Case Study: Gloucester MGP Remediation

In 2010, GZA was asked to evaluate a former manufactured gas plant facility that had since been redeveloped on a busy part of one of New England’s historic shorelines. What began as a preliminary site investigation became one of our most challenging and rewarding projects; one for which we’d have to marshal staff across seven of our 31 offices to work the two 12 hour shifts a day, seven days a week through two New England winters.  This work was made all the more challenging as engineering solutions were required on a real-time basis to address difficult field conditions encountered on land and in the water.

Hidden Problems On A Busy Waterfront

The project area was in a busy commercial harbor, which included a U.S. Coast Guard base, city park facilities, the city Harbormaster, a utility company, a local non-profit museum, a restaurant by the ocean, and local fishing operations. The assessment found non-aqueous phase liquid (NAPL) in soil, bedrock, and the sediments in the harbor. There were also buried structures, foundations, and abandoned underground tanks and pipes.

GZA wrote a remediation plan that, after extensive review by local stakeholders and state and federal regulatory agencies, recommended a number of remedial actions to improve the site. Contaminated soil near the harbor would be removed, where possible, and soil that couldn’t be removed would be isolated beneath an engineered barrier. Groundwater from the site would be treated with a vertical sand/Organoclay barrier and coal tar NAPL would be removed via automated pumping wells to address potential impacts to the harbor. Additionally, seven acres of contaminated sediment also needed to be dredged, which was done under the direction of Anchor QEA of Beverly, Massachusetts,.

Not Just Remediation

In addition to the abandoned infrastructure of the gas plant, there were existing seawalls, several built in the 1800s, that needed to be reconstructed to modern safety standards and with historical accuracy in mind. Because it was an active shoreline, piers, docks, and an active marine railway that had been in continuous operation since the 1800s needed to remain open for business while the work was performed. The construction was restricted to between Labor Day and Memorial Day, which meant much of the concrete placement and dredging would have to be performed during freezing winter conditions. Even the ocean and earth were uncooperative;  all work needed to be carefully planned around the rising and falling of a ten-foot tide, and 14-foot diameter boulders in the glacial soil severely impeded all forms of subsurface work.

These challenges were solved with creative thinking and innovative but practical solutions. One of the new seawalls, for example, was built with multiple components, each to address a specific purpose.  The main purpose was to provide a vertical groundwater treatment barrier that could be serviced, if necessary, in the future.  This resulted in construction of a geotextile wrapped-face wall just 18” landward of the back side of a new self-supported gravity granite block seawall, with the sand/Organoclay treatment barrier sandwiched in between; the seaward face of the new granite block seawall was constructed from block saved during the demolition of the original wall to match the waterfront’s historical architecture. Another adjacent seawall was built for groundwater containment, and was constructed of reinforced concrete and faced with granite blocks as well.The concrete pours for this wall were performed during below freezing conditions with water temperatures below 40 degrees, requiring special techniques to achieve concrete curing/strength.  These wall improvements will allow the public to more easily and safely access the historic waterfront for many years to come.

Meanwhile, 30,000 cubic yards of sediment were dredged and transported by barge for treatment. The dredge water was treated both on land and by a barge-mounted system, with the treated water returned to the harbor.

Today, you’d never know a harbor where tourists wander, fisherman offload their catch, and the Coast Guard scuds out to sea for search and rescue was in need of such extensive remediation. While automated systems now continue to unobtrusively recover coal tar NAPL from below the surface, GZA has not only orchestrated major improvements to the environment, but has done so in a way leaving new granite block seawalls looking completely consistent with Gloucester’s historic shorelines dating back to the 1800s.