Biofilm growth in porous media changes not only the hydrodynamic properties of the medium (reduction in porosity and permeability, and increase in dispersivity), but also the transport itself (breakthrough curves display increasingly fast first arrivals and long tails). These features are well reproduced by multicontinuum models (Multi-Rate Mass Transfer, MRMT) which can be used to describe reactive transport in heterogeneous porous media and facilitate the simulation of reactions that are localized within biofilms. Here, we present a conceptual and numerical model of biochemical reactive transport with dynamic biofilm growth based on MRMT formulations. Mass exchange between mobile water and immobile biofilm aggregates is represented by a memory function, which simplifies definition of MRMT parameters. We successfully tested this model on two sets of laboratory data and found that (a) a basic model based on the growth of uniformly sized biofilm aggregates fails to reproduce laboratory tracer tests and rate of biofilm growth, while a fractal growth model, which we obtain by integrating the memory functions of biofilm aggregates with a power law distribution, does; (b) the biofilm memory function evolves as the biofilm grows; and (c) the early time portion of eluted volume tracer breakthrough curves are independent of flow rate, whereas the tail becomes heavier when the flow rate is decreased, which implies that both advection controlled and diffusion controlled mass exchange coexist in biofilms. These findings imply that porous media biofilms are essentially different from those developing in human tissues or open spaces.

The authors acknowledge the financial support of projects RESTORA (Agència Catalana de l'Aigua, ACA210/18/0040), ConMimo (Spanish Research Agency, AEI, number TED2021-131188B-C31), and Ciencia de Frontera CONAHCYT CF-2023-G-904. Additional funding was obtained from the Generalitat de Catalunya (2017 SGR1485) and the Spanish Ministry of Science and Innovation (Center of Excellence Severo-Ochoa, CSIC-IDAEA, CEX2018-000794-S). Financial support from National Natural Science Foundation of China (Grant 42307271) is acknowledged. J. W. is also sponsored by Shanghai Pujiang Program (Grant 23PJ1413200). We thank two anonymous reviewers for their approval and constructive comments that significantly helped improve the manuscript.

Peer reviewed