EPA/600/R-10/126

This report evaluates and summarizes dredge residuals and dredge removal efficiency in support of GLNPO’s objectives for this joint project. This evaluation is restricted to pre-dredging characterization studies conducted in 2006 and post-dredging residuals information generated after completion of dredging operations in 2007. Long-term residuals data produced from follow-on post-dredging field studies in 2009 are still being analyzed and are not included in this report. Investigative results of immediate and long-term effects of dredging operations on ecosystem health and restoration using biological indicator, food web, and surrogate sample data will be presented in a subsequent report.

The primary goals of this phase of the project were to develop: 1) an understanding of sediment residual formation during dredging operations on the Ashtabula River, and 2) methods to assist in generating more realistic predictions of post-dredging residual mass/volume and contaminant concentrations. To achieve these goals, extensive monitoring studies and physical and chemical measurements were carried out prior to, during, and after dredging. Field efforts focused on:

1. estimating the volume and concentration of contaminated sediment residuals remaining after completion of dredging in a selected study area within the dredge zone, and
2. comparing pre- and post-dredging sediment mass and concentration characterization data to assess the PCB concentration relationship of the residual sediment to the contaminated material removed.

A weight of evidence (WOE) approach was developed by ORD to characterize dredging residuals. The WOE approach relied on multiple lines of evidence (LOEs) to measure residuals generated during dredging. Six LOEs consisting of chemical and physical measures were used to evaluate dredge residuals in an attempt to address study objectives. These LOEs included:

1. vertical alignment and physical examination of pre- and post-dredge sediment cores,
2. physical parameter analysis of the sediment,
3. sediment PCB chemistry and data analysis of core segments,
4. two- and three-dimensional PCB modeling of pre- and post-dredge areas,
5. bathymetric surveys prior to, during, and following dredging , and
6. dredge cutter head horizontal and vertical positioning.

The above tools, methods, and interpretations drawn were not applied on a site-wide basis, but rather to ORD’s selected study area of a 1,100-ft long stretch of the river from River Station 181-00 to River Station 170-00. These approaches are being evaluated on spatial and temporal scales as necessary for their development, and, therefore, cannot be used to evaluate the success of dredging for the entire site.

The residuals data produced in the project study area indicated consistent sediment and PCB mass removals equal to or in excess of 95%. The data also revealed that the sediment residuals layer was composed of more highly contaminated sediments originating from higher elevations in the vertical sediment profile, rather than the lower-concentrated sediments removed from immediately above the final post-dredge sediment surface.

This final report is presented as Volume I of the Ashtabula River dredge residuals project. Due to the large amount of data generated on this project, an additional data report (Volume II) will be issued by ORD as an extended set of Appendices with additional data tables and figures. A subset of these data is included in this report to illustrate the approaches, methods, and interpretations applied on this project.

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Marc Mills

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