ADVANCES IN UNDERSTANDING SORPTION AND TRANSPORT PROCESSES AFFECTING THE FATE OF ENVIRONMENTAL POLLUTANTS IN THE SUBSURFACE Hrissi K. Karapanagioti 1, David Werner 2, Charles J. Werth 3 1 Department of Chemistry, University of Patras, Patras, Greece, 2 School of Civil Engineering and Geosciences, Newcastle University, UK, 3 Department of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign, USA, The attachment of environmental pollutants to solid matrices in soils and sediments is a well- investigated physicochemical process of fundamental importance for predicting pollutant fate, and the risk that pollutants pose to human and ecological receptors. The continuing refinement of our mechanistic process understanding challenges us to improve the representation of sorption in computational pollutant fate models in order to provide more realistic predictions of pollutant fate in complex soil and aquifer systems, and cleanup times for alternative soil and groundwater remediation technologies. We have edited this special issue entitled “Sorption and Transport Processes Affecting the Fate of Environmental Pollutants in the Subsurface” with the following objectives: a) to present studies of processes that are normally not included or are simplified in large-scale computational models because of the constraints on computation time or scale differences between experimental settings and field work, b) to discuss thoroughly the importance of the new findings related to sorption processes in terms of their wider environmental significance, and c) to strengthen the ties between sorption and transport processes through advances in experimental and field work as well as modelling studies. We feel that it is valuable to collect in one issue papers that would otherwise appear in a range of issues / journals, and we hope that this special issue will prove valuable for those at all stages of inquiry into this topic. Nine papers were accepted. They are separated into four main topics as follows: a) new experimental methodologies and parameter evaluation, b) implications for pollution remediation and reclamation methods, c) challenges in modifying and selecting existing models to interpret experimental results, and d) subsurface properties that affect sorption and transport. Pollutants studied include: biocolloids, metals (arsenic, chromium, nickel), organic compounds such as hydrocarbons, chlorinated hydrocarbons, micropollutants (PAHs, PCBs), and pesticides (glyphosate, 2,4-D). Overall, our special issue indicates that existing models need modification to fit experimental data, and there remain gaps in our understanding of the underlying mechanisms controlling sorption and transport. There is still much work to be done in this fascinating area! Acknowledgements The authors would like to thank David A. Sabatini for helpful discussions and guidance. This presentation is funded by the University of Patras and by the University of Patras Research Committee Program “Karatheodori 2009” Jeppu et al. uses a modified batch reactor system to study equilibrium- reactive transport problems of metals. This paper provides a novel experimental system for investigating one-dimensional transport problems involving equilibrium geochemical reactions. The proposed system uses a set of sequentially coupled batch reactors that directly mimic a typical one-dimensional numerical grid. Jeppu and Clement propose a new isotherm model that combines modified traditionally used isotherms and can be used to simulate pH-dependent metal adsorption. This analytical isotherm model, is identified as the modified Langmuir- Freundlich (MLF) isotherm. The isotherm was tested using adsorption data collected for arsenic and goethite-coated-sand systems. Syngouna and Chrysikopoulos use column studies along with theoretical approximations to evaluate the combined effects of grain size and pore water velocity on the transport of three biocolloids in water saturated porous media. The main objective of this study is to evaluate the combined effects of grain size and pore water velocity on the transport in water saturated porous media of three waterborne fecal indicator organisms (Escherichia coli, MS2, and  X174) in laboratory-scale columns packed with clean quartz sand. AC amendment is an innovative in-situ method for the remediation of sediments polluted by hydrophobic organic compounds, but the performance depends on the microscale spatial distribution of the AC in the polluted sediment. Modelling and 5 year field monitoring data confirm the long-term benefits of AC amendment and illustrate the importance of achieving a good sorbent distribution during the initial AC placement. Cho et al. evaluate a sorbent-based sediment remediation method considering site-specific conditions such as small scale heterogeneity by bringing together monitoring results and modelling. Ranieri and Young through a field study point to the importance of clogging and also sorption as mechanisms affecting the effectiveness of sub-surface flow constructed wetlands. Chromium (Cr) and Nickel (Ni) removal was also assessed as a function of increased clogging that occurred in the cws over a four year research program. Proportionality between increasing clogging and sediment accumulation of metals was observed, especially for Ni. Adsorption to the original matrix and the accumulated sediment is a removal mechanism consistent with available data. Linear free energy relationships (LFERs) were established which estimate stable carbon and hydrogen isotope enrichment factors for equilibrium sorption to geosorbents from equilibrium vapor-liquid isotope effects. For benzene in the figure above it is illustrated that the 13C and 2H-benzenes will sorb more weakly because their vapor pressures are higher. The LFERs are useful since very few experimental data for fractionation by sorption are available. Höhener and Yu demonstrate the ability of linear free energy relationships (LFERs) to predict slight shifts in the isotopic ratio of groundwater pollutants which are due to sorption. Magga et al. test possible modifications that improve the reliability of kinetic models and their parameter values during the evaluation of pesticide sorption experiments. A thorough review paper by Cheng et al. presents the impact of mineral micropores on the transport and fate of organic contaminants especially when the porous geological media have very low organic carbon contents. Organic contaminants can adsorb into the hydrophobic micropore spaces present in porous geological media through displacement of the sorbed water, and their desorption rate is very slow due to hindered molecular diffusion Because of the inhibitory effect on abiotic and biotic transformations, micropore sorption can cause preservation of anthropogenic organic contaminants in the subsurface and may increase their persistence to the time scale of geological ages under appropriate conditions. Werner et al., discuss the effect of grain- scale sorption rate limitations on the groundwater pollutant fate based on experimental results and the Damköhler number. Relatively high groundwater velocities in combination with a low sorption coefficient Kd and slow diffusion limited sorption kinetic rates are necessary conditions to justify the implementation of grain-scale sorption rate limitations in groundwater contaminant fate models.