Project Description

Following are the steps most often required by the New York State Department of Environmental Conservation in the investigation and remediation of a former MGP site.

Remedial Investigation/Feasibility Study
Interim Remedial Measures
Record of Decision
Design and Construction

The Sag Harbor Site project has completed the Design and Construction Phase and is now under Site Management.

Remedial investigation Summary:

  • Contaminants detected in soil and groundwater are consistent with those expected for a former MGP site (primarily the BTEX complex – Benzene, Toluene, Ethylbenzene and Xylene; PAH’s – polycyclic aromatic hydrocarbons.
  • The contaminants associated with the site were observed to be migrating off-site in a north-northwesterly direction, under an adjacent public roadway and privately owned properties, in the direction of Sag Harbor Cove. However, sampling of sediments and surface water in the Cove did not show the presence of contaminants associated with the former MGP operations
  • There are no data to suggest that the public was being exposed to the chemicals present in surface soil or groundwater either on the site or in the community, although the potential for exposure – in both current and potential uses – existed absent remedial measures. There were also no data to suggest that the contaminants were having an adverse impact on fish or the transient wildlife in the community.

Interim Remedial Measures:

Prior to its acquisition by National Grid, KeySpan undertook an Interim Remedial Measure (IRM) to minimize any current impact of the environmental conditions related to the historic use of the site.

  • Early in the project, KeySpan undertook a “cut-and-plug,” capping underground pipes that could act as conduits for the migration of contaminated groundwater or source material.
  • In addition to the IRM, National Grid has completed the demolition of the Hortonsphere and its related structures on the property, and the relocation of utilities in the areas where excavation or construction will take place.

Record of Decision:

The Record of Decision described the remedial goals and specific remedial requirements for the site to achieve those goals. For the Sag Harbor site the goals are:

  • to eliminate or reduce to the extent practicable exposures of persons at or around the site to PAHs and BTEX in soil, groundwater and soil gas vapors;
  • environmental exposures of flora or fauna to PAHs and BTEX in soil, groundwater, soil gas vapors and sediment;
  • the release of contaminants from soil into groundwater that may create exceedances of groundwater quality standards; and the release of contaminants from surface soil, subsurface soil, soil gas vapors, groundwater, and sediment into surface water, indoor air, ambient air, sediment, and soil gas vapors through storm water erosion, vaporization, wind borne dust, and groundwater discharge.

Further, the remediation goals for the site include attaining to the extent practicable:

  • ambient groundwater quality standards and
  • recommended soils cleanup values for surface soils

The remedial requirements for the Sag Harbor site were:

  • Excavation of visibly tar-contaminated soil to approximately 10 feet below ground surface (bgs).
  • Installation of passive tar collection wells to remove deeper tar that cannot be removed by excavation.
  • Evaluation and, if necessary, installation of vapor mitigation systems under existing or new structures constructed on or adjacent to the site to control potential migration of contaminated soil gas into these structures.
  • Covering all vegetated areas with clean soil and all non-vegetated areas with either concrete or a paving system.
  • Development of a site management plan to address residual contamination and any use restrictions.
  • Provide certification of the institutional and engineering controls

Remedial Construction Plan:

Description of Work:

The remediation work consisted of excavation of contaminated soil in the top 10 to 15 feet of the site and on several surrounding properties. Constructing and completing the remedy consisted of several activities:

  • Prior to excavation, a soil mix wall was installed around the perimeter of the excavated area. The purpose of the wall is to provide stability during the excavation and to assist in dewatering the hole. A soil wall was chosen instead of sheet piling to reduce the impacts of noise and vibration on the community. The wall extends approximately 1000 feet around the perimeter and is 10 to 15 feet wide.
  • After the wall was installed, a temporary fabric structure (tent) was erected on the site, and placed over the areas being excavated. This structure helped control the release of vapors and dust during the excavation activities. The tent operated under negative pressure, air from within the tent being continually evacuated and treated using carbon filtration prior to release. The tent was moved from one area to another through the course of the excavation activities. Approximately 20,000 cubic yards of material were excavated from the site. A trucking plan to minimize traffic impacts was developed with the Village Police Department and worked effectively.
  • During excavation activities a dewatering system was used to lower the groundwater levels at the site, allowing more efficient and complete removal of impacted soils. Water was removed from the subsurface using a series of pumps and wellpoints. The water was treated in a temporary facility built on site to remove site related contaminants to meet state permit limits prior to discharge. The rates of pumping ranged from 500,000 to 1,000,000 gallons per day. The treated water was discharged through a pipe to an area of Sag Harbor Cove specifically chosen because minimal impact to water salinity would occur there. The pipe was located and clearly marked to not interfere with marine navigation. Current measurements show that there was no measurable impact on salinity or other environmental factors in the Cove.
  • Soil was excavated and stockpiled under the tent prior to loading into trucks for transport and disposal. The truck beds used to transport the soils were lined with a plastic liner and the tops of the trucks covered to prevent leaks or spills in transit. No spills or leaks were reported during the excavation project. Soils excavated from the site were sent offsite for thermal treatment at a permitted disposal facility.
  • The hole was backfilled using certified clean soil from a local source, and the site returned to current grade. All of the equipment used during the remediation will be demobilized.

Other Key Elements:

  • All work related to construction of the remedy was undertaken between September 2008 and Memorial Day 2009. There was no work during the “season” to minimize disruption to community activities. The project was finished on time.
  • A Community Air Monitoring Plan was in place throughout the project as well as continued testing of the water influent and effluent from the dewatering system. Any exceedances of dust or chemical emissions from the site were transient and at levels below health or environmental impacts.

Current Status:

National Grid is developing a Construction Completion Report and finalizing a Site Management Plan for the former Sag Harbor MGP Site. The Site Management Plan, once finalized and approved by the NYSDEC and NYSDOH, will be implemented on National Grid property and the adjoining areas and residences.

The site is now in the Operations, Maintenance and Monitoring phase with periodic groundwater sampling.