Special Section on Mass Transport and Environmental Risk Assessment

Modeling and prediction of the mass transport in subsurface environments has been a very challenging issue in both scientific research and engineering practice. Traditionally, the study of mass transport in geological media focused more on fluid flow, because the principal purpose was to assess and exploit natural resources, like groundwater, petroleum, natural gas, and geothermal energy. With the growing importance of environmental issues in recent years, the study of mass transport and the fate of different kinds of hazardous and toxic contaminants, such as radionuclides, heavy metals, volatile organic compounds (VOCs), and carbon dioxide in both engineered materials and geological media has become more and more important, and significant progress has been made in the research area. This special section is devoted to recent studies on mass transport associated with various applications as well as environmental risk assessment from different viewpoints.

The environmental fate and transport of hazardous and toxic contaminants in subsurface environments is very complex. Although transport of contaminants in a medium, such as a soil or rock matrix, or an artificial material is primarily controlled by the mechanisms of advection, dispersion, sorption, chemical or biochemical reactions, and chain decay, many factors may affect the models for describing mass transport and the magnitudes of parameters related to transport properties. In addition, studies on mass transport and environmental risk assessment have many practical implications, such as geological disposal of radioactive waste, geological storage or sequestration of carbon dioxide, remediation of polluted soil and groundwater, land fill, and so on.

Zhu et al. performed a quantitative study on the sensitivity of advective contaminant arrival time to the porosity of hydrogeological units for groundwater transport from the Nevada Test Site (NTS) to Yucca Mountain (YM), a site which had been evaluated as a potential deep geological repository for the disposal of spent fuel and high-level radioactive waste in the United States. This study illustrated that the uncertainty in porosity of hydrogeologic units is an important factor on advective travel time and its uncertainty, and the study on sensitivity of advective contaminant arrival time may provide practical considerations for designing monitoring networks and remediation strategies for contaminated sites and determining the associated risks of groundwater pollution. Sun introduced an improved Ensemble Kalman Filter (EnKF) approach to identify the geological fault network geometry, an important factor that affects the subsurface flow and mass transport, from production data. The feasibility and applicability of the modified EnKF approach was demonstrated through a series of numerical experiments.

Liu et al. performed a modeling study to evaluate the sensitivity of acid gas evolution in a deep saline aquifer. A series of parameters, such as relative permeability hysteresis, aquifer heterogeneity, salinity of formation water, permeability of cap-rock and leakage wells, and formation depth, were examined through three-dimensional numerical simulations. Li proposed a coupled reactive transport model to investigate the heat and density-driven flow associated with ${\mathrm{CO}}_2$ storage in saline aquifers. Numerical simulations illustrated that the diffusivity and reaction rate has strong effects on the flow patterns. The convection flow transports ${\mathrm{CO}}_2$ into saline aquifers rather than diffusion flow for short and middle terms, whereas mineral trapping is prominent for the long term.

Asada et al. investigated the effects of subsoil hardpan on leaching of zinc and copper fractions in the soil amended with swine manure and chemical fertilizer. The study provided detailed knowledge about the leaching of high mobility heavy metals in a layered soil and can be useful for evaluating environmental risk from the manure treatment. Zhang et al. investigated the efficiency of chemical stabilization on soil residuals after soil washing. A series of laboratory batch experiments were performed on a clayish soil contaminated with Cu, Cr, Ni, Pb, Zn with lime, ${\mathrm{Na}}_2{\mathrm{HPO}}_4$, or a mixture containing ${\mathrm{FeSO}}_4$ under various concentrations. This study indicated that metal stabilization involves various mechanisms depending on the metal’s properties and species as well as the interference between metals.

Sharma et al. studied the transport and deposition of suspended soil-colloids in saturated sand. This study illustrated that the coupled effect of solution chemistry, colloid sizes, and medium surface properties has a dominating role in particle-particle and particle-collector interactions in colloid transport and deposition. Wickramarachchi et al. investigated the gas transport properties of compacted reddish brown soil for the use in Sri Lankan landfill final cover. Soil-gas diffusivity and air permeability were determined experimentally for differently compacted soil samples. The results obtained from this study may offer practical information for designing cover soils for optimally low water and high oxygen exchange while minimizing climate and toxic gas emissions between waste layer and atmosphere.

The study on mass transport and environmental risk assessment has many practical applications, especially in the field of geoenvironment. Because of the variety of applications, the complexity of the problems, the difficulty of modeling, and the lack of knowledge related to the corelationship between mass transport properties and affecting factors, many technical issues still remain to be resolved. The editor is looking forward to further advances in this area and new submissions of papers on such research topics to our journal in the future.