Sevenson Completes Gowanus Canal Pilot Project

At the staging area on the left bank of the canal the black piles of sediment treated with Portland cement are curing and tested periodically for stability.  Beyond them a bray tarp covers some of the debris that was removed from the 4th Street Turning Basin.  This view is of the main canal. The 4th Street Basin is farther upstream. Photo by Michael Cykoski.

Sevenson Environmental Services has completed a project to remove debris and collect sediment from a segment of the Gowanus Canal. The project is part of a pilot study required in the Environmental Protection Agency’s (EPA’s) Record of Decision (ROD) for the cleanup of the 1.8-mile-long 100-foot-wide canal in Brooklyn New York. The pilot study began the last week of October and ended on November 14 of this year.

A worker prepares to wash down debris removed from the 4th Street Basin in front of the Whole Foods store.  All items removed from the basin have been stockpiled at a nearby staging area and will be evaluated for archeological value before being hauled to a disposal site.  Photo courtesy of USEPA.

The Gowanus Canal Remedial Design Group composed of potentially responsible par-ties (PRPs) is conducting the study and Seven-son is the dredging contractor.  

The pilot study was conducted in the 4th Street Turning Basin a short spur off the main canal. An initial sonar scan located the debris prior to removal and Sevenson used floating cranes to remove more than a century’s worth of debris including sunken boats sheet piles tires concrete rubble timbers and general trash. After washing the debris down aboard the barges the crew transported it to a nearby staging area for temporary storage and evaluation for archeological importance.   

Sevenson then dredged several feet of silt with each of four digging tools testing them for possible use in the final canal cleanup. The dredged silt was transported to the staging area where Portland cement was added as a stabilizer prior to stockpiling onshore. The stockpiles are being monitored for possible use as capping material.  

A second pilot study took place in the sum-mer of 2015 to test the stability of the native material for use as a cap in tidal conditions.

“The pilot studies resolve the issues that come up so when we get to the main canal all issues and technical questions will be answered” said Christos Tsiamis EPA Region 2 Remedial Project Manager. “We are now in the design stage” he said.  

“The project went smoothly. We achieved our goals for the debris removal” Tsiamis said.  Included in the evaluation was the impact of the project on the surrounding community and control of turbidity. A full-scale dredging test will take place in the 4th Street Turning Basin next summer. 

The pilot projects are being paid for by the Gowanus Canal Remedial Design Group led by the City of New York and National Grid. The latter is the owner of former manufactured gas plants (MGPs) lining the canal. On March 21 2014 EPA issued the unilateral administrative order (UAO) to the PRPs to conduct the remedial design for the dredging portion of the cleanup at a cost pf $35 million.

The Gowanus Canal is one of the nation’s most seriously contaminated water bodies. It is a tidally influenced dead-end channel that opens to Gowanus Bay and Upper New York Bay.  Built in the mid-1800s it was used as a major industrial transportation route to flush away sewage and receive stormwater. Manufactured gas plants pa-per mills tanneries and chemical plants operated along the canal and discharged wastes into it. In addition the canal receives overflows from sewer systems that carry sanitary waste from homes rainwater from storm drains and industrial activities. More than a dozen contaminants including polycyclic aromatic hydrocarbons polychlorinated biphenyls and heavy metals including mercury lead and copper are found at high levels in the canal’s sediment. 

The ROD describes the sediment in the canal as ranging in thickness from approximately one foot to greater than 20 feet with an average of about 10 feet. The thickest deposits are found at the head of the canal and within the turning basins. The soft sediment consists generally of a dark gray to black sand/silt/clay mixture that contains variable amounts of gravel organic matter (e.g. leaves twigs vegetative debris) and trash. Odors described as “organic” “septic-like” “sulfur-like” and “hydrocarbon-like” were commonly detected in the soft sediment during investigation as were visible sheens. The soft sediments are underlain by the alluvial and marsh deposits of the Gowanus Creek complex that were present prior to the canal’s construction. 

The canal is near several existing and planned residential neighborhoods. A Whole Foods Market with adjacent park and walking path was built on a brownfield site and on the opposite bank a 700-unit apartment complex was recently opened.  

Sevenson Environmental Services is the dredging contractor on the Gowanus cleanup. Geosyntec is the primary technical consultant and Engineer of Record for Gowanus Canal Re-medial Design Group with a goal of establishing a remedial design that is constructible sustain-able cost-effective and permanent.

DESCRIPTION OF REMEDY

Once the remedial design has been finalized dredging in the main canal can begin. Approximately 588000 cubic yards of sediment will be removed treated and transported for a final cost estimated at $506 million. 

The selected remedy addressing the contaminated sediment in the Gowanus includes the following components: dredging the entire column of hazardous substance-contaminated sediments (referred to as “soft sediments”) which have accumulated above the native sediments in the upper and mid-reaches of the canal; in-situ stabilization of those native sediments in select areas in the upper and mid-reaches of the canal contaminated with high levels of non-aqueous phase liquid (NAPL); construction of a multilayered cap in the upper and mid-reaches of the canal to isolate and prevent the migration of polycyclic aromatic hydrocarbons (PAHs) and residual NAPL from native sediments; dredging of the entire soft sediment column in the lower reach of the canal; construction of a multilayer cap to isolate and prevent the migration of PAHs from native sediments in the lower reach of the canal; off-site treatment of the NAPL-impacted sediments dredged from the upper and mid-reaches of the canal with thermal desorption followed by beneficial reuse off-site if possible; off-site stabilization of the less contaminated sediments dredged from the lower reach of the canal and the sediments in the other reaches not impacted by NAPL followed by beneficial reuse off-site; excavation and restoration of approximately 475 feet of the filled-in former 1st Street turning basin; excavation and restoration of the portion of the 5th Street turning basin beginning underneath the 3rd Avenue bridge and extending approximately 25 feet to the east and the installation of a barrier or interception system at the eastern boundary of the excavation; implementation of institutional controls incorporating the existing fish consumption advisories (modified as needed) as well as other controls to protect the integrity of the cap; periodic maintenance of the cap and long-term monitoring to ensure that the remedy continues to function effectively; combined sewer overflow (CSO)4 controls.  

Point source pollution is being addressed prior to and in phased coordination with the sediment cleanup.