Remediation of Coal Tar Impacted Intertidal Sediment in the Fore River, Portland Harbor, Maine



Remediation of Coal Tar Impacted Intertidal Sediment in the Fore River, Portland Harbor, Maine

Presenter:  John  Collins (AquaBlok, Ltd.)
Authors:  John A. Collins (AquaBlok, Ltd.), Jason Provencher, Richard Egan, and Peter Thompson  (Amec Foster Wheeler Environment & Infrastructure),
Thomas Murphy (Unitil Service Corporation)


Background/Objectives: From 1852 to 1965 the Portland Gas Light Company operated a manufactured gas plant (MGP) on the shore of the Fore River in Portland Harbor, Maine.  Inland portions of the former MGP site (gas and tar holders) were remediated under the Maine Department of Environmental Protection (MEDEP) Voluntary Response Action Program (VRAP) prior to 2008.  A shoreline parcel impacted by coal tar DNAPL was remediated in 2015.  The coal tar DNAPL seep that impacted intertidal sediments presented the several unique remedial design and construction challenges requiring a combination of remedies that were implemented under the operational constraints inherent to shoreline sediment removal in a busy marine port.  The remedial objective required elimination of sheen caused by discharge of coal tar DNAPL to the water column from disturbance of the impacted intertidal sediments by wave action.  The remedy combined a grouted sheet pile barrier wall to prevent future migration of coal tar DNAPL from upland sources with excavation of sediment behind an engineered turbidity / fish exclusion barrier.  The remedial design included a vertical barrier of AquaBlok®  behind the grouted sheet pile as a secondary means of containment to prevent future lateral migration of coal tar should imperfections ever occur in the grouted sheet pile wall. This approach is similar conceptually to a composite liner where imperfections in a geomembrane are constrained by a low permeability soil.   During remedial excavation of sediment, densely spaced deep timber piles were discovered in an area adjacent to the exterior of the sheet pile wall that severely limited ability to access contaminated sediment for removal.  This portion of the remediation was redesigned in the field to incorporate an armored subaqueous AquaBlok® cap to contain and prevent migration and exposure of the coal tar.  This change was successfully implemented due to the ability of AquaBlok Ltd to quickly produce and ship a custom product that could be placed in an integrated manner in conjunction with other remedial construction activities.        Approach/Activities: High resolution delineation of the upland coal tar seep (via TarGostTM direct push investigations) and visual confirmation of the extent of the intertidal seep provided an optimized design for the sheet pile wall location. Geotechnical stability of the barrier wall and depth of excavation on the exterior of the wall were critical design and constructability factors. At the sheet pile wall, the elevation of the seep source was backed with a vertical AquaBlok barrier on the exterior of the sheet pile wall as a secondary containment measure. Careful placement of the material behind removable steel plates was a critical QA/QC component of the construction.   The dredged area was restored with clean backfill materials and excavated sediments were transported off-site for beneficial re-use.


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