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 several different entities. 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, followed by design development and construction management and oversight of a multi-season upland, nearshore, and harbor remedy implementation.
The selected remedial alternative included the following elements:
- Removal of contaminated 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 to treat upwelling porewater
- Isolating deep MGP-impacted soil via the construction of an engineered barrier
- 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;
- Dredging sediments surrounding piles that supported an active, historical marine railroad
- An expedited timeline to meet short winter working windows, as established by regulators and local restrictions
- Concrete design and QA/QC monitoring of concrete pours for/during freezing marine conditions
- Requirements to work multiple shifts, up to 24/7, to meet schedule and work within tidal cycles
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.