Subsurface colloids can enhance the movement of strongly sorbing contaminants, a phenomenon called colloid‐facilitated contaminant transport. In the presence of mobile subsurface colloids, contaminants may move faster and farther than in the absence of colloids, thereby bypassing the filter and buffer capacity of soils and sediments. Fate and transport models neglecting colloid‐facilitated transport therefore often underpredict contaminant movement. Long‐term predictions of contaminant fate and transport as well as risk assessment rely on an accurate representation of subsurface processes, and in the case of strongly sorbing contaminants, need to consider mobile colloids as potential contaminant carriers. The purpose of this review is to discuss the current knowledge and recent developments of modeling colloid‐facilitated contaminant transport in the vadose zone. The main part of this review is devoted to the discussion of conceptual models used to describe colloid‐facilitated contaminant transport in the vadose zone and their mathematical implementation. Modeling of colloid‐facilitated contaminant transport involves various interactions, including colloid attachment to and detachment from the solid matrix and the air–water interface, contaminant adsorption to and desorption from colloids and transport with mobile colloids, and contaminant adsorption to and desorption from the solid matrix. Most of these processes in colloid‐facilitated contaminant transport models have been described by first‐ or second‐order kinetics. The unique feature of the vadose zone is the presence of an air phase, which affects colloid and contaminant transport in several ways. Colloids can be trapped in immobile water, strained in thin water films and in the smallest regions of the pore space, or attached to the air–water interface itself. The modeling of colloid‐facilitated contaminant transport in the vadose zone has mostly been theoretical, and tested only with column experiments; field applications are still lacking.

Abstract:Traditionally a streambed is treated as a layer of uniform thickness and low saturated hydraulic conductivity (K) in surfaceand ground-water studies. Recent findings have shown a high level of spatial heterogeneity within a streambed and such heterogeneity directly affects surface-and ground-water exchange and can have ecological implications for biogeochemical transformations, nutrient cycling, organic matter decomposition, and reproduction of gravel spawning fish. In this study a detailed field investigation of K was conducted in two selected sites in Touchet River, a typical salmon spawning stream in arid south eastern Washington, USA. In-stream slug tests were conducted to determine K following the Bouwer and Rice method. For the upper and lower sites, each 50 m long and 9 m wide and roughly 20 m apart, a sampling grid of 5 m longitudinally and 3 m transversely was used. The slug tests were performed for each horizontal coordinate at 0Ð3-0Ð45, 0Ð6-0Ð75, 0Ð9-1Ð05 and 1Ð2-1Ð35 m depth intervals unless a shallower impenetrable obstruction was encountered. Additionally, water levels were measured to obtain vertical hydraulic gradient (VHG) between each two adjacent depth intervals. Results indicated that K ranged over three orders of magnitude at both the upper and lower sites and differed between the two sites. At the upper site, K did not differ significantly among different depth intervals based on nonparametric statistical tests for mean, median, and empirical cumulative distribution, but the spatial pattern of K varied among different depth intervals. At the lower site, K for the 0Ð3-0Ð45 m depth interval differed statistically from those at other depth intervals, and no similar spatial pattern was found among different depth intervals. Zones of upward and downward water flow based on VHG also varied among different depth intervals, reflecting the complexities of the water flow regime. Detailed characterization of the streambed as attempted in this study should be helpful in providing information on spatial variations of streambed hydraulic properties as well as surface-and ground-water interaction.

No abstract