PNNL

Abstract

A number of waste trenches at cribs at Hanford’s BC Cribs and Trenches site, which received about 10 Mgal of scavenged tank waste with elevated concentrations of technetium-99 and nitrate, are currently being evaluated for remediation. The objective of this study was to investigate the influence of fine-scale heterogeneity (i.e. horizontal laminations, cross-bedding) on the large-scale transport behavior of mobile contaminants through the vadose zone with the purpose of developing a remedial strategy. The vertical heterogeneity structure, conditioned on grain size distributions and geophysical logs (water content and natural isotopes), was developed from a single borehole at the site. Geostatistical methods were used to impose the 3-D spatial correlation structure from the nearby well-characterized experimental site to merge the heterogeneities at various scales. Flow and transport properties were derived using physically-based property transfer models. The STOMP simulator was then used to predict contaminant transport through the vadose zone and into a 5-m thick confined aquifer during the period of trench operations (1956-1958) and to present time. Simulation results show that the fine-scale heterogeneity inside the large-scale lithologic units has considerable impact on the large-scale transport behavior of contaminants. The fine-scale heterogeneity enhanced the lateral flow and mixing and limited vertical penetration in the vadose zone. Model results are in excellent agreement with the vertical contaminant profile obtained from a borehole installed in the 216-B-26 trench. The simulated 2-D distribution of nitrate and electrical resistivity sounding curves also agreed well with results of field-scale resistivity surveys. These results suggest that installation of an engineered surface barrier would reduce the threat to ground water by reducing the mass flux of contaminants to the water table and increasing the residence time in the vadose zone.