Vadose Zone Contaminant Fate and Transport Analysis for the 216-B-26 Trench

Abstract

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 ...

Experimental and Numerical Investigations of Soil Desiccation for Vadose Zone Remediation: Report for Fiscal Year 2007

“…Of the available technologies, very few are applicable to the deep vadose zone. Ward et al (2004) investigated the feasibility and efficacy of engineered barriers for the BC Cribs and trenches site, an area of concern because of elevated 99 Tc concentrations at a depth of 90 m below the surface. In relation to the same site, Truex reviewed six technology alternatives for minimizing downward contamination transport, of which soil desiccation ranked highest.…”

“…(a) An expert panel review of the work by Ward et al (2004) and that of Truex concluded that 1) there was a need for supplemental technologies for use with engineered barriers, and 2) there were a number of knowledge gaps that would need to be overcome before this desiccation could be deployed.…”

“…In heterogeneous formations, the extent of lateral air migration will depend on the horizontal correlation lengths or connectivity of the layers (Ward et al 2004) as well as the air-injection rate and the range of permeabilities. The modeling effort therefore considered both homogenous and heterogeneous formations.…”

“…One set of simulations was conducted in a heterogeneous formation composed of alternating layers of sand and silt loam, intended to mimic the lithology at the BC Cribs and Trenches Site (Ward et al 2004). All of the simulations were conducted with a 25-× 25-m spatial domain discretized with a variable grid spacing ranging from 0.30-m to 1-m grid cells.…”

“…At sites with complex hydrogeology and those where contaminants are located beyond the practical depth of excavation, the options are even more limited (Ward et al 2004). Truex (a) reviewed six technology alternatives for minimizing downward contamination transport: these were in situ gaseous reduction, vadose zone bioreduction, vadose zone permeable reactive barrier remediation, redox manipulation, in situ grouting, and soil desiccation.…”