Reactive Transport Modeling of Biogeochemical Processes in Groundwater and Soils – A Tool for Data Integration and Building Conceptual Understanding.

The interactions between transport, geochemical and biological processes in aquifers, the vadose zone, and soils are characterized by a high degree of complexity. As a result, flexible model formulations are required that allow consideration of problem-specific transport and reaction processes. In addition to coupling solute transport and geochemical reactions, substantial progress has been made over the past decades by extending the capabilities of reactive transport models. More comprehensive model formulations are now available that allow simulation of plant-soil interactions, vadose zone gas transport processes, microbial growth and decay, atmospheric boundary conditions, and density coupling between water flow and solute transport, possibly involving highly saline solutions. At the same time major advances have been made in terms of data acquisition and analysis techniques, both in the laboratory and the field. Nowadays, models can be used most effectively in tandem with laboratory experiments or field observations to further our conceptual understanding and to quantitatively integrate data sets. Several examples are provided to demonstrate the use of reactive transport models for this purpose. The examples focus on elemental cycling in soils, carbon sequestration in ultra-mafic mine tailings under arid climate conditions, and the natural attenuation of organic contaminants in the vadose zone at crude oil and biofuel spill sites. The value of mineralogical characterization, flux, and rate measurements to constrain models and to develop a quantitative understanding of processes is discussed. In addition, an outlook on future modeling needs for simulating reactive transport in soil environments and the vadose zone is provided.