Understanding and predicting colloid transport and retention in water‐saturated porous media is important for the protection of human and ecological health. Early applications of colloid transport research before the 1990s included the removal of pathogens in granular drinking water filters. Since then, interest has expanded significantly to include such areas as source zone protection of drinking water systems and injection of nanometals for contaminated site remediation. This review summarizes predictive tools for colloid transport from the pore to field scales. First, we review experimental breakthrough and retention of colloids under favorable and unfavorable colloid/collector interactions (i.e., no significant and significant colloid‐surface repulsion, respectively). Second, we review the continuum‐scale modeling strategies used to describe observed transport behavior. Third, we review the following two components of colloid filtration theory: (i) mechanistic force/torque balance models of pore‐scale colloid trajectories and (ii) approximating correlation equations used to predict colloid retention. The successes and limitations of these approaches for favorable conditions are summarized, as are recent developments to predict colloid retention under the unfavorable conditions particularly relevant to environmental applications. Fourth, we summarize the influences of physical and chemical heterogeneities on colloid transport and avenues for their prediction. Fifth, we review the upscaling of mechanistic model results to rate constants for use in continuum models of colloid behavior at the column and field scales. Overall, this paper clarifies the foundation for existing knowledge of colloid transport and retention, features recent advances in the field, critically assesses where existing approaches are successful and the limits of their application, and highlights outstanding challenges and future research opportunities. These challenges and opportunities include improving mechanistic descriptions, and subsequent correlation equations, for nanoparticle (i.e., Brownian particle) transport through soil, developing mechanistic descriptions of colloid retention in so‐called “unfavorable” conditions via methods such as the “discrete heterogeneity” approach, and employing imaging techniques such as X‐ray tomography to develop realistic expressions for grain topology and mineral distribution that can aid the development of these mechanistic approaches.

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First, we discuss network models and techniques for determining the pore-scale properties and characteris-While network models of porous materials have traditionally been tics; second, we describe the regular and random porous constructed using regular or disordered lattices, recent developments allow the direct modeling of more realistic structures such as sphere media systems used to validate and evaluate the techpackings, microtomographic images, or computer-simulated materi-niques we are proposing; third, we present the improveals. One of the obstacles in these newer approaches is the generation ments made to two network generation techniques of network structures that are physically representative of the real based on their known limitations; fourth, we validate systems. In this paper, we present and compare two different algothese algorithms using regular packings; and finally, we rithms to extract pore network parameters from three-dimensional evaluate and compare the network representations of images of unconsolidated porous media systems. The first approach, random packings generated by these techniques. which utilizes a pixelized image of the pore space, is an extension to unconsolidated systems of a medial-axis based approach (MA). The second approach uses a modified Delaunay tessellation (MDT) of the BACKGROUND grain locations. The two algorithms are validated using theoretical Early models developed to account for pore-scale packings with known properties and then the networks generated properties idealized the pore space as collections of from random packing are compared. For the regular packings, both capillary tubes and provided simple analytical solutions methods are able to provide the correct pore network structure, including the number, size, and location of inscribed pore bodies, the num-to predict continuum-scale properties such as permeber, size, and location of inscribed pore throats, and the connectivity. ability. However, these models failed to incorporate the Despite the good agreement for the regular packings, there were interconnectivity of the pore space. Thus the idea of differences in both the spatial mapping and statistical distributions in representing the pore space as a two-or three-dimennetwork properties for the random packings. The discrepancies are sional network emerged from the pioneering work by attributed to the pixelization at low resolution, non-uniqueness of the Fatt (1956a,b,c). Due to the complexity of the poreinscribed pore-body locations, and differences in merging processes space morphology, the pore bodies and throats are usuused in the algorithms, and serve to highlight the difficulty in creating ally represented by simplified shapes. Pore bodies have a unique network from a complex, continuum pore space.

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River deltas all over the world are sinking beneath sea-level rise, causing significant threats to natural and social systems. This is due to the combined effects of anthropogenic changes to sediment supply and river flow, subsidence, and sea-level rise, posing an immediate threat to the 500–1,000 million residents, many in megacities that live on deltaic coasts. The Mississippi River Deltaic Plain (MRDP) provides examples for many of the functions and feedbacks, regarding how human river management has impacted source-sink processes in coastal deltaic basins, resulting in human settlements more at risk to coastal storms. The survival of human settlement on the MRDP is arguably coupled to a shifting mass balance between a deltaic landscape occupied by either land built by the Mississippi River or water occupied by the Gulf of Mexico. We developed an approach to compare 50 % L:W isopleths (L:W is ratio of land to water) across the Atchafalaya and Terrebonne Basins to test landscape behavior over the last six decades to measure delta instability in coastal deltaic basins as a function of reduced sediment supply from river flooding. The Atchafalaya Basin, with continued sediment delivery, compared to Terrebonne Basin, with reduced river inputs, allow us to test assumptions of how coastal deltaic basins respond to river management over the last 75 years by analyzing landward migration rate of 50 % L:W isopleths between 1932 and 2010. The average landward migration for Terrebonne Basin was nearly 17,000 m (17 km) compared to only 22 m in Atchafalaya Basin over the last 78 years (p < 0.001), resulting in migration rates of 218 m/year (0.22 km/year) and <0.5 m/year, respectively. In addition, freshwater vegetation expanded in Atchafalaya Basin since 1949 compared to migration of intermediate and brackish marshes landward in the Terrebonne Basin. Changes in salt marsh vegetation patterns were very distinct in these two basins with gain of 25 % in the Terrebonne Basin compared to 90 % decrease in the Atchafalaya Basin since 1949. These shifts in vegetation types as L:W ratio decreases with reduced sediment input and increase in salinity also coincide with an increase in wind fetch in Terrebonne Bay. In the upper Terrebonne Bay, where the largest landward migration of the 50 % L:W ratio isopleth occurred, we estimate that the wave power has increased by 50–100 % from 1932 to 2010, as the bathymetric and topographic conditions changed, and increase in maximum storm-surge height also increased owing to the landward migration of the L:W ratio isopleth. We argue that this balance of land relative to water in this delta provides a much clearer understanding of increased flood risk from tropical cyclones rather than just estimates of areal land loss. We describe how coastal deltaic basins of the MRDP can be used as experimental landscapes to provide insights into how varying degrees of sediment delivery to coastal deltaic floodplains change flooding risks of a sinking delta using landward migrations of 50 % L:W isopleths. The no...