Assessment of sediment disturbance during application of a thin-layer cap in Peninsula Harbor, Canada

Abstract

Thin-layer caps (TLC) can be an effective method for managing in situ–contaminated sediments. However, there is very little information in the literature on the amount of mixing between the sediments and the applied cap material that should be estimated during design. This study examined the amount and extent of disturbance resulting from the placement of a cap material as evidenced by visual observations and verified with vertical chemical gradients profiles. Pollucite, a zeolite mineral tracer enriched with cesium, was entrained in the top 2 cm of the sediment surface by divers as a marker of the sediment-water interface prior to cap construction. Diver-collected cores were extracted 24–48 h after tracer application to determine the initial concentrations of pollucite. Post-cap construction, cores were collected from each of the test plots by divers during two subsequent sampling efforts, 4 months and 1 year after the tracer was applied to assess the degree of mixing. The cores were then split, photographed, and sectioned into 1 cm intervals which were analyzed for particle size distribution and a suite of trace metal elements including cesium. Each of the sectioned cores collected had a discrete and distinct visual interface between the original sediments and the cap material with no mixing observed which was consistent with the X-ray images. All the cores had a well-defined Cs signal/depth profile with only trace levels being detected in the samples of cap material that were in direct contact with sediment. This mixing zone, as determined by the Cs signal, was generally constrained to within 2 cm of the interface between the capping material and contaminated sediment and is consistent with the initial depth that the pollucite was entrained in the sediment. The chemical analyses also confirmed that there was no measureable transfer of mercury or other contaminants from the sediment to the cap material. All lines of evidence indicate that there was very little disturbance or entrainment of the in situ sediments into the cap material in this study. While these results are specific to the site studied, some general insights and approaches to assess sediment disturbance during cap construction have resulted. This study has also successfully expanded the use of a tracer, such as pollucite, to assess the disturbance of in-place sediments during the construction of a thin-layer cap.