Review of thin-layer placement applications to enhance natural recovery of contaminated sediment

This article provides a review of thin-layer placement applications to enhance the natural recovery of contaminated sediment. Three principal case studies are presented in which thin-layer placement has been implemented as a component of enhanced monitored natural recovery (EMNR). EMNR is defined as the application of engineered means such as thin-layer placement or broadcasting of capping material to accelerate natural recovery processes in locations not appropriate for application of monitored natural recovery (MNR) alone. Case studies examine factors affecting the implementation of EMNR, including the impact of site conditions on stable and successful thin-layer placement of clean sediment or other capping material, as well as the challenges in development and implementation of monitoring plans that chart progress toward achieving remedy success. Pilot-scale or demonstration studies of thin-layer placement of clean sand or sediment are subsequently assessed to highlight a range of potentially successful strategies for placement and post-placement monitoring. The primary difference between the pilot-scale or demonstration sites and the 3 primary EMNR case studies is that monitoring at the demonstration sites has focused more explicitly on understanding mechanisms of material placement and/or chemical migration, rather than assessing longer-term or more comprehensive remedial action objectives (RAOs) such as reductions in human health or ecological risk. All sites discussed in this review appear to have demonstrated reductions in the surface sediment concentration of at least some chemicals of concern following thin-layer placement; however, the achievement of human and ecological risk reduction has been inconsistent or is still under evaluation. Effective monitoring as an integral component of EMNR continues to represent a challenge. For cap material stability, monitoring typically focuses on surface sediment chemistry and the persistence of the cap material, whereas monitoring of ecological recovery tends to be limited or difficult and is not always correlated with successful placement of the thin-layer, especially in the short term. Recontamination of the newly placed cap material has been a relatively common occurrence in many of the sites considered herein, and has led to exceedance of remedial targets. However, in no case did recontamination return surface sediment chemical concentrations to pre-placement levels. Where the placement of cap material is stable and there is no evidence of chemical migration through the cap, recontamination signals a need to update conceptual site models to better reflect sediment and contaminant transport processes in areas in which EMNR has been implemented.