U. Owing to the potential risk, remediation is being accelerated at this site. Developing an effective remediation plan and evaluating remedial options for the site requires a detailed analysis of fate and transport for this site and such an analysis must rely on numerical modeling. The purpose of this study was to develop a conceptual model for contaminant fate and transport at the 216-B-26 Trench site to allow interpretation of the current contaminant distributions and to support identification, development, and evaluation of remediation alternatives.In developing the conceptual model, the presence of large concentrations of 99 Tc high above the water table implicated stratigraphy in the control of the downward migration. The resulting conceptual model therefore included 1) small-scale stratigraphy and changes in physical and chemical properties, 2) tilted layers to accommodate the natural slope to the formation, and 3) lateral spreading along multiple strata with contrasting physical properties. Flow and transport properties were derived using physically based pedotransfer functions that were coupled with high-resolution neutron moisture logs taken on a vertical spacing of 0.076 m. Heterogeneity in the longitudinal and transverse horizontal directions was incorporated by using geostatistical methods to overlay the spatial correlation structure of flow variables from the well-characterized 299-E24-111 test site on to the simulation domain. Two approaches were used to compare no-action and capping remedial alternatives. The first approach, based on a simple analytical convolution solution to the convective-dispersive equation, assumed steady downward flow and allowed for spatially averaged or variable water content. This approach was combined with a soil screening protocol to calculate soil screening levels (Tables S1 and S2) and contaminant concentrations reaching hypothetical receptors down gradient of the site. In the second approach the STOMP simulator was used to predict contaminant transport through the vadose zone and into a 5-m thick confined aquifer during transient multidimensional flow.Both modeling approaches show that leachate concentrations reaching the water table would exceed the MCLs under the no-action alternatives with hypothetical recharge rates of 25 and 3.5 mm/yr. Soil screening levels were exceeded under the no-action alternatives and concentrations of mobile contaminants reaching hypothetical receptor wells consistently exceeded the MCLs (Tables S3 and S4). Owing to the large inventory of 99 Tc and NO 3 -, the high mobility of the two contaminants, and the long half life of 99 Tc, additional measures appear necessary to meet the appropriate soil screening levels. In this respect, on-site capping was evaluated simply by reducing surface recharge to mimic the design performance of hypothetical surface barriers. Both modeling approaches show capping to be an effective technology for remediating the site. Capping successful removed the threat to ground water through a reduction in mass flux ...