Formatted Title:

A Novel CapSim Application for Predicting Post-Remediation Porewater Concentrations

Background/Objectives:

The surface weighted average concentration (SWAC) approach has been applied at several sites for meeting performance targets in bulk sediments for bioaccumulative contaminants such as polychlorinated biphenyls (PCBs). However, the application of the SWAC approach for meeting performance targets in the porewater phase has been limited. This paper presents a study for a 3.7-acre tidal mudflat located in Washington, DC, with PCB-impacted sediments, wherein the remedial approaches needed to achieve a stringent post-remediation SWAC of 0.64 ng/L for PCBs in porewater. Several remedial approaches such as capping, dredging, in situ treatment, and combinations thereof were evaluated as part of a focused feasibility study (FFS) to meet this performance target by integrating CapSim simulations with site-specific chemical and geochemical data.

Approach/Activities:

As part of the Remedial Investigation and Feasibility Study, concentrations of PCBs and the organic carbon content in bulk sediments were measured. In addition, concentrations of PCB congeners in the porewater of surface sediments was measured at several locations at the site using ex situ passive sampling methods. Concentrations of total PCB congeners in bulk sediment and porewater were used to derive site-specific partitioning coefficients. Site-specific chemical and geochemical data were used as inputs to the CapSim model. Utilizing an innovative approach, CapSim model runs were conducted only for a subset of the area to develop linear correlations between initial and model-predicted porewater concentrations for alternatives such as capping, dredging, in situ treatment, and combinations thereof. The linear correlations were subsequently applied to the remaining area to predict post-remediation SWACs for each of the alternatives. These correlations were also applied for optimizing the extent of dredging and in situ treatment. Predicted post-remediation SWACs were compared with the 0.64 ng/L performance target to identify effective alternatives and develop remedial alternatives for the site.

Results/Lessons Learned:

The modeling results predicted that capping (with organic-rich or AC-amended sand), in situ treatment using activated carbon (AC), and dredging and subsequent capping (with AC-amended sand) could meet the post-remediation porewater target of 0.64 ng/L on a SWAC basis. Results also showed that dredging of the surface sediments and subsequent capping was only effective when AC-amended or organic-rich sand was used as the capping material. Conducting CapSim model runs for only a subset of the data and use of linear correlations reduced the total number of model evaluations needed by 75%. The approach presented here also facilitated easier optimization of the dredging extent. Based on the above evaluations, in situ treatment with AC was the recommended remedy for the site. This study presents a novel approach that combines passive sampling measurements with CapSim modeling that allows for evaluation of remedial alternatives against SWAC-based performance targets, as well as optimization of remedial components to suit site-specific requirements.

Jamie Hill

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A Novel CapSim Application for Predicting Post-Remediation Porewater Concentrations