish a link between effective reaction rates and various structural features of porous media which can be directly observed or measured. This link is urgently needed because explicit resolution of heterogeneities within large-scalereactivetransport models is still limited by the available computational capacities.

The present study introduces areactivetransportmodelingapproach to determine the influence of pore-scaleheterogeneities on biogeochemicalprocesses in porous media. For this purpose, apore network model, which simulates flow and advective–diffusive transport of chemical species in heterogeneous pore networks is developed and coupled to the Biogeochemical Reaction Network Simulator (BRNS). The resulting coupled model (PNBRNS) is able to simulate the reactivetransport of solutes in heterogeneous pore assemblies. The PNBRNS model is applied for the simulation of a test case of bioavailability and effective biodegradation rate of a dissolved contaminant in different pore networks, built using a discrete set of geostatistically derived pore-size or biomass distributions. Results show that the heterogeneity of the pore-size distribution has a significant impact on bioavailability while the heterogeneity of the biomass distribution only leads to minor effects. The model also includes intra-pore bioavailability restrictions using diffusion-limited biodegradation kinetics. The results indicate that intra-pore limitations lead to extra constrains on the biodegradation of contaminants, even in the presence of larger-scale structural heterogeneities