In 2010, when coal tar was detected in the harbor adjacent to a former manufactured gas plant (MGP) site, GZA was retained to analyze the preliminary site investigation data previously collected. The redeveloped site now spans upland and in-water properties owned and operated by seven different entities, including the federal government, a local municipality, a utility company, a marine museum, a harbor-side restaurant, and commercial fisheries-related operations. GZA’s services expanded as client’s needs grew on this complex site.


Based on the initial data review, GZA developed a Conceptual Site Model that formed the basis for subsequent assessment and remedial actions.

The assessment phase of the project involved:

· Geophysical surveys to identify the locations of buried structures, gas holder foundations, DNAPL tar tanks, etc.;

· Drilling of soil borings in the upland, and marine borings in the harbor;

· Installation of multi-level groundwater monitoring wells to assess the nature and extent of groundwater contamination, including the extent of NAPL in the upland and harbor;

· Collection and analysis of soil, bedrock, groundwater, sediment, surface water, pore water, soil vapor, and indoor air samples;

· Assessment of tidal influences on groundwater flow and marine porewater upwelling, including via piezometers placed in off-shore harbor sediments, sediment sampling and pore water extraction/testing to identify the location/extent of porewater upwelling zones in the harbor;

· In-situ hydraulic conductivity testing to support contaminant flux computations;

· Assessment of the risk to human and ecological receptors, including benthic organisms, for each individual property at the site; and

· Preparation of a Comprehensive Site Assessment (CSA) and Human Health and Ecological Risk Characterization Report.

Following the completion of the CSA, GZA prepared a Remedial Action Plan (RAP) that presented an initial screening followed by a detailed evaluation of potentially applicable remedial technologies for each property within the site boundaries. The RAP resulted in the selection of a comprehensive remedial action alternative that was presented to regulatory agencies and all affected property owners for third-party review. The selected remedial alternative included the following elements:

· Excavating and disposing of contaminated upland soil (source material) adjacent to the harbor, requiring earth support and dismantling and rebuilding historical granite block seawalls;

· Constructing a vertical concrete and sheet pile barrier to prevent migration of impacted groundwater and NAPL, requiring stabilization/ underpinning of walls and structures;

· Dredging approximately seven acres of contaminated harbor sediments;

· Placing an armored, active marine cap (both as unconfined granular layers and Marine Armor Mattresses (MAMS)) to treat upwelling porewater;

· Isolating deep MGP-impacted soil via the construction of an engineered barrier; and

· Automated NAPL recovery from overburden and bedrock.

Site-specific challenges considered during remedial design included:

· The presence of former MGP infrastructure, including buried seawalls within the upland subsurface and debris within the harbor;

· Granite boulders and bedrock, which limited the ability to drive sheets for Support of Excavation (SOE);

· The instability of existing granite block seawalls, which were constructed in the late 1800s;

· Minimum draft requirements established by the City harbormaster and the U.S. Coast Guard (USCG);

· A requirement for continued access to piers and docks for ongoing operations at the various properties, including USCG search-and-rescue, and minimal impact to a federal navigation channel;

· Dredging sediments surrounding piles that supported an active, historical marine railroad;

· An expedited timeline to meet short winter working windows, as established by the Division of Marine Fisheries and local restrictions;

· Concrete design and QA/QC monitoring of concrete pours for/during freezing marine conditions; and

· Requirements to work multiple shifts, up to 24/7, to meet schedule and work within tidal cycles.


The remedial design phase of the project included:

· Evaluating the stability of the existing seawalls and multiple buildings founded on and immediately behind the seawalls;

· Geotechnical borings in the upland area to support design of new seawalls and associated SOE;

· Geotechnical borings in the harbor for design of temporary and permanent piers and rock-socketed, epoxy-coated steel pipe piles for a new municipal floating dock system;

· Evaluating materials (Organoclay, AquaBlok, Bentomat, Reactive Core Material) for upland vertical barriers and in-water capping;

· Pilot testing NAPL recovery systems;

· Reviewing treatability studies for sediment stabilization and suction dredging; and

· Design/specification and monitoring of treatment systems to allow water from sediment dewatering to be treated for direct discharge into the harbor.

GZA prepared drawings and specifications designed to achieve remedial objectives while limiting disruption of ongoing site operations, based on the geochemical, geotechnical, and hydrogeological information collected during the assessment and remedial design phases. Key design elements included a geosynthetic-wrapped face wall fronted by a drained granite block seawall with a replaceable Organoclay/sand groundwater treatment chimney between the two walls, and an adjacent impermeable concrete seawall with subsurface sheetpile wall as a vertical barrier to NAPL migration through/below the seawall. These seawalls were designed to replace the unstable granite block walls and to meet historical commission requirements for consistency with the existing wharves in the harbor.

The design also included the placement of specific backfill types with varying permeability behind the seawalls to control/direct post-construction groundwater flow, the use of an Organoclay/sand chimney and Reactive Core Mats as hydrocarbon-migration barriers, and the construction of an engineered barrier (horizontal cap) over a portion of the impacted soil within the park. GZA also provided design services for various in-water marine structures, including a temporary docking system with wave attenuation barriers for use by Coast Guard vessels during the remediation, a steel pile-supported floating dock system for the City, and reconstruction of wooden piers.

In conjunction with the design, GZA submitted permit applications to various regulatory agencies with jurisdiction over waterfront and in-water work. Key considerations identified during the permitting phase included the need to restore habitat in the harbor following dredging and a requirement to limit impact to operations within the federal navigational channel.

Beginning in 2015, GZA has provided construction management services and full-time field oversight for the approximately $30 million project. At one property, an SOE system consisting of steel micropiles was drilled and grouted in place behind the existing granite block walls, with wood lagging and steel plates installed between the piles. The SOE was tied back to a series of concrete deadmen, after which the granite seawalls and associated contaminated soils were excavated in the wet. A new seawall system consisting of a geosynthetic-wrapped face wall with a vertical Organoclay/sand barrier atop a six-foot-thick tremie concrete foundation was constructed and fronted by a granite block wall visually consistent with the historical waterfront.

To support a new seawall footing, a secant pile wall was then drilled at the outer edge of the proposed seawall footing to serve as a hydraulic cutoff wall. Steel sheeting with mastic-filled interlocks was driven into the secant wall alignment and was toed into either the grouted secant piles or into an underlying clay layer. The tops of the steel sheets were keyed into the concrete wall footing, and angle iron joint seals over the interlocks were filled with AquaBlok to provide a secondary seal against migration of contamination past the cutoff wall. A reinforced concrete stem wall was then constructed during freezing winter conditions requiring special concrete formulations including crack-control fiber reinforcement, insulation, and strength gain monitoring. The wall was finished with a granite façade to match the historical seawalls along the waterfront, and a concrete cap was placed over the fill behind the wall to form a new walkway providing public access to the waterfront.

At the USCG property, a concrete stability wedge was constructed in the harbor adjacent to the seawall to stabilize the existing granite block wall and provide a portion of the porewater cap. Sand was placed atop the wedge to create new benthic habitat.

Within the harborapproximately 30,000 cubic yards of impacted sediment was dredged and transported by barge to an off-site facility. Mechanical dredging with HYPACK-controlled environmental buckets was conducted across most of the site, with localized suction dredging beneath piers and around sensitive waterfront structures, including an active historical marine railroad.

Dredge water was treated in both upland and barge-mounted treatment systems and discharged back to the harbor under permit. Clean fill material was placed across most of the nearshore areas to manage dredge residuals and restore habitat. A granular porewater cap consisting of an Organoclay/sand layer overlain by a filter layer and armor stone was placed in one area of the harbor, while replaceable MAMs with similar layering were used in areas with restrictive minimum draft requirements.

Following the dredging, GZA oversaw the installation of an array of 70-foot-long concrete-filled, epoxy-coated steel pipe piles, drilled into the bedrock beneath the harbor to withstand tidal and wave action, to support a deep-water draft City-owned floating dock system.

In the upland area, stainless steel well screens with oleophilic inserts were placed in vertical crushed stone chimneys constructed just outboard of the SOE walls to remove LNAPL sheens and extend the longevity of the Organoclay/sand chimneys constructed just inboard of the granite block walls as LNAPL barriers to protect the harbor. In addition, an automated DNAPL recovery system was installed across multiple properties and will be operated by GZA until no additional DNAPL recovery is feasible from the overburden and bedrock.


This technically-challenging assessment and remediation project benefitted from GZA’s expertise in a wide range of environmental, engineering, ecological, and construction management disciplines, resulting in successful integration of the NAPL collection system and integration into the seawall construction.