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The research community is actively studying subsurface fluid flow and reactive mass transport, where both the reactions and the transport are processes of primary importance. There are formidable challenges to overcome in advancing knowledge in this area. The sub-surface has a challenging level of complexity in its physical and chemical properties. It is difficult to characterize this complexity because of many challenges presented in acquiring subsurface data. This difficulty in characterization creates, in turn, challenges in developing conceptual models for physical processes and biogeochemical reactions. The relevant length and time scales span a large range. There is nonlinear interplay between influences of heterogeneity in physical and chemical properties and the forces driving reactions between adjacent zones. Material properties may themselves be evolving with the chemical evolution of the system.

Advances in our understanding of flow and reactive transport in physically and chemically heterogeneous media will likely come through a nexus of some of the following: (1) advancing the body of observational data from which processes can be characterized and modeled, including data on physical, chemical, and biological reactions and interactions; (2) combining information from experimentation at different scales: laboratory bench, mesoscale tanks, and in-situ at field sites; (3) advancing the body of observational data from which the heterogeneity of physical and chemical properties associated with these processes can be characterized and modeled; (4) developing upscaling methods to represent complexity at spatial scales below the model grid scale; (5) developing more efficient algorithms for solving large systems of coupled equations, and for estimating parameters; (6) using benefits of exponential growth in the speed of computational processing and digital storage when running simulations; and (7) using advances in visualization tools, which facilitate the interpretation of simulation results.

To present the state of the science, a full-day session was organized for the 2004 Geological Society of America Annual Meeting, titled ‘‘Modeling Flow and Transport in Chemically and Physically Heterogeneous Media.’’ The session included 23 presentations (Geological Society of America Abstracts with Programs, v. 36, no. 5). One of the striking aspects of the session was the degree to which advanced scientific visualization tools were used in conveying ideas, including the use of animated results from interpretive simulations. Some of these findings could not have been conveyed as effectively if published in printed pages within the traditional scientific literature. At the same time, the Geological Society of America was introducing Geosphere, this entirely electronic publication, motivated in large part by the desire to indeed include the state of the art in scientific visualization in peer-reviewed publications. It was clear to us that the research community would appreciate having full peer-reviewed articles published from among some of the talks given in the session, and furthermore, that some of the graphic and animation-intensive talks would be especially appropriate for presentation in Geosphere. In this way, our idea for this special volume of Geosphere was born.

Eight articles were contributed to the special volume of papers on modeling flow and reactive transport in physically and chemically heterogeneous media. These are divided among two issues of Geosphere (April and June). The articles collectively represent the majority of the research areas listed above. The four articles within this issue include one that uses animated graphics (Samper and Yang). Two of the four articles in press for the June issue also contain animated graphics (Scheibe et al. and Carle et al.).

We would like to thank the editor, Randy Keller, for his support and assistance in developing the special volume. We thank all those who contributed talks to the session, those who developed manuscripts from their talks for this volume, and those who participated in the peer review.